A Fine C. Plath Vernier Sextant

4 12 2011
Previous posts in this category include:  “A C19 Sextant Restoration” , “Making a Keystone Sextant Case” , “Restoring a C. Plath Drei Kreis Sextant” , “Heath Curve-bar sextant compared with Plath” , “A Drowned Husun Three Circle Sextant”, ”Troughton and Simms Surveying Sextant” , “A Sextant 210 Years On” , “A fine sextant by Filotecnica Salmoiraghi”, “A British Admiralty Vernier Sextant”, “An Hungarian Sextant via Bulgaria” and “A Half-size Sextant by Hughes and Son”.
 
Clicking on the figures will enlarge them and allow you to see more detail, while clicking on the back arrow (top left) will restore the post.
 
Until the late 1930s, sextants seem to have been provided with a variety of viewing  accessories, whereas by the 1950s this had been reduced to a single star or Galilean telescope, perhaps with the addition of a sighting tube. The Japanese firm of Tamaya continued until late in providing a Galilean telescope and high power inverting ‘scope, usually ten or twelve power. The size of the kit in pre-war sextants probably depended more on the depth of the buyer’s pocket than on any particular utility of the optics provided. I recently acquired a Plath vernier sextant, dated no later than 1923 that has a more-or-less standard kit, with the addition of a pair of binoculars. A later post will describe an approximate British equivalent of about the same date which has an even more elaborate set of optics.

The Plath instrument was in generally good condition except for the paintwork, which showed the expected wear and tear of a seventy year-old instrument and my first task was the by-now routine one of stripping down the whole sextant to its component parts and repainting the frame, index arm, shades and so forth. I polished screw heads, cleaned out the slots, replaced screws that had  damaged heads and re-assembled the parts to give the appearance of a near-new instrument. A few shrinkage cracks in the case required refilling and the sextant pocket had disintegrated, so this had to be re-assembled and made fast again to the floor of the case. I also cleaned up the exterior brass work and lacquered it. There were no particular difficulties in restoring the instrument, so I will confine myself to a description of it as it now is, working from the outside in.

The case is made of high quality quarter sawn mahogany, from an era when such precious woods were available solid  in substantial widths (Figure 1). The corner s have box comb joints and the top and bottoms are glued and screwed to the sides. The handle is of typically elegant C Plath form. Hook latches hold the lid closed and there is also a two lever box lock, used more as an insurance against the lid falling open than as a theft deterrent. The wise (or obsessional) mariner might also carry it with an index finger over the lid or with the lid against the side of his leg. The brass keyhole escutcheon carries an engraved “Sunseeker” C Plath logo (Figure 2).

Figure 1 : Exterior of case

 Figure 2 also shows the Sunseeker logo on the name plate inside the lid of the box and engraved on the front end of the limb. The latter also carries the serial number, dating the sextant to no later than 1923 and the stamp of Deutsche Seewarte, the German Hydrographical Institute that, like the National Physical Laboratory in Britain, assured the quality of nautical instruments. A final detail shown is the stop screw that limits the movement of the index arm. Many makers omitted this detail, allowing the base of the horizon shades mounting at one end and the telescope mounting at the other to halt the movement of the index arm.

Figure 2 : Sunseeker logos on lock escutcheon, name plate and limb

 
Figure 3 shows the sextant in its case. The ladder-pattern frame is of bronze with a silver arc of 175 mm radius and the vernier scale is divided to ten seconds, while the index arm and other attached parts are of brass or are small bronze castings. The index mirror, small by modern standards, measures 44 x 32 mm, with an horizon mirror of 32 x 30 mm. The Galilean or “star” telescope is 2 1/2 power x 26 mm and the Keplerian or “inverting” telescope is 6 power x 17 mm. An unusual feature is the provison of a pair of Gallean 3 x 26 mm binoculars, complete witha  rising piece that allows them to be used in conjunction with the sextant. This is probably an advantage when the horizon is indistinct, as approximately one and a half of the binoculars views the horizon while the other half views the reflected body. This is probably an oversimplification of the actual state of affairs. It is accepted that the brain does not receive twice as much information when binoculars are used but 1.414 times as much (the square root of 2 times as much). The instrument is held in the case in a wooden pocket with cross bar. This is not altogether a satisfactory method, as the grain of the wood is apt to give way and set the sextant adrift. The chain that prevents the lid from falling backwards is something that I now add to all fine sextant cases, as it prevents the hinges from being strained if the lid falls open.
Figure 3 : Contents of case.
The remainder of this account is concerned with design details, starting with the mirror adjusting screws. Referring to Figure 4, which shows the rear of the index mirror bracket, the screw that adjusts the mirror for perpendicularity passes through a threaded hole in a brass strip and then through a threaded hole in the back of the mirror bracket. A second screw passes through a clearance hole in the strip and into a threaded hole in the bracket. One end of the strip is bent to form a foot and when this second screw is tightened, it tends to lock the adjusting screw, as clearances in the thread of the latter are taken up. Some sextant manufacturers, Tamaya in particular, copied this set up and often appear to have omitted to form a foot, making locking a hit or miss affair, especially when a flat strip bearing a round nut was simply attached tightly to the back of an aluminium bracket.
 
 

Figure 4 : Mirror adjusting device

 
Until the Second World War forced makers to make economies in materials and time, the rising pieces of sextants was usually provided with some form of fine adjustment for height, often together with an adjustment to allow the optical axis of the telescope to be made exactly parallel with the plane of the arc. Figures 5 and 6 show one such arrangement.

Figure 5 : Rising piece.

 
 The adjusting knob  and feed screw are held captive in the bracket, and when the screw rotates it causes a rectangular nut to move along it. Attached at one end to the nut is a flexible metal strip or clip, which has a hole in its other end for a button on the telescope rising piece. The clip has a longitudinal slot through which the locking screw passes into the bracket. When the screw is unlocked, the feed screw can cause the clip with the telescope rising piece to move towards or away from the frame of the sextant, to allow the telescope to see more or less of the horizon. The telescope ring can be made to tilt in the rising piece to bring the axis of the telescope parallel to the frame of the sextant, by means of a pair of adjusting screws (see my post of 2 September 2011 : Tamaya Collimation Blunder for details  ).

Figure 6 : Rising piece exploded.

 The lower end of the index arm is conventional (Figure 7), with a Ramsden-type magnifier to allow the vernier to be read easily and a diffusing screen to reduce glare when doing so. The slow motion adjustment does not differ in any essential respect from that described by G. W. Heath of the British instrument makers Heath and Co., in their patent application granted 10 March 1910 (British Patent  no. 17840). However, well before this date in about 1907, C Plath had invented the release catch and slow motion adjustment of a micrometer sextant that was later to become the standard arrangement used by almost every other maker except Heath and Co.  There exists a Plath instrument that was almost certainly made before 1907 and that has the Heath arrangement. It is not clear quite why they continued to make it as late as 1923, when their new arrangement was  easier to manufacture and superior in use. Possibly there were conservative mariners who continued to want vernier sextants at a time when the micrometer sextant was less than fifteen years old.
Figure 7 : Index arm details.
Figure 8 shows some details of the rack and worm. The worm shaft is mounted in bearings on a swing arm or plate and end float of the shaft is prevented by a pre-load leaf spring (Heath used a cone-ended screw and lock nut). The plate itself is mounted between centres, and when the release catch is squeezed  the plate swings away from the limb of the sextant against a spring between the two button of the catch. This brings the worm out of engagement with the rack and the index arm can then be swung rapidly to any required position before releasing the catch, so that the final fine adjustment can be made. The worm has a pitch of about 0.5 mm so that its threads and the teeth of the rack are rather delicate and prone to injury if the worm is accidentally dragged agains the teeth. However, there is no need for great accuracy in cutting the worm and rack, in contrast to the requirements of a micrometer sextant.
 

Figure 8 : Rack and worm.

 
 If you have found this account of the details of a sextant of ninterest, you will find many more similar details in my book “The Nautical Sextant”, available through bookstores, Amazon and direct from the joint publishers, Paradise Cay Publications and Celestaire.
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C Plath Sun Compass

16 10 2014
Figure 1: Dan LaPorte's sun compass.

Frontispiece: Dan LaPorte’s sun compass.

This post was preceded by  “A Fleuriais’ Marine Distance Meter” A Stuart Distance Meter”;“A Russian Naval Dip Meter”; and  “An Improvised Dip Meter”

Sometimes, kind people lend me their precious instruments for me to deconstruct and examine so I can post details on this site. I invited Dan LaPorte to contribute a “guest blog post” and he has kindly obliged. Dan’s contribution is in blue, and my comments and additions are in black.

Last year I obtained a WWII Plath sun compass via an e-bay purchase.    At the time I really didn’t know exactly what I had bid on and won, but I did know it was something out of the ordinary.

First, a little about me and why I would be remotely interested in such an item.  I am a retired  US Merchant Mariner having sailed for some thirty-five years at sea, twenty of those as a Ship’s Master.  During that time, I acquired many skills and interests, one of them being magnetic compass correction.  For years I’ve used an Abrams and an Astro sun compass for such duties.  Both work on the same basic principal of local time or hour angle to obtain a true bearing of the sun or other celestial body.  Hence I was immediately interested in the Plath sun compass.

Upon delivery of the item I was saddened to find that the clock work no longer functioned (the Plath uses a Junghans 30 clock work  with optical sight for taking a bearing of the sun).  In fact the Junghans 30 movement was also used in the ME 109 fighter of the same era.   The idea is to set the correct time (more on this later), so the sun compass will track the sun’s path and hence a constant bearing using the sun can be obtained.  When functioning and set correctly a true bearing can be recorded of a landmark to obtain a position, or drive (or fly) from a known position to another by following the desired course.  Another use of the Plath would be to check and correct an aircraft’s magnetic compass while on the ground.  

After a bit of research I found that this model was used almost exclusively by  German troops deployed to the  North Africa corps during WWII.   Of course all this would have been unknown to me if not for the assistance of Mr Malcolm Barnfield.   By contacting Malcolm via his web site: http://www.sundials.co.za , I was able to obtain a wealth of information on the Plath.   Malcolm is without a doubt one of the most knowledgeable people in the world on the topic of sundials, sun compasses and their use.   Without Malcolm’s expertise on the topic I would have certainly been lost for much longer, and perhaps forever.  Malcolm was also good enough to put me in contact with other very talented men, highly regarded on the same topic, such as Mr Konrad Knirim who provided a manual for the Plath, and Mr Kuno Gross, who translated it from German to English for me.  These highly accomplished men in their fields were good enough to assist me in my search for information regarding the Plath. 

While history of the Plath was very interesting, it did nothing to solve the one large remaining problem – it simply didn’t keep time and thus was nothing more than an interesting item to marvel at and only ponder at its use.  

Enter Bill Morris.   Bill and I had communicated for months on various topics related to celestial navigation both air and sea.  Bill is regarded by all that know him as one of the most knowledgeable people in the world where navigation instruments and their structure are concerned.   Bill has written books on the topic and provides detailed manuals for repair of several  sextant types both aeronautical and nautical.   His manuals are truly works of art and allow the layman to repair and bring sextants back to working order.  Bill had in the past repaired an old A10A aircraft sextant for me that works perfectly to this day.  Given his talent for repairs, knowledge of machine tools and ability to work on intricate and complex antiques with a sure touch, I asked if he would be good enough to have a look at the workings of the Plath.   I should state at this point that I was, and remain very protective of the Plath and would not allow just anyone to begin repairs on it.  Bill was my first and only choice that I would trust enough to allow any attempt at repairs.  As luck would have it Bill was to travel to Katy in Texas, not all that far from my home.  Add to this I would be able to finally meet Bill face to face.  In short it was a perfect and fortunate turn of events. 

I was able to meet Bill and his lovely wife for a visit in August of this year.  We enjoyed a very nice chat and lunch, covering topics ranging from navigation to what should be seen while in Texas.   I left the Plath safely in Bill’s hands, with hopes he could repair it.  A mere week later I had the Plath in my hands and working perfectly.

At this point the Plath was repaired, and I had a basic knowledge of how to use it.   When the Plath arrived I at once set it to local standard  time adjusted with the EQT ( Equation of time) from the nautical almanac.  To my dismay it did not point to North or any other direction.  In fact is seemed to be some 15 degrees off to the East at best.  That is, I would have to be another time zone to the East for the Plath to be anywhere near correct in obtaining a true bearing. Adding to my frustration, I was not entirely clear on how to orientate it to obtain a true bearing (the manual giving scant information in the translation). I set both the Abrams and Astro Compass in a hope to clarify the situation, this only proved to entangle my thoughts even more, at least for the moment. 

A few words about the equation of time are perhaps appropriate. Our daily life is governed mainly by the sun and its passage across the sky is not perfectly regular. It slows at some times of the year and speeds up at other. This is partly because the Earth’s orbit is slightly elliptical, so that it speeds up when nearer the sun and partly because the Earth’s axis of rotation is inclined at about 23 1/2  degrees to the plane of its orbit, so that the component of the Sun’s apparent velocity parallel to the equator  varies with the seasons. It is very difficult to make clocks to follow these variations, so the concept of mean solar time was invented, the average time for the Sun’s apparent rotation around the Earth. The difference between the apparent time on a given day and the mean solar time is known as the equation of time, often shown as a graph as in Figure 2, and in the bottom right hand corner of the daily pages of the Nautical Almanac. In the sun compass, it has to be applied as a correction on a given date to the mean time so that the alidade will point correctly to the sun.

Figure 2: The equation of time.

Figure 2: The equation of time.

 After further study I found the problem. To outline what the problems was I first have to explain the use of the two sun compass types I was more familiar with.   As stated previously, my tools used for obtaining a bearing of the sun or other celestial body was the Abrams or Astro Compass.   The Abrams uses local standard time, adjusted east or west of the standard time meridian, the observer’s approximate latitude and an adjustment for EQT provided on the face of the sun compass.  When all these details are known and set the instrument will provide the desired bearing by using the shadow of the sun.  The instrument has to be updated by moving the time marker one mark on the scale every four minutes.  This of course is due to the movement of the sun covering one degree of longitude every four minutes.  Simple when you know how.  The Astro Compass uses the settings of:  Local Hour Angle (LHA), declination of the body and latitude of the observer.  The declination of the body is obtained from the Nautical Almanac, LHA is calculated from your known longitude and applying it to the GHA of the sun or other celestial body.  Latitude of the observer would also need to be known and set on the instrument.  As with the Abrams the Astro needs to be constantly updated by moving the LHA scale in keeping with the sun’s motion across the sky.  

Why am I boring the reader with these details?  Simply to drive home the use of the Plath and the ingenious setting of the unit.  Unlike the Abrams and Astro the Plath is set to GMT standard time (not DST).  The user would also need to apply EQT to the time setting in order to obtain solar time with the EQT sign ( -/+) reversed due to the correction from a local time to GMT.  Once set to GMT – Solar time (GMT with EQT applied) the user then simply sets his latitude and longitude on the Plath.  No further corrections  and no almanac entries are needed.  As long as the Plath keeps correct time, and the user updates the estimated position of latitude and longitude, the unit will continue to function.  My mistake was in setting the Plath to local time.  I had wrongly assumed local time would be used as with my other instruments.  The Plath’s use of GMT is a perfect solution when one has time to reflect on the subject.  

Needless to say when the details of correctly setting the Plath were known and understood another test was in order.   So, one afternoon with the sun high and bright in the sky I set the Plath.  It should be noted that as with all other sun compasses it needs to be mounted securely to a stable platform and levelled with the provided spirit level. I also set the Abrams and Astro compass at the same time, a kind of a sun compass smorgasbord if you will.    To my amazement the Plath indicated true north as checked by my Abrams and Astro Compass (any course could have been selected for the test).  Given that the ultimate test of the Plath was to maintain a true bearing for hours or even throughout the night a test for the rest of the evening continued. I allowed the Plath to run for a few hours checking it now and then.   As per the design, the Plath displayed a constant true bearing until sunset due to the clock works keeping time and following the transit of the sun across the sky.  The Abrams and Astro compass would have had to be manually corrected continually for the entire event.  The value of the Plath became even more clear when you imagine using it in a desert with no natural land marks.  Given the successful test I personally would not have a problem using it for land navigation across a desert to this day if I knew what course I needed to travel from my location to destination.  No need for GPS signals or the like.  Just simple old style navigation would serve the user very well indeed. In fact I’d prefer to use the Plath instead of the Abrams or Astro compass due to the Plath’s ability to constantly maintain the required bearing, thanks of course to the Junghans clock works. 

The final test was the following morning.  As the sun rose in the East the Plath tracked perfectly still displaying the true bearing of North as she was set the evening before.  Perfect!   After seventy plus years the Plath with the assistance of Bill Morris worked as she worked many years ago in the North African desert.

 I wish to thank Dr Bill Morris,  Mr Malcolm Barnfield, Mr  Kuno Gross, and  Mr Konrad Knirim with having similar interest, assisting me, answering my questions, being patient,  and at times commiserating with me on this project.   These men:  doctor, military historian, engineers, authors, experts in their fields, took the time to assist a retired sea Captain with his quest to restore an antique sun compass to operational status.  Simply put, without them and their assistance the Plath would have remained locked away in my study with other odd instruments, not used, not understood and in a non-functional condition.  It would have been an unfortunate end for such a fine instrument.  As it turns out she may well run another seventy plus years.

 Captain Dan LaPorte (ret)

Figure 3: General arrangement.

Figure 3: General arrangement.

Now for a few anatomical details. The compass is mounted on the vehicle or aircraft via a universal joint, which can be quickly locked or unlocked in order to level the base plate (which I have labelled “compass card” in Figure 3 above) using a circular level in the centre of the plate. The base plate index corresponds to the lubber’s line in a magnetic compass and the line can be correctly aligned with the fore and aft axis of the vehicle using a sort of iron sight. In Figure 3 this can be seen as a thin vertical rod in a gap in the trunnion above the W of the base plate. On the other side is a point, just visible in Figure 1. The two trunnions support the horizontal axis of the compass and this axis is provided with a latitude scale on one end and a knurled locking knob on the other. A moment’s thought shows that when the scale is set to the local latitude, the equatorial axis, which I have drawn as a red line, will be parallel to the Earth’s axis. The horizontal axis bears a clock with a twenty-four hour dial and the clock can be rotated inside an equatorial mounting ring provided with a longitude scale and locked at the local longitude.

Figure 4: Equatorial mounting ring and longitude scale.

Figure 4: Equatorial mounting ring and longitude scale.

The watch is provided with a rather unusual hour hand in the form of an alidade (Figure 5a) and also has a conventional minute hand.

Figure 5a: Structure of the alidade.

Figure 5a: Structure of the alidade.

The alidade is made of Perspex (Lucite). One end has a vertical cylindrical lens that projects an elongated image of the sun on to a ground screen at the other end. The screen has two vertical setting lines and a lower, transverse extension to help in initial setting. When the time is set to the correct Universal or Greenwich Mean time, adjusted for the equation of time,  and the latitude and longitude scales set to their local values the base plate is rotated so as to bring the elongated image of the sun between the setting lines. If the vehicle is pointing north, the base plate index will then indicate north. If the vehicle turns and the base plate re-adjusted to bring the sun back between the setting lines, the direction of travel will be indicated by the base plate reading. The clock will keep the alidade tracking the sun correctly as long as the direction of travel does not change and if the direction of travel  does change  it is necessary only to bring the alidade back into alignment to get a correct indication of the new direction of travel. Amateur astronomers will recognise this as an adaptation of the familiar equatorial telescope mounting. The whole is enclosed by a protective Perspex cover. Figure 5b gives another view

Figure 5b

Figure 5b: Further view of alidade and scales.

I do not propose to give many details of the clock mechanism except to point out that, somewhat unusually, it is wound up by rotating the spring barrel rather than by rotating the spring arbor. This allows some simplification of the internal power train and also allows setting of the hands by means of a co-axial arbor (Figure 6).

Figure 6: Winding and setting knobs.

Figure 6: Winding and setting knobs.

Figure 7 shows how power is transmitted to the train from the spring via the central winding gear.

Figure 7: Winding gear detail.

Figure 7: Winding gear detail.

Dan and I would be glad to know of any errors or significant omissions and to hear from other owners about their experience with this ingenious instrument.

Bill Morris

Pukenui

New Zealand

 

 

 

 





A Fine Sextant from Spencer, Browning and Co.

23 01 2014

Previous posts in this category include:  “A C19 Sextant Restoration” , “Making a Keystone Sextant Case” , “Restoring a C. Plath Drei Kreis Sextant” , “Heath Curve-bar sextant compared with Plath” , “A Drowned Husun Three Circle Sextant”, ”Troughton and Simms Surveying Sextant” , “A Sextant 210 Years On” , “A fine sextant by Filotecnica Salmoiraghi”, “A British Admiralty Vernier Sextant”, “An Hungarian Sextant via Bulgaria” ,  “A Half-size Sextant by Hughes and Son” and “A Fine C Plath Vernier Sextant”, “Heath and Co’s Best Vernier Sextant.” and “An Early C19 Ebony Quadrant Restored”.

Recently, I was made the guardian of an exceptional nineteenth century sextant made by Spencer, Browning and Co. The owner, whose father had bought it in an antique shop fifty years previously, did not wish to see it on e-bay, and I am flattered that he took the trouble to send it half-way around the world to me for me to look after.

The firm of Spenser, Browning and Co. has a long history going back even further than Richard Rust, who was apprenticed  to his master, possibly a John Rust, for a term of seven years. Rust in turn was apprentice-master to William Spencer, who joined him at the age of about fifteen in 1766, to Samuel Browning in 1768, and to Ebenezer Rust in 1770. Ebenezer was probably Richard Rust’s nephew. Spencer and Browning formed a partnership in 1778 which lasted to 1781 and in 1784 the partnership was reformed by the inclusion of Ebenezer Rust. The members of the partnership were described as “grocers”, which at that time meant “a trader in gross quantities” and included such trades as mathematical instrument makers. Originally trading from 327 Wapping High Street in London, they later moved to 66 Wapping High Street, a site now occupied by modern apartments.

Sam Browning married Spencer’s sister, Catherine, and their sons Richard, William and Samuel were in turn apprenticed to their father. William Spencer and his wife Anne had no children, but their nephews Samuel, John, Anthony and William Spencer were apprenticed to their uncle William. Ebenezer Rust’s son, also Ebenezer, was apprenticed to his father in 1795. With so many family members involved we may surmise that the firm of Spencer Browning and Rust was very successful, as witness to which is the relatively large number of their instruments that survive in museums and collections. Of the original partners, Ebenezer Rust died in 1800, William Spencer in 1810 and Samuel Browning in 1819. Upon the death of the younger Ebenezer in 1838, the firm became Spencer, Browning and Co., which continued to trade until 1870.

Figure 1: Restored sextant and case.

Figure 1: Restored sextant and case.

Thus, the instrument, shown restored in Figure 1, can be placed between 1838 and 1870. Its keystone case, I would suggest, places it in the earlier part of this period, although one with a similar serial number in a collection is provided with the more convenient rectangular case. What is clear is that the sextant, readable to 135 degrees, is a high-end instrument. Immediately obvious (Figure 1) is the handle of ivory and , inside the case, the large kit of telescopes. (You can enlarge all the figures by clicking on them. Return to the text by using the back arrow)

Less obvious is the arc of gold inlaid into a limb of silver (Figures 2 and 3), but in case anyone might be in doubt, the words “Gold in silver” appear at the left end of the limb.

Figure 2: Face of limb and arc.

Figure 2: Face of limb and arc.

Figure 3: End view of limb and arc.

Figure 3: End view of limb and arc.

The structure of the limb follows eighteenth century practice, as one would expect of a company whose seniors trained at the end of that century and whose successors were their relatives. Usually, a limb of brass was screwed and sweated on to the face of the bronze frame  and a dovetailed segmental groove machined in it. A strip of silver would then be hammered into the groove and machined flat (See “A Sextant 210 years on”). The reason for using silver is well-known. The brass of the time was hammered and rolled into sheets and there were frequent hard spots in it which could divert the scriber of the dividing engine. Silver, by contrast, was available in  a pure and soft state.  The reason for attaching the brass limb to the bronze casting of the frame is less clear. The bronze of the day was perhaps higher in tin content than in later instruments and was sometimes described as “bell metal”, a hard and tenacious metal which would have been very difficult to machine at such a large radius on the sort of lathe likely to be available to instrument makers of the time. It is easy to forget that thee was no powered machinery and lathes had to be turned by hand or foot. It is true that large and rigid  lathes were coming into being in the first half of the nineteenth century, but these were to be found in the workshops of steam engine builders and the like. Interested readers can find more details in LTC Rolt’s book “Tools for the Job.” I surmise that attaching the softer brass limb was a means of getting around a machining difficulty.

I have occasionally seen sextants with the usual arc of silver but having a gold vernier. The combination gives improved contrast of the scales, but this is the first time I have seen one which has a gold arc too. As well as perhaps reflecting the wealth of the owner, there is a more practical reason for having a golden arc: gold does not tarnish as does silver and so never needs to be cleaned, with the attendant risk of damaging the almost impossibly fine graduations. Even if polishing does not remove the graduations altogether (and I have seen a sextant in which the arc has been reduced to unreadability for this reason), it tends to round their edges, making it more difficult than usual to read a vernier against them. Figure 4 shows how beautifully sharp the graduations remain. Also visible in this figure at the top left of the limb is the ghost of a screw head, which has been used to attach the limb, its head then being riveted into the limb and machined flush. Sextants with platinum arcs are occasionally seen. Surprising as it might seem, this was at first an economy measure, as it was cheaper than silver in the early nineteenth century, because no use could be found at the time for this relatively chemically un-reactive and difficult-to-work metal.

Figure 4: Close-up view of graduations.

Figure 4: Close-up view of graduations.

Figure 5 shows the index mirror bracket. This uses the familiar three-point bearing for the back of the mirror, and the next figure, Figure 6, shows the clip which holds the mirror to the bracket directly over the points. A screw through the centre of the back of the clip and bearing on the back of the bracket pulls the clip back on to the mirror. In 1772 Peter Dollond claimed to have invented this system. At any rate, he was granted a patent for it, though he may not have invented it, as in the eighteenth century patents were about monopoly rather than originality.

Figure 5: Index mirror bracket.

Figure 5: Index mirror bracket.

Figure 6: Index mirror clip.

Figure 6: Index mirror clip.

The shades and the horizon mirror follow a pattern that became quite standard in the first half of the twentieth century. Indeed, the horizon mirror is perhaps remarkable for the absence of the complications that characterize the adjustments of many nineteenth century instruments (Figure 7).  It has the three point mounting for the mirror with two adjusting screws to correct for side and index error, with the added refinement of screw caps to protect the screws, one of the requirements for the sextants of British naval cadets in the latter part of the century (they also had to have class A certificates from Kew Observatory).

Figure 7: Index mirror and shades.

Figure 7: Index mirror and shades.

Figure 8 shows structures in the region of the magnifier. The magnifier itself is a Ramsden compound type. It is not clear to me what purpose was served by the large surrounding disc unless perhaps to cut down glare. Many sextants have a ground-glass diffusing screen, but this one is hinged and can be folded down flat. Engineering students are (or more likely nowadays, were) taught to read a vernier scale by arranging the light to shine along the graduations, but this seems to fail with the sextant, in my hands, since the main scale always looks much darker than the vernier, sometimes to the point of unreadability. I find the scales much easier to read when the light shines across the graduations, but even so I have yet to come across a vernier divided to ten seconds in which I can definitely say which of three or four pairs of lines coincide, even using a x30 microscope with axial illumination. The best I can do is to choose the two pairs of lines which just do not coincide and to choose the middle value between them.

Figure 8: Scale magnifier.

Figure 8: Scale magnifier.

The telescope rising piece and collimation ring, both of conventional form, needed only cleaning and re-greasing, but the same was not true of the index arm. After dismantling it I found that the index arm expansion was bent and twisted, so that the vernier scale could not be brought to lie flush with the arc. This seems to be a common consequence of dropping or bumping the instrument as the cut-outs in the index arm expansion severely weaken it and little force is needed to bend it. Figure 9 shows the tapered gaps quite clearly as well as showing the state of the paintwork before restoration. Tightening of the index arm clamp only made matters worse, so I had to reassemble the clamp and carefully first correct the twist and over-correct the longitudinal bend until the feather edge of the vernier sat squarely on the arc when the clamp was tightened.

Figure 9: Bent index arm.

Figure 9: Bent index arm.

Figure 10 shows the index arm clamped, with the fault corrected.

Figure 10: Bent index arm corrected.

Figure 10: Bent index arm corrected.

The rest of the restoration consisted of the by-now-usual dismantling down to the last screw, cleaning all the parts, polishing and lacquering screw heads and other brass parts usually left bright and spraying other parts with black lacquer to reproduce as closely as possible the original finish. The case seemed to have suffered from modern central heating, which tends to dry out wood and cause it to shrink and crack where it is restrained by screws. The large shrinkage crack in the top is shown in the un-restored case in Figure 11. I decided to fill the crack with mahogany paste, but if it had been much larger, I would have been tempted to remove the top, complete the crack with a saw cut, plane the edges and glue in a strip of approximately matching wood. This process applied to the base of a sextant case will be shown in my next post.

Figure 11: Unrestored case.

Figure 11: Un-restored case.

Happily, what appeared at first sight to be worm holes in the bow front of the case turned out to be fly deposits (flies alight on surfaces after feeding and regurgitate their fluid meal, prior to sucking it up again…). The French polish on the sides was much battered and dented, and would have required major repairs, but I felt it worthwhile to strip the top and re-polish it to show off the grain of the wood. to better effect.

The interior of the case too was battered and some of the blocks for retaining the telescopes and sextant were absent, so I made this good and renewed all the felts. The original interior finish in matt black paint was in poor condition, so I repainted it as close to the old finish as possible (Figure 12). The lacquer on the telescopes and sighting tube was flaking and decayed so this too I renewed. I imagine some antique dealers might be horrified at this loss of “valuable patina”, but in my view, the instruments do not need it to be placed in their era. No one, after all, objects to antique cars being refinished, indeed they are much less valuable in a rusty and battered state.

Figure 12: Interior of case with telescopes.

Figure 12: Interior of case with telescopes.

The rule with telescope kits seems to have been that “more is better”. There is a 4 x 18 mm Galilean or “star” telescope, a 6 x 18 mm inverting telescope with an additional eyepiece giving a x 10 magnification, a zero magnification sighting tube and a 18 x 18 mm telescope with additional lenses to give an erect image. It is difficult to see what practical  the purpose the latter telescope served except perhaps to add prestige to the whole kit. The kit is completed by a single deep red eyepiece shade that screws on to any of the eye pieces. The star telescope would probably have received the most use, by day or night, while the higher magnifications would have been reserved for use with an artificial horizon on land in remote places to check the rate of the chronometers. The 6 x 18 inverting scope may have been used in calm seas for sun sights, while the sighting tube would be reserved for rough conditions and for taking angles between shore objects.

If readers have comments, or corrections, or have some question about this instrument, I am happy to receive them and to comment in turn when appropriate.





Eighty Years of Carl Plath Sextants

13 11 2012

See also my posts for 18 December 2010 and 6 December 2012

In this post I will be surveying some C Plath sextants in my possession. I have given detailed coverage already to the Plath Dreikreis or three-circle sextant in January 2010 and to a 1953 ladder pattern sextant in December 2011, so I will be only summarizing details of those instruments. I also looked at Plath’s standard wartime sextant of WWII in the context of detecting fakes in my post of 14 July 2010  and I will give a general overview of that instrument. The last production instrument was Plath’s Navistar Professional and as I have not previously given any cover to that, I will take a detailed look at it. The National Maritime Museum (NMM) at Greenwich has a few Plath nautical sextants  and the excellent photographs of the instruments allows one to follow the evolution of the micrometer sextant. Following this link will take you to their sextant collection: http://collections.rmg.co.uk/collections.html#!csearch;authority=subject-90227;collectionReference=subject-90227;makerFacetLetter=p;makerReference=agent-17323;start=0  ; and I will give the reference numbers to allow more rapid retrieval. Most of the photographs in this post will stand enlarging to 200 percent by right clicking on the figures. Use the back arrow to return.

Carl Christian Plath took over the business of David Filby in 1862. Philby was an instrument maker, but just as with clock and chronometer makers, he did not actually make the sextants he sold. However, Plath bought a dividing engine from Repsold in about 1865 and his own  first sextants probably date from  shortly after that date. As with most early instruments, few of them survive. It seems that they were regarded simply as tools of the trade by their users and most often unsentimentally discarded when their owners retired. No doubt, some were lost at sea and others found their way to attics and basements, only to be thrown out  as unidentifiable junk after the owner’s death. The earliest Plath sextant that I have dates from about 1909.

Figure 1: Front of Dreikreis vernier sextant

The main interest of the Dreikreis (three circle) sextant, apart from its relative rarity, is in the ribbed three circle frame, also adopted by Heath and Co and Hughes and Son, the two leading makers in Britain. That shown in Figure 1 is a vernier instrument. The sliding block arrangement (Figure 2) is rather more complex than usual and is covered in more detail in my post for 24 January 2010. Around about 1907, Plath began to advertise a micrometer sextant whose mechanism  scarcely changed for the remainder of the time that sextants were manufactured. A three circle micrometer sextant by Plath is illustrated in the NMM on line collection, reference NAV1130, and the photographs have sufficient detail to allow one to compare the micrometer mechanism with later ones.

Figure 2: Tangent screw mechanism of Dreikreis vernier sextant

Plath were still producing vernier instruments in 1920, the date of the sextants shown in Figure 3, but by then was using a fine adjustment system that was on its way to being a micrometer, in that the “endless tangent screw” operated on a fine rack cut into the back of the limb, as shown in Figure 4. More details will be found in my post of 4 December 2011. This layout was patented by Heath and Co in 1910 and required only the addition of a micrometer drum and the adoption of a suitable pitch for the rack, which was what Heath and Co did, but Plath had already placed the rack of their micrometer sextant on the edge of the limb and this was the pattern adopted by most other makers.

Figure 3: Plath vernier sextant with “endless tangent screw”.

Figure 4: Rack of sextant shown in Fig 3

A micrometer sextant with a ladder pattern frame of smaller radius dating from 1917 is in the NMM collection, reference number NAV1250 and this design of sextant remained standard until about 1942 when, on a war footing, an instrument that was easier to produce and used less strategic metals was standardised. However, as the magazine cover in Figure 5 shows, many earlier instruments were still in use on the Germany Navy or Kriegsmarine by December 1943. Note that the aperture of the telescope has also increased from 28 to 40 mm, more than doubling the light grasp, with a corresponding increase in the size of the mirrors. This increase in light grasp is important in conditions where contrast of the horizon is poor. When Plath resumed sextant production in the early 1950s it was this pattern that they used, with a bronze frame, rather than the technologically superior wartime sextant which behaved equally as well. Figure 6 gives a closer look at a 1953 sextant of the  type in use in Figure 5. The colour of the drum is different and is divided to whole mnutes rather than half-minutes, but otherwise the two sextants are the same

Fig 5: “Die Kriegsmarine” magazine, December 1943.

Figure 6: Standard ladder pattern sextant.

Figure 7 shows the micrometer mechanism in close-up. The worm is conical so that the end of the shaft that bears the drum is further away from the frame, allowing the use of a larger drum than would have been the case had a cylindrical worm been used. End play in the worm shaft bearings is taken up by the axial preload spring that bears on the rear end of the shaft. A radial pre-load spring keeps the worm in engagement with the rack and a keeper on each end of the expanded lower end of the index arm prevents the mechanism from lifting away from the front of the limb. The worm in its bearings  is attached to a swing arm that rotates about a bearing. When the release catch is operated, the worm swings out of engagement with the rack in the plane of the sextant frame, allowing the index arm to be swung rapidly to any desired position. When the catch is released, the worm swings back into engagement with the rack under the influence of the radial pre-load spring, ready for fine adjustments to be made via the worm. As will be seen, this mechanism was later simplified, probably to reduce production costs, before a final burst of simplification in the Navistar Professional model.

Figure 7: Standard Plath micrometer mechanism

While the standard sextant had a bronze frame with bronze mirror brackets and weighed with its telescope a hefty 1.9 kg (4.19 lbs), the wartime standard sextant had an aluminium alloy triangulated frame (Figure 8) with alloy brackets weighing a mere 1.2 kg (2.65 lbs). Once the very expensive moulds for the frame and brackets had been made, many thousands of the parts could be turned out rapidly by the pressure die-casting process; and the serial numbers increased by about 4,500 between mid 1942 and the end of the war in mid 1945. While aluminium alloy frames were perceived to be inferior to bronze ones by mariners, in fact they are rather more rigid and stable and at least as corrosion resistant. Figure 9 shows the instrument in use during wartime as illustrated by the magazine Die Woche for 14th April 1943. The 0.7 kg reduction in weight makes using this model of sextant a pleasure and it has not been my experience that a lighter sextant is more difficult to use when there is a strong wind.

Figure 8: WWII Kriegsmarine standard sextant

Figure 9: Sextant in use during WWII (1943).

As noted above, when Plath resumed sextant manufacture in the early 1950s, they produced their standard pattern bronze-framed instrument, but by 1975, while retaining the ladder pattern, it was slightly better designed with sharp corners eliminated. I give a general view of the front of this “Navistar” sextant in Figure 10 of this post. A battery handle with scale illumination via light guides had been added and the handle was canted at a  more ergonomic angle. The mirror brackets and shade frames were now of aluminium alloy and the classical tapered index arm bearing was replaced by a plain parallel one of white metal running directly in the frame. Its weight with batteries remained about the same at 1.9 kg. The telescope in older instruments can be dismantled for cleaning or drying out, but in the newer instruments, the objective lens was glued into place in the telescope body and the eye lens was glued into the plastic eyepiece, making any servicing rather problematic. Figure 11 of my post of  18 December 2010  shows how problematic it can be.

Figure 10: Front view of early Plath Navistar sextant

The shades were retained by a non-standard nut requiring a special tool (Figure 11 of this post) to tighten it if the shade friction was insufficient. Such a tool cannot be bought, so any one wishing to dis-assemble the shades for cleaning and greasing had to make one of their own. There was no question of simply sending the shades assembly to the makers for adjustment, as the bracket was now part of the frame (Figure 12), rather than being attached with screws as previously, so the whole instrument had to be sent.. It appeared as though a sextant too expensive to throw away had been made for a throw-away age.

Figure 11: Non-standard pin wrench to adjust shades friction.

Figure 12: Rear view of sextant in Figure 10

Figure 13 gives details of the micrometer, from which it can be seen that its manufacture had been considereably simplified without, one hopes, sacrificing quality. While the release catch itself was an alloy die casting, the fixed part of the release catch was an extension of the plastic of the light guide and was easily broken off.

Figure 13: Plath Navistar micrometer construction

These instruments were no longer supplied in mahogany cases like pre-war sextants, but in heavy, moulded, black bakelite cases with various retention systems that did not always survive heavy handling, though the cases themselves are almost indestructable (Figures 14 and 15).

Figure 14 : Case of Navistar sextant

Figure 15: Retaining latch and pocket of Navistar sextant

In 1977, Plath introduced their Navistar professional sextant, just in time for it to be made obsolete withn a few years by the advent of the Global Positioning System. The frame was a hefty triangle of aluminium alloy 14 mm (0.55 in) thick. While this undoubtedly made it cheaper to manufacture, it had the unfortunate corollary that there was nothing by means of which one could pick it up, except for the handle, so it was provided with a light moulded plastic case in which it sat handle uppermost. This led to a further difficulty: because it was not provided with legs, this is the only place that it can be put down and then only with the index set at – 6 degrees. Figure 16 shows a front  view of the instrument and Figure 17 shows it in its case. What appear to be stubby legs on the rear of the limb serve only to act as stops for the index arm.

Figure 16: Front view of Navistar Professional sextant.

Figure 17: Navistar Professional in its case, rear view.

Extensive use was made of high-impact moulded plastic, in the handle, the telescope body, the shades mountings. the lower end of the index arm and release catch and the micrometer drum.  The latter was a mere 15 mm (0.6 ins) in diameter with figures of a size to tax aging long-sighted eyes (Figure 18). The same slightly inadequate drum was used in the Plath Navistar “Traditional” in which the frame took the form shown in Figure 10. The shades, two each for the horizon and index mirrors, were mounted so that they rotated around their mirrors (Figure 19). They are easy to use, but if the sextant is placed carelessly on a flat surface face downwards it is the shades that will suffer. The two mirrors were identical so only one size was needed for replacement, but because the horizon is viewed directly rather than through glass as in a traditional full glass horizon mirror, overlap of images is rather limited. The mirrors were first surface with overcoating.

Figure 18: Micrometer drum and arc.

Figure 19: Horizon and index shades.

The index arm journal took the form of a bronze bush surrounding a boss extending from the bottom of the index mirror bracket and the bronze-coated journal ran directly in the frame, being secured by means of a C-ring and a nylon washer (Figure 20).

Figure 20: Index arm bearing.

The micrometer mechanism was even simpler than that shown in Figure 12. There was no longer a swing arm. The worm shaft bearing rotated about a boss formed on its base and an arm extended down wards. A spring at the end of the arm abutted against one wall of the plastic enclosure for the mechanism while a U-shaped wire link attached the arm to  the release catch, which formed another wall of the enclosure made springy by slotting the base of the wall. Axial preload for the worm was provided by a spring clip (Figure  21).

Figure 21: Micrometer mechanism of Navistar professional.

The scales were illuminated by a system of light guides within the black plastic covering to the lower end of the index arm. The source of the light was simplicity itself, being simply a wire bulb-holder that also embraced the battery. An orange cap when depressed pushed the positive end of the battery against the central contact of the bulb and thus compeleted the circuit (Figure 22). A ridge on the cap latched against an internal cut-out to prevent the whole falling out, but  slots cut in the cap to allow the ridge to spring into the cut-out had sharp corners which in my specimen have already led to a crack forming, and I anticipate having to machine a brass  replacement some time in the future. Meanwhile, I have drilled a hole at the end of the crack to reduce stress concentration.

Figure 22: Lighting system.

The 4 x 40 Galilean telescope and its integral bracket were of high-impact plastic and there was no provision for servicing the internal surfaces of the lenses as both are cemented into place. An interesting telescope could be supplied which has a front filter that allows normal viewing of extended sources but which acts as an astigmatiser for point sources (Figure 23).  Almost needless to say, this filter cannot be removed for servicing of the lens beneath.

Figure 23: Astigmatising telescope.

The sextant is quite easy to use and is no doubt very rigid for its weight of 1600 G (3.5 lb), but the placing of the handle at 120 degrees means that when it is turned to view the micrometer drum it is out of balance. The telescope has a fairly limited field of view and the micrometer is difficult to read. Instrumental accuracy was given as better than 20 seconds. I doubt that this sextant found much favour with traditonalists, but by the time my specimen was sold (1988) sextants were already falling into disuse.

If you enjoy reading about navigational instruments and technology of the sea, you will probably enjoy reading the book which gave rise to this web site, The Nautical Sextant as well as my more recent The Mariner’s Chronometer, both of which are available via the amazon web sites  in North America and Europe. The Nautical Sextant is also available from the joint publishers, Paracay and Celestaire.





A Fine Sextant by Filotecnica Salmoiraghi of Milan

5 10 2010

The preceding posts covers “A C19 Sextant Restoration” , “Making a Keystone Sextant Case” , “Restoring a C. Plath Drei Kreis Sextant” , “Heath Curve-bar sextant compared with Plath” , “A Drowned Husun Three Circle Sextant”, “Troughton and Simms Surveying Sextant”  and “A Sextant 210 Years On”.

A little while ago, I felt privileged to be able to overhaul an uncommon sextant, made by La Filotecnica Salmoiraghi of Milan in Italy. A first glimpse of the case hinted that the instrument inside would be of very high quality, as attention to detail had been paid in making the case. A heavy cast brass latch and two chromium plated brass hooks held the case closed, the slots in the screw heads that held the top of the lid in place were all lined up parallel to the edges and the hinge was a piano hinge extending nearly the whole breadth of the case. As final details, the brass key bore the maker’s initial (Salmoiraghi) and a brass plate was let into the front for the owner’s name.    

Figure 1 : Exterior of case

  La Filotecnica was founded in 1865 by Ignazio Porro ( 1801 – 1875), the inventor of the prismatic binocular. Porro served in the Italian Army before retiring with the rank of major, in 1842. He worked in Paris for about fifteen years, producing a variety of surveying and optical instruments before returning to Italy in 1861. He founded the Officina Filotecnica at first in Turin and then, in about 1865, in Milan. The factory combined production with what we would probably call nowadays a school for apprentices. Porro seems to have had a poor head for business and in 1870 one of his pupils, Angelo Salmoiraghi (1848 – 1939), took over the ownership of the business after a short period in management. Under Salmoiraghi as La Filotecnica Salmoiraghi,  the firm became one of Italy’s leading producers of optical and precision instruments. Today, the firm, trading as Salmoiraghi e Vigano, manufacturers principally spectacles and markets optical goods made by other firms..    

Figure 2 : Name plate.

Opening the case reveals a clever way of  retaining the sextant in the case. Unusually, the instrument can be placed directly in the case without letting go of the handle. A further very practical aspect to the design of the case is that the sextant can be replaced with the telescope in place. Closing the lid brings pads of very fine brown felt into play to secure instrument and accessories securely in place. A finely crafted stand receives the sextant, location being ensured by large wooden spigots that fit into the circles of the frame, while the telescopes fit into well-thought-out pockets (Figure 3).    

Figure 3 : Contents of case

  The mirrors, shades, the index arm and its bearing are all conventional. The horizon mirror is fully silvered so that the horizon is viewed viewed directly, without passing through an unsilvered part of the mirror. This would be a disadvantage with a Galilean (so-called “star”) telescope, in which each half sees its own field of view, so that without reflection from the clear glass of a half silvered mirror there is little overlap of the direct and reflected images. However, this sextant is provided only with a 6 x 30 mm prismatic monocular and a 10 x 17 mm inverting telescope, both Keplerian in principle, so that there is very adequate overlap of the images. There is a sighting tube for use when surveying or taking sights in heavy weather. In what follows, I will describe only what is unusual or unconventional.    

Figure 4 ; Frame of sextant.

The bronze frame (Figure 4) is of about 162 mm radius and is of three-circle pattern. It resembles superficially the three-circle instruments of Hughes and Son (see my post of 18 Feb 2010) and of C Plath (see my post of  24 Jan 2010) but there are differences in detail. It is heavier than other frames of its radius, weighing in at a hefty 750 G compared to the Hughes frame of 600 G. Sailors who like a bit of “heft” in their sextants will not be disappointed by the Filotecnica as, equipped for viewing with the monocular, it weights 1750 G (3lb 14oz) versus the Hughes at 1500 G (3lb 5 oz). Both are equally and very resistant to flexing. With the exception of the arc, which is chromium plated, the face of the frame is painted semi-gloss black, while the rest is painted in a black wrinkle finish. The index arm bearing is attached with screws from the back.    

In a general view of the instrument (Figure 5), one is immediately struck by the length of the micrometer shaft. Unlike almost every other sextant of this pattern, the micrometer worm is not tapered. While it is easier to make a cylindrical worm, the penalty is that either the micrometer drum has to be smaller to fit beneath the curve of the limb or the shaft has to be longer and therefore more vulnerable to damage.    

Figure 5 : General arrangement, front view.

 The worm shaft runs in plain parallel bearings mounted on a swing arm chassis, which pivots around the ends of two cone-pointed screws attached to the back of the index arm expansion (Figure 6). These latter screws can be adjusted to remove all end float and then locked in place. In this instrument, the release catch swings the worm at right angles to  the plane of the arc (Figure 7), in contrast to most other sextants, which swing it out of engagment parallel to the plane of the arc. There is no  adjustment for end float of the micrometer shaft, however, and absence of float, equally as undesirable here as in the swing arm chassis, depends entirely on skilled fitting, rather than using a preload spring like most other instruments .

Figure 6 : Micrometer mechanism

 

Figure 7 : Swing arm detail

Figure 8 shows how thrust in either direction is received on collars on the shaft and the inner ends of the plain bearings. Upon testing by directing an autocollimator on to the index mirror and approaching zero with the micrometer reading increasing and  decreasing, I could detect no difference in the autocollimator readout, indicating a complete absence of measurable backlash (unwanted longitudinal shaft movement) with a precision of better than 1 arcsecond. This absence of backlash, besides indicating immaculate fitting of the shaft and bearings, is of course necessary to make meaningful the non-adjustable error readings shown in Figure 9. Almost needless to say, these are quite exceptional results for a micrometer sextant of 1948 date. The pitch of the worm is 1.4 mm giving a pitch circle radius for the rack of (1.4 x 720)/2 pi = 160.43 mm.

Figure 8 : Micrometer worm shaft bearings.

Figure 9 : Chart of non-adjustable errors

The method chosen to mate the telescope and sighting tube to the sextant seems somewhat over-engineered. The tubes are fitted with  heavy brass collars that have  coarse interrupted threads on the outside. The rising piece has the usual arrangement of a tiltable bush for collimating the telescope, with the addition of a large captive nut having a female interrupted thread on the inside. A stout pin projects from the face of the bush to locate in a hole on the telescope collar, so that the thread starts correctly and attaches the ‘scope to the rising piece with one sixth of a turn. Figures 10 I hope make this clearer.

Figure 10 : Telescope attachment

But there is further complication in the telescope bracket. It has a female vee into which the vee on the rising piece fork fits and a thumb screw to lock it into place, but there is also a captive thumbscrew whose thread passes through a short rectangular nut, to which is attached a leaf spring. In the end of the leaf spring is a hole that engages with a short nib on the rising piece. The whole is intended to act as a slow motion adjustment for the position of the rising piece (Figure 11). I have seen a somewhat similar slow motion adjustment on a 1940s Tamaya that is an almost exact copy of a pre-war C Plath instrument and may even be a C Plath, sold to have a Tamaya trade mark added. Most post WW II sextants of course managed very well without this complication.

Figure 11 : Exploded telescope mounting

The whole arrangement strikes me as being at the same time clumsy and yet an elegant solution to a problem that did not really exist. It must surely have been cheaper simply to provide each scope and tube with its own rising piece, manufactured so that no collimation adjustment was necessary, as was done for the prismatic monocular (Figure 12). This could surely have posed few problems for a factory capable of achieving the almost impossibly precise fit of the micrometer shaft bearings.

Figure 12 : Monocular and its rising piece.

 The battery handle also shows an elegant and complex solution to a relatively simple problem (Figure 13). The positive pole of the single AA cell makes contact with a screw head to which is soldered the wire to the lamp. The negative pole makes contact with the lid of the compartment via a spring. The hinge of the lid is attached via a tongue let into the side of the handle and the tongue is prolonged as a piece of brass rod with a switch contact on the end. A second piece of brass rod (shown dotted in brown) with a similar contact passes through a hole from above and a single screw that attaches the upper pillar passes through a threaded hole in the rod. When the switch button is pressed, the contacts are bridged and current flows from the negative pole to the frame of the sextant, completing the circuit to the lamp. A second screw , which does not form part of the circuit, passes through the handle into a threaded hole in the upper pillar base and the head of the screw forms the third point of rest when the sextant is set down with its face up. How the very narrow slot for the tongue that attaches the lid was made still eludes me.

Figure 13 : Battery handle

The over-all impression is of a very finely made instrument having some unnecessary complications. It cannot have been cheap to produce, but professional mariners continued to pay a premium for perceived quality even when cheaper instruments able to do the job just as well became available.

If you enjoyed reading this post, you will very likely enjoy reading my book, which goes into similar detail about the structure of The Nautical Sextant.





A turn-of-the-century French Sextant

11 08 2015

Previous posts in this category include: “A Half-size Sextant by Lefebvre-Poulin”, ” A Fine Sextant by Spencer, Browning and Co”,  “A C19 Sextant Restoration” , “Making a Keystone Sextant Case” , “Restoring a C. Plath Drei Kreis Sextant” , “Heath Curve-bar sextant compared with Plath” , “A Drowned Husun Three Circle Sextant”, ”Troughton and Simms Surveying Sextant” , “A Sextant 210 Years On” , “A fine sextant by Filotecnica Salmoiraghi”, “A British Admiralty Vernier Sextant”, “An Hungarian Sextant via Bulgaria” ,  “A Half-size Sextant by Hughes and Son” and “A Fine C Plath Vernier Sextant”, “Heath and Co’s Best Vernier Sextant.” and “An Early C19 Ebony Quadrant Restored”.

Figures may be enlarged by right clicking on them. Return to the text by using the back arrow.

A rather dusty and neglected little French sextant came into my hands a few weeks ago. The kinship of French sextants is quite difficult to sort out. This one bore a label in which the name of A Hurlimann is given prominence and the names of his successors, Ponthus and Therode, are given less prominence (Figure 1). The limb is engraved in copperplate Lorieux, A Hurlimann, succr (successor) à Paris.

Figure 1 : Label in lid of case.

Figure 1 : Label in lid of case.

A. Hurlimann succeeded a distinguished line of French instrument makers. Two pupils of the renowned Henri Gambey founded a firm in 1845. Possibly both originally named Schwartz (Black), they were known as Lenoir (Black) and Lorieux, and managed by Lorieux and then Hurlimann. In 1900 they were succeeded by Ponthus and Therode. At the turn of the century in about 1902 the firm moved from 43, Passage Dauphine, Paris, to 6 rue Victor Considerant. It was then taken over by Albert Lepetit , possibly in 1914, and moved to Montrouge at 204 avenue Marx Dormoy, eventually passing into the hands of Roger Poulin in about 1950. Thus, I surmise that the sextant dates from between 1902 and 1914.

Figure 2 shows the instrument in its case as received. There is a large shrinkage crack in the lid and floor. Not clearly visible are a Keplerian telescope of about 6 power in a pocket at the front of the case and a screwdriver at the back. The label in the lid counsels against using alcohol to clean the instrument, suggesting that it is painted with a shellac-based lacquer (which dissolves in alcohol).

Figure 2: Sextant as received.

Figure 2: Sextant as received.

The frame is a bronze casting with inlaid silver arc, beautifully finished and otherwise unremarkable except for its relatively small radius for the time, of only 142 mm.  The arc is divided to 20 minutes and the vernier reads to a realistic 30 seconds. The divisions are crisp and the numerals are hand engraved in italics. A more usual radius for vernier sextants is about 180 mm and many of these instruments have verniers divided to 10 seconds, though it is usually quite impossible in these cases to say with certainty which pair of lines coincide.

There are two main areas of interest in the design: a) the mirror adjustments are complicated, compared to what we may think of as the modern method of springs on the front of the mirrors opposing the action of screws behind the mirror (in fact this dates from the 18th century and was invented by John Dolland); and b) the mode of mounting of the tangent screw, which seems to have had a German influence, as it is also seen in vernier sextants made by Frederick Ernest Brandis, a German immigrant to the New York in 1858.

The index mirror is held against the upright of a bracket by means of a clamp whose clamping screw bears against the back of the upright as shown in Figure 3 and Figure 4.

Figure 3: Front of index mirror clamp.

Figure 3: Front of index mirror clamp.

Figure 4: Index mirror in place.

Figure 4: Index mirror in place.

The mirror is adjusted for perpendicularity by rocking the whole bracket on the two radiused front feet by means of an adjusting  screw held captive in the horizontal part of the bracket and engaging with a threaded hole in the top of the index arm (Figure 5). The screw can be locked by tightening the screws that pass through the brass clamp bar that holds the screw captive. An earlier (and simpler) method was to have the hole in the foot tapped for a screw, the end of which simply bore on the face of the index arm, though there was a tendency for the thread to strip in clumsy hands.

Figure:

Figure 5: Index mirror bracket with screw in place.

The horizon mirror adjustment system is even more complex. A large cylindrical boss on the underside of the bracket (Figure 6) passes through a hole in the sextant frame and is secured by a large brass locking screw and washer (Figure 7). A screw held captive in the frame enters the threaded hole in the tongue and rotates the mirror to correct for index error (Figure 8). This movement is locked by means of the knurled clamp screw behind the frame, seen in Figures 7 and  8.

Figure 7: Horizon mirror bracket and clamp.

Figure 6: Horizon mirror bracket and clamp.

Figure 8: Horizon mirror locking screws.

Figure 7: Horizon mirror locking screws.

Side error is taken care of by a similar captive screw that opens or closes the slot in the base of the bracket, as seen in Figure 6 and 8. These adjustments are easy to use and effective, but are at the expense of a good deal of complication.

Figure 9: General view of horizon mirror adjustments.

Figure 8: General view of horizon mirror adjustments.

The telescope mounting again achieves a good result at the expense of complexity (Figure 9). The rising piece is triangular in  section and is close-fitting in a triangular hole in a bracket that passes through a hole in the telescope frame and which is secured by a large knurled nut. A countersunk screw passes through the frame into the bracket to secure it against rotation. A large knurled adjusting thumb screw is held captive in the bracket and its thread passes up the middle of the rising piece to make it rise or fall.

Figure 9:mounting of telescope rising piece.

Figure 9: Mounting of telescope rising piece.

Figure 10 shows the mounting exploded.

Figure 10: Exploded view of telescope mounting.

Figure 10: Exploded view of telescope mounting.

The tangent screw  follows F E Brandis’ practice quite closely (or vice-versa). It has a knurled knob on each end and runs in a spherical bearing at the front end. The threaded part passes through a spherical nut which is held captive in the sliding block by a cap and prevented from rotating by the slender boss on its underside, that passes into a hole in the spherical seat on the face of the sliding block (Figure 11).

Figure 11: General view of tangent screw mechanism.

Figure 11: General view of tangent screw mechanism.

Figure 11 shows the tangent screw bearing exploded and it can also be seen that the nut cap is a similar device to the bearing cap.

Figure 12: Tangent screw bearing.

Figure 12: Tangent screw bearing.

The sliding block is held in the rectangular window cut into the expanded part of the index arm by a leaf spring and a clamp spring. When the clamp is tightened, the block can no longer slide over the limb, so that when the tangent screw is rotated it is the index arm that moves. Thus, although I have named the part the sliding block (since no one else seems to have troubled to give it a name) in truth it is the index arm expansion that does the sliding.

Figure 13: Underside of sliding block

Figure 13: Underside of sliding block and clamp.

The shades mountings are unremarkable in design (see Figure 7) except that they have no provision for isolating the movement of one shade from the next by means of, for example, keyed separating washers. Friction is provided by Belleville washers, patented in 1867 by Julien Belleville, a Frenchman.

The telescope kit comprises the usual “zero magnification” sighting tube, a 3 x 26 mm Galilean “star” and a 6 x 15 Keplerian or “inverting” telescope (Figure 14), supplemented by two eyepiece shades of more or less equal density though of slightly different colours (neutral and deep orange).

Figure 14: Sighting devices.

Figure 14: Sighting devices.

The case (Figure 15) is of a light coloured wood with a grain similar to that of mahogany, with dovetailed corners, and both top and bottom were glued and nailed on with steel pins. I find it strange that a few francs were saved by using steel instead of brass pins or, better, brass screws, given that the whole instrument probably cost several months salary for a merchant officer of the time. On the other hand, the brass handle seems on the heavy side for the neat little case into which all the parts fit snugly, perhaps too snugly, and the instrument is held in a pair of felt-lined pockets by a pillar in the lid which passes through the frame and sits on the sextant handle (Figure 2, above). The latter is of the traditional pear shape. The hinges and hooks are of brass and there is a brass lock with ebony escutcheon.

Figure 14: Case exterior.

Figure 15: Case exterior.

It was possible to remove both top and bottom for re-gluing and the crack in the bottom was simply closed up, leaving a minor cosmetic deficit at the back of the case. The crack in the top was much wider and I dealt with it after re-gluing and nailing by filling it with epoxy cement and adding a layer of coloured filler above and below. It was not possible to save the label with its cleaning advice and I replaced it by a modern one using a matching typeface and lay out. Figure 16 shows the final result of the restoration of sextant and case.

Figure 17: Restored instrument in its case.

Figure 16: Restored instrument and case.

Post script: Come to think of it, the card pasted inside the lid, suggests a time of sale after 1902, but the inscription on the limb of the sextant itself suggests that it was made before the Pontus and Therode take-over.





A Half-size Sextant by Lefebvre-Poulin

29 06 2014

Previous posts in this category include:  ” A Fine Sextant by Spencer, Browning and Co”,  “A C19 Sextant Restoration” , “Making a Keystone Sextant Case” , “Restoring a C. Plath Drei Kreis Sextant” , “Heath Curve-bar sextant compared with Plath” , “A Drowned Husun Three Circle Sextant”, ”Troughton and Simms Surveying Sextant” , “A Sextant 210 Years On” , “A fine sextant by Filotecnica Salmoiraghi”, “A British Admiralty Vernier Sextant”, “An Hungarian Sextant via Bulgaria” ,  “A Half-size Sextant by Hughes and Son” and “A Fine C Plath Vernier Sextant”, “Heath and Co’s Best Vernier Sextant.” and “An Early C19 Ebony Quadrant Restored”.

A few months ago, I acquired an unusual little sextant, but was only recently able to collect it in Europe and bring it home to New Zealand for cleaning and overhaul. According to the seller, it had been found in the attic of a merchant seaman who had been active in the 1950’s, but there was no other information about its origins. It bears the name “Lefebvre-Poulin, Montrouge, but the naming of French sextants is rather confusing. Poulin made sextants in the second half of the 20th century in Montrouge in the south-west suburbs of Paris, but Jules Lefbvre was active in central Paris in the latter half of the nineteenth century, so it is unlikely that there was an association between the two makers, and the name of Poulin is sometimes associated with Blanchet.

Apart from its small size, there are one or two other unusual features such as its handle and its micrometer drum, which spans two degrees. Figure 1 shows a front view of the un-restored instrument and Figure 2 gives a rear view. You can get a magnified view of all figures by clicking on them. Return to normal by using the back arrow.

 

Figure 1: General arrangement, front view.

Figure 1: General arrangement, front view.

Figure 2: General arrangement, rear view.

Figure 2: General arrangement, rear view.

To help readers who have not yet had the wisdom to buy my book, “The Nautical Sextant“, in Figure 3 I show the restored sextant with its main parts labelled.

Figure 3: Main parts of the sextant.

Figure 3: Main parts of the sextant.

At the heart of any sextant is the frame and its bearing for the index arm. In this case, the frame is of an aluminium alloy with a cast-in bronze rack for the micrometer and a bronze bearing for the index arm. The latter also serves as a point of attachment for the unusual handle (Figure 4).

Figure 4: To show handle.

Figure 4: To show handle.

The hexagonal alloy handle screws over the index arm bearing (Figure 5) and is locked in place by two Allen grub screws. The bearing itself is attached to the frame by three brass screws.

Figure 5: Index arm bearing, and handle attachment.

Figure 5: Index arm bearing, and handle attachment.

The vast majority of sextants ever made had tapered index arm bearings, but as micro-finishing of plain parallel bearings advanced in the second half of the twentieth century, C Plath and Observator made use of the new technology in their sextants. As Figure 6 shows, Poulin followed suit in this sextant. The parallel steel journal rotates in the bronze bearing (strictly speaking, the bearing is the enclosure in which the shaft or journal rotates), and is secured against axial movement with a phosphor-bronze spring washer and a brass screw. No provision is made for adjustment as no wear is to be expected in the slow-moving, lightly-loaded bearing.

Figure 6: Exploded index arm bearing.

Figure 6: Exploded index arm bearing.

Figure 7 shows the rack with a view of the micrometer mechanism. The bronze rack appears to be cast in to the frame and its pitch is relatively large for its radius of about 80 mm, so that one turn of the micrometer worm advances the sextant reading through two degrees. Henry Hughes and Son in their WWII half-size seaplane sextant reduced the pitch of the rack  so that they could use most of the components of the full-sized micrometer mechanism, needing to modify only the pitch of the worm to match the rack. Their micrometer advances the sextant reading through one degrees per rotation of the worm (see my post of 26 September 2011).

Figure 7: To show the rack.

Figure 7: To show the rack.

The micrometer mechanism is of conventional design (Figure 8). The lower end of the index arm carries a post for the bearing of a swing-arm chassis to which the plain parallel bearing of the micrometer worm shaft are attached. A large leaf spring (radial preload spring) holds the worm in contact with the rack and a simple cam allows the worm to be swung in the plane of the frame out of contact with the rack against the spring pressure. This allows the index arm to be moved rapidly into position before releasing the catch to re-engage the worm for final adjustment. A smaller leaf spring (axial preload spring) bears on the end of the worm shaft to take up any axial play in the bearings. Also shown in the figure are the two keepers, which prevent the index arm from lifting away from the front of the frame.

Figure 8: Micrometer mechanism.

Figure 8: Micrometer mechanism.

Since one rotation of the worm with its attached micrometer drum advances the reading through 2 degrees, the drum is divided 0 to 60 minutes twice with subdivision to minutes (Figure 9). The main scale on the limb and the micrometer drum are divided to single degrees and minutes respectively, with alternate tick marks being long (Figure 10). At first sight, this gives the impression that half-degrees and minutes are being shown and it might have been easier to read had the normal practice been followed of making the 5  tick mark longer than the others.

Figure 9: Micrometer drum divisions.

Figure 9: Micrometer drum divisions.

Figure 10: Main scale close-up view.

Figure 10: Main scale close-up view.

The shades are unremarkable except that there are only two index shades instead of the more usual three or four and one horizon shade instead of two or three. No provision is made to prevent the index shades rotating together, but as there are only two of them this does not pose much of a problem. The two shades together give adequate reduction for viewing full sun and the single horizon shade also gives adequate reduction of glare beneath the sun.

The mirrors and their brackets are conventional. The horizon mirror is fully silvered, but there is adequate overlap of the direct and reflected images when viewed through the 4 x 25 mm Galilean telescope; though the field of view is somewhat restricted, I found no difficulty in finding the sun, though with star sights the story might be very different.

I have not been able to discover for whom this instrument was intended. Its small size and simplicity suggests it might have been aimed at yachtsmen, for whom storage space may be at a premium. It may also have found the occasional sale among surveyors and explorers. A merchant marine officer is unlikely to have wished to be seen using anything other than a full-sized instrument. It came to me without a case. Monsieur Hervé Le Bot has kindly provided me with a photograph of the case interior, which is shown in Figure 11. Unusually, the sextant is stowed in its case upright, in a socket which accepts the hexagonal handle and which has cheeks to prevent it from rotating.

Figure 11: Interior of case (courtesy of M. H Le Bot)

Figure 11: Interior of case (courtesy of M. H Le Bot)

 

 

 





An Early C19 Ebony Quadrant Restored

11 03 2012

Previous posts in this category include:  “A C19 Sextant Restoration” , “Making a Keystone Sextant Case” , “Restoring a C. Plath Drei Kreis Sextant” , “Heath Curve-bar sextant compared with Plath” , “A Drowned Husun Three Circle Sextant”, ”Troughton and Simms Surveying Sextant” , “A Sextant 210 Years On” , “A fine sextant by Filotecnica Salmoiraghi”, “A British Admiralty Vernier Sextant”, “An Hungarian Sextant via Bulgaria” ,  “A Half-size Sextant by Hughes and Son” and “A Fine C Plath Vernier Sextant” and “Heath and Co’s Best Vernier Sextant.”

Many of the sextants in my collection began as wrecks that I have usually been able to restore without too many challenges, but after Christmas I bought an ebony quadrant or octant that had been sold “for parts or restoration” and found that I had to make many new parts in the style of 19th century instruments. It arrived as a frame and a plastic bag of screws and other bits and pieces, including a co-axial cable socket!  Had I not known that the maker was Crichton of London (Figure 1), I doubt that I would have bothered to proceed.

Captain Lecky’s advice in his famous book “Wrinkles in Practical Navigation” was to own  both an octant and a sextant. The octant was usually a wooden instrument capable of measuring up to 90 degrees with a precision of about a minute while the sextant was an altogether more expensive and finely made item with a bronze frame and an arc divided  to read  often to ten seconds (a sixth of a minute). The sextant would be used to measure angles of up to 120 degrees when, for example, making lunar distance observations, while the octant would be used for coarser observations that required no great accuracy. No doubt, there were mariners in poorly paid positions who could not afford to follow Lecky’s advice and for whom the octant was their only instrument. By the second half of the nineteenth century, lunar distance observations were becoming obsolete, displaced by the marine chronometer, so that paradoxically, as the quality and accuracy of sextants increased the need for extreme accuracy fell. However, even by the turn of the twentieth century, there were still ships that did not carry chronometers, whose navigating officers did not make lunar distance observations and whose celestial navigation might be confined to noon altitudes of the sun and pole star observations for latitude. Perhaps it was on one such ship that my battered octant found its home.

There was a James Crichton working in Glasgow who probably spent two years, from 1774 to 1776, in London, but our Crichton was John Crichton who flourished in London between 1831 and 1865 and who showed off some of his scientific instruments in the Great Exhibition of 1851. He was based in Leadenhall Street (Figure 2) which is in the centre of London and which joins Fenchurch Street, the home of the famous dynasty of instrument makers, Henry Hughes and Son.

Figure 2 : Leadenhall Street in mid C19

  Figure 3 shows the contents of the bag of bits and pieces while Figure 4 shows the frame and index arm. From left to right in Figure 3, the traditional pear-shaped handle shows a mixture of quality. The upper mounting has plainly been fabricated from turned parts, while the lower mounting is a somewhat crude sand casting in brass where little effort has been made to remove evidence of the casting flash, the casting metal that creeps between the two halves of the mould box. The index arm journal had parted company with the index arm. It was a matter of moments to resolder it to the disc that carries the index mirror bracket. The latter, together with the horizon mirror mounting, lacked the clip to hold the mirror to the bracket. Sawing out sheet brass, folding it to shape and soldering the joints took the better part of a morning. Two of the index shades were loose in their mounts and one had parted company with its mount, but it took only a few minutes to tighten them up by bending over the retaining lips, a process called swaging.

Figure 3 : Loose parts

As mentioned, the index arm and its journal had parted company, and the tangent screw mechanism had somehow migrated from the back, where it belongs, to the front (Figure 4). The bedraggled frame was in an even worse state, having lost its legs and the arc. The latter in wooden-framed instruments was sometimes made of boxwood but more commonly of ivory, which is now, for practical purposes, unobtainable except perhaps by scavenging from the keys of a wrecked piano, and even then the material may be ivorine, an early plastic related to celluloid. Obtaining something with the appearance of ivory was the subject of some experiments, but dividing the arc posed few problems to a well-equipped workshop. My account of how I produced the arc will have to be the subject of another blog post. I show the results in Figure 5 and hope that my scribing of the figures approximately matches the style of the hand-scribed name plate shown in Figure 1

Figure 4 : Frame and index arm.

Figure 5 : Detail of arc.

The clamp and its screw were missing from the tangent screw mechanism. Figure 6 shows a piece of parent brass being tried for size in the mortice of the piece that I have chosen to call the sliding block (since nobody else seems to have done so in print), and Figure 7 shows the clamps screw being checked before final turning. By this stage the clamp has acquired a strip of “well hammered” springy brass.

Figure 6 : Trying clamp for size.

Figure 7 : Clamp screw being checked.

Figure 8 shows the completed mechanism. When the clamp screw is tightened, the clamp fixes the sliding block to the limb of the octant. The threads of the tangent screw, which is held captive in a bearing attached to the back of the index arm, pass through a nut in the sliding block. When the tangent screw is rotated, the index arm moves slowly over the limb and allows fine adjustment of the position of the index arm. Thus, it is really the index arm that slides, rather than the sliding block.

Figure 8 : Tangent screw mechanism.

In a cheap instrument, time would probably not have been wasted turning legs to a tapered shape but I had made three new legs before the thought occurred to me, so the rejuvenated instrument has tapered legs. Figure 9 shows one of these being turned. At some later date, I may make new legs that are more in keeping with the class of octant. At the top left of Figure 6 can be seen holes where legs once resided. Selley’s “Steel Knead It™” is useful for filling the holes, as it dries to a black that is a fair match for ebony and can also be drilled and tapped to allow new legs to be screwed into place.

Figure 9 : Taper-turning a leg.

Figure 10 shows the front of the index mirror clip and Figure 11 the back. When the screw is tightened its tip bears on the back of the upright of the mirror bracket, pulling the claws on the front of the clip against the mirror so that the latter is pressed against three small projections on the front of the bracket, opposite the claws. This method of holding the mirror at three points that are supported on the other side of the mirror ensures that the glass is not distorted by asymmetrical strains and was first described, if not invented, by John Dolland in the mid 18th century.

Figure 10 : Front of index mirror clip.

Figure 11 : Rear of index mirror clip.

Before a sextant or octant can be used, the index mirror has to be made perpendicular to the plane of the arc. There are several ways of doing this: by tilting the mirror in its mount as in most modern sextants; by making the bracket perpendicular in the first place as in a few high class modern sextants; or by tilting the bracket with its attached mirror. The latter seems to have been the solution favoured by 18th and early 19th century makers. Figure 12 shows how it was done. The mirror bracket is fastened to the index arm by two screws that pass through clearance holes in the bracket into the index arm. A third screw  passes through a tapped hole in  heel of the bracket and its end bears on the index arm. Tightening the screw tilts the bracket and mirror forward and loosening allows them to tilt back. It is a satisfactory method except for the ham-handed, who are liable to tighten the third screw without slackening the fastening screws a little to allow movement. Perhaps there were many ham handed mariners, those “overhandy gentlemen” remarked on by Troughton and quoted by Raper, as the method seems to have died out in the second half of the 19th century.

Figure 12 : Perpendicularity adjustment of index mirror.

The horizon mirror is secured to its base by a similar clip (Figure 13). It has to be possible to bring the horizon mirror perpendicular to the plane of the arc and to bring it parallel to the index mirror when the instrument reads zero. Navigational texts refer to removing side and index error respectively. Again, modern practice is simply to move the mirror in its mount by means of screws bearing on its back, but 18th and early 19th century makers favoured more complex approaches.

Figure 13 : Horizon mirror clip.

The horizon mirror clip holds the mirror against a bracket atop the tilting base (Figure 14). The latter has two nipples that sit in depressions in the rotating sub-base and can be tilted against spring pressure by means of the side error adjusting screw. The rotating base has a tapered shaft that passes through the frame, where it is secured in a square hole in a swing arm (Figure 15). The securing screw allows play in the bearing to be taken up, while the square ensures that no rotational forces are transmitted to the securing screw.

Figure 14 ; Horizon mirror adjustment 1.

The end of the swing arm has a half-nut at its end that engages with a worm that is held captive in a fabricated brass mounting (Figure 15). Rotating the worm against the half-nut moves the end of the swing arm and in turn causes the mirror base to rotate. The swing arm can be locked in position by a locking screw and washer. Figure 16 shows the mechanism assembled.

Figure 15 : Horizon mirror adjustment 2.

Figure 16 : Horizon mirror adjustment 3.

An octant of this quality and period might well have been provide with only a pin hole sight, but as I had a spare 19th century telescope, I elected to make a mounting for it. A thread had to be machined inside a brass ring to suit the telescope (Figure 17) and then it was simply a matter of turning a pillar of the correct height and silver-soldering the ring to it. Figure 18 shows the completed article.

Figure 17 : Screw-cutting telescope ring.

Figure 18 : Completed telescope mounting.

The bag of parts unfortunately did not contain any horizon shades, but as there was a hole for them in the brass mounting plate I could scarcely pretend that none had ever been fitted and I had to set to and make the shades together with their mounting, using traditional methods. I began with the mounts themselves by cutting out three pieces from 3 mm brass plate using a piercing saw having first very carefully marked out the centres. I then clamped all three plates together and drilled through them so that they could all be filed to the same shape and size. Modern laser cutting machinery would make this the work of minutes, but I had to use traditional “cheaters”, discs of hard metal that are clamped either side of the work piece to guide a file. Figure 19 shows the rough sawn parts ready for coarse filing and Figure 20 shows them filed nearly to size along side a fine file.

Figure 19 : Cheaters in place and coarse file.

Figure 20 : Fine filing close to completion.

Figure 21 shows the parts separated, the exterior shape round enough to be held in a lathe chuck for drilling and boring while Figure 22 shows the mounts completed and ready to receive discs of coloured glass which, I confess, only appear to have been swaged into place. Modern acrylic cement unites them to the mounts.

Figure 21: Mounts ready for drilling and boring.

Figure 22 : Shade mounts ready to receive glass.

Examining several mountings suggested that they are fabricated from several parts : a pillar, a base (shown with machining nearly complete in Figure 23) and two discs for cheeks that will receive the tapered cross pin that holds the mounts in place and which are silver soldered to the semi-circular groove machined in the base (Figure 23).

Figure 23 : Base for pillar and cheeks.

Figure 24 shows all the various parts assembled. Once the cheeks were in place, spacing washers and the tapered cross pin could be made to size and then a tapered hole reamed through the sandwich of cheeks, washers and shades. I made  the holes through the washers a little undersize so that when the pin was pulled home by a screw (seen in Figure 24), the washers would not turn on the pin and motion of one shade would not be transferred to its neighbours.

Figure 24 : Shades aseembled in mounting.

Figure 25 shows the structure of the index arm bearing. It is simply two brass or bronze washers let into each side of the frame and secured there by two pins whose heads have been rivetted over. The slightely tapere journal of the index arm fits in the bearing and is adjusted by the familiar screw and washer fitted to a square on the end of the journal (Figure 26).

Figure 25 : Index arm bearing and journal.

Figure 26 : Fitting of index arm bearing.

When restoring an antique instrument it is sometimes hard to decide how far to go. Common sense suggests that the exposed brass parts were probably painted or lacquered black, or sometimes chemically blackened. Sometimes traces of the original finish can be found, but not in this case, so for the time being I have compromised and lacquered only the optical parts. I do not myself feel that doing so devalues the instrument. Its age speaks for itself from its design and structure, so that there is no need for verdigris and thick layers of “antique” dirt. Perhaps readers will let me have their opinions. Figures 27 and 28 show front and back views in its (for the time being…) finished state. All that remains is to make a case.

Figure 27 : Front (left-hand) view.

Figure 28 : Rear (right hand) view.

If you have enjoyed reading this account, I am sure you will enjoy reading my book “The Nautical Sextant“, and your purchasing it will help me to ensure that more sextants are restored to a normal life.





Heath and Company’s best vernier sextant

10 12 2011

Previous posts in this category include:  “A C19 Sextant Restoration” , “Making a Keystone Sextant Case” , “Restoring a C. Plath Drei Kreis Sextant” , “Heath Curve-bar sextant compared with Plath” , “A Drowned Husun Three Circle Sextant”, ”Troughton and Simms Surveying Sextant” , “A Sextant 210 Years On” , “A fine sextant by Filotecnica Salmoiraghi”, “A British Admiralty Vernier Sextant”, “An Hungarian Sextant via Bulgaria” ,  “A Half-size Sextant by Hughes and Son” and “A Fine C Plath Vernier Sextant.”

Clicking on the figures will enlarge them and allow you to see more detail, while clicking on the back arrow (top left) will restore the post.

Several years ago, when I had first started to restore nautical sextants, I bought a Brandis vernier sextant on e-bay. I was dismayed when it arrived to find that it appeared to be loose inside a case that did not  belong to it and, worse, the case was jammed shut, perhaps explaining why the seller had not followed my usual request to put packing around the sextant inside the case. Eventually, I was able to get the case opened without damaging it and found that, improbably, the Brandis sextant had escaped all damage. The rosewood case, bound in brass, belonged to a Heath and Co pillar sextant that, as befits such a high-end product, had been provided with every possible accessory, though the only one present was an early 10 x 20 prismatic monocular. I restored the case and put it aside, against the day, yet to come, when I could acquire the sextant to go with it. However, a few weeks ago I acquired a somewhat later Heath and Co top-end product, an 8 inch (200 mm) radius vernier sextant, equipped with their patent “Hezzanith” endless tangent screw automatic clamp and a set of telescopes that was complete except for a prismatic monocular and the rising piece to go with some of  the other telescopes.  The sextant had its own case, so I still have a spare case for a Heath and Co Pillar sextant, and could be persuaded to part with it if offered the right price…

Heath and Co were granted a patent for their automatic clamp in 1910, so the sextant was no earlier than that, but it also had a Class A inspection certificate from the National Physical Laboratory in Teddington, dated January, 1921, so that its date can be fixed to within a dozen years (see Figure 1)

Figure 1 : Inspection certificate.

The mahogany case (Figure 2) had been protected from much damage by being contained in a stout cowhide satchel. It came as no surprise that most of the stitching had rotted and given way, nor that the leather of the lid hinge had dried out and parted company with the rest of the satchel. I spent a few quiet afternoons restitching the case by hand and gluing strips of leather to repair the broken hinge. Nothing can be done to restore the finish, however, and illustrating the satchel will have to await a post script. While the top of the case had, as is usual, been attached with glue and screws, I was surprised to find that shortcuts had been taken with the bottom: it had been attached by glue and brass panel pins, both of which, after over seventy years, had given way in places. Some of the drawer dovetails at the corners had also given way, so I re-glued everything and replaced the panel pins with brass screws. The “furniture”: brass handle, keyhole escutcheon, piano hinge and hook latches, responded to 600 grit emery paper, followed by metal polish.

Figure 2 : Exterior of case as found.

The details of the hook latches are a little interesting, as they incorporate a safety lock (Figure 3), similar to those found in some early post WW II Tamaya sextant cases. A springy brass sector plate is screwed to the case underneath the hook and when the hook is swung into the closed position, the plate springs up behind the hook, so that it cannot be accidentally un-latched without first depressing the plate.

Figure 3 : Safety hook latches.

A “belt and braces” (belt and suspenders in US) approach was taken to securing the sextant in its case. The pocket and boxwood latch is commonplace, but Heath and Co added the refinement of a brass pillar that  locates the handle in the pocket, and which has a spring-loaded tongue that projects above the handle to secure it. Pressing a button at the rear of the case (Figure 4) withdraws the tongue and releases the handle. The figure also shows that the legs rest upon a springy brass plate that protects the bottom of the case from the legs and also prevents the instrument rattling within its bonds.

Figure 4 : Release knob.

Figure 5 shows the sextant in its case before restoration. At some time, the original black lacquer had been over coated with black paint which had begun to flake off.  Beneath the paint was widespread verdigris that fortunately had progressed no further than a light surface coating. The frame, mirror brackets, shades mountings and legs are all of bronze, while the index arm is a single plate of heavy brass. Catalogues often describe sextants as having brass frames, but brass is an alloy of copper and zinc, without the resistance to corrosion of the copper and tin alloy that is bronze. The silver arc has a radius of about 200 mm (8 inches) and weighs a hefty 1.8 kg (4 lbs) without any telescope mounted. The size of the mirrors is large for the era. The index mirror measures 38 x 57 mm while the horizon mirror is 30 x 40 mm.  The large star telescope “sees” a relatively small area of the reflected image, but has a wide view of the horizon through and around the unsilvered part of the horizon mirror.

Figure 5 : Interior of case as found.

There is a substantial set of telescopes (Figure 6). Of especial note is the 4 x 52 mm Galilean or “star” telescope that, despite its impressively large objective lens, has a measured field of view of only 3.5 degrees. The other star telescope is only a 3 1/2 x 19 mm instrument that is very little different from those in use a hundred years earlier. While lacking the light grasp of the large star telescope, the 4 x 30 inverting telescope has more than twice the field of view to compensate. The 11 x 19 mm inverting scope again belongs to another era and even by 1921 was probably very seldom used. The kit is completed by a zero magnification sighting tube and a pair of eyepiece shades, to which I have added the 10 x 20 mm prismatic monocular with its field of view of about 3 degrees.

Figure 6 : Telescope kit.

 Those telescopes not provided with a forked rising piece have interrrupted screw threads, to allow them to be mounted on the instrument  thread with less than one sixth of a turn. The rising piece for these ‘scopes was missing, so I had to make a new one from scratch. This can be seen in Figure 7 , below, but I have saved the description of how to make it for my next post, under the “Interesting Overhaul Problems” category.  The plain fork fits into a substantial and close-fitting slot in the telescope bracket and is retained there by a nut and a large knurled washer. The washer has a short slot cut in it at 45 degrees to a radius and could presumably have engaged with a button on the telescope fork to act as a crude way of making fine adjustment to the position of the fork, by rotating the washer. However, the large star telescope has no such button and only traces of the button remained on the prismatic monocular, following its adaptation to another instrument.

Figure 7 : Telescope mounting.

The index arm bearing is typical. A slender bearing fits closely in the frame  and a tapered shaft or journal rotates within it. The end of the shaft bears a square that fits inside a square in a washer, while a screw adjusts fit and removes end play. It is worth noting (and repeating) that this screw is used for taking up play only until the faintest trace of resistance to rotation is felt and is then slacked off a little. It must not be screwed up hard as this will very likely cause the bearing to seize, if it does not first twist off the head of the screw. The purpose of the square is to prevent rotation of the shaft being transmitted to the head of the screw. A cover acts as a third leg for the sextant.

Figure 8 : Index arm bearing.

  The mirror mountings are standard, following the pattern described by Peter Dollond in a letter of 1772 addressed to the Astronomer Royal, Nevil Maskelyne. In the letter, Dollond describes how the mirrors are supported at only three points at the back and are retained in their brackets by three spring clips that bear on the front directly over the points. Dollond claimed to have devised the system. Whatever the truth of this, he was granted a patent for it on 22 May 1772 (no. 1017), though one should bear in mind that in the eighteenth century at least, patents were not about priority of invention but gaining a monopoly of use. One of the screws on the index mirror mounting allows it to be brought perpendicular to the plane of the arc and on the horizon mirror, one screw brings it parallel to the plane of the arc while the other one makes it parallel to the index mirror when the sextant reads zero. In this instrument Heath have made a slight refinement to protect the thread of the adjusting screws by providing a counterbore which fits over a boss at the rear of the bracket and which can be filled with a soft rubber washer or with grease (Figure 9). A front view of the clips is shown in Figure 10.

Figure 9 : Horizon mirror mounting.

 

Figure 10 : Horizon mirror clips.

Figure 11 shows how the horizon shades are mounted and the same arrangement is used for the index shades. The shades are mounted on a tapered shaft and are separated by washers which also have tapered holes in them. When the shaft is inserted into the bracket and through the sandwich of shades and washers, it is prevented from turning by a pin that passes through its head into the bracket. As the adjusting screw is tightened, the washers and shades are forced further up the taper, thus increasing the friction. There is enough friction between the washers and the shafts to prevent them from turning, so that rotational forces from moving one shade are not transmitted to the next. Unusually, in addition to the four index shades, there is an astigmatiser. This is a weak primatic lens that draws out the image of a star into a fine line. In some circumstances, this can make it a little easier to bring a star down to the horizon and, if correctly mounted, can indicated whether the frame of the istrument is tilted relative to the horizon. However, its main use was probably when employing an artificial horizon, when the line of the reflected image would be made to bisect the round direct image of the star, or the image of the bubble when using  a bubble artificial horizon. The latter had only recently been invented at the time this sextant was made.

Figure 11 : Horizon shades

Cheaper vernier sextants generally simply mounted the magnifier at approximately the correct viewing angle and focussing was carried out by sliding the magnifier up or down in a sleeve at the end of a swing arm centred about one third of the way up the index arm. Heath’s rather elaborate and delicate swing arm carries trunnion bearings that allow the magnifier to be tilted so that the view through the magnifier can be centred at any point along the vernier scale (Figure 12).

Figure 12 : Scale magnifier.

Figure 13 (below) shows the intact catch fitted to the rear of the index arm expansion on the left and the exploded structure on the right. A swing arm plate carries the bearings for a worm and its shaft and is itself carried on trunnions that run in bearings mounted on the index arm. Click on the photo to see an enlarged view. These bearings also double as keepers that prevent the index arm from lifting off the front of the limb. Close inspection of the right hand side of the illustration will show that these keeper-bearings have bosses that fit into bushes within holes on the index arm. The holes in the bushes are eccentric, so that the position of the bearings of the swing arm plate can be adjusted to remove end float of the plate and to bring the worm into correct engagement with the rack. End float of the worm itself is removed by adjustment of a cone-ended screw that engages with a centre in the end of the worm and that is locked by a knurled lock nut

Figure 13 : Release catch mechanism.

When the release catch button is squeezed, the worm and its mounting is swung out of engagement with the rack so that the index arm can be placed rapidly and approximately in position, after which the worm is used to make fine adjustments. Because it is so short, the pitch of the worm is rather difficult to measure, but it appears to be of around 0.8 mm (32 t.p.i.). After receiving their patent (No 17,840 of 10th March, 1910), it seems that it took Heath and Co another fifteen years or so to make the obvious next step and make the pitch such that one turn of the worm moved the index arm through half a degree, or 1 degree of sextant reading. This probably had more to do with conservatism than with technique, as the rise of the motor industry around the turn of the century had stimulated the production of  accurate gear hobbing machines. There is some evidence that C Plath of Hamburg had produced a very similar release catch mechanism somewhat before Heath did so, and they certainly continued to do so into the 1920s, until their micrometer sextant gained popularity and ousted the vernier instrument. Neither firm could of course claim priority for the worm and rack which was certainly known to 1st century Greeks. Heath’s claim was for the method of mounting  a “spring urged plate upon which the traversing screw is mounted…in such manner that the traversing screw can be taken and held out of gear...”  Had Plath patented their micrometer sextant in 1907, when they first advertised it, this is probably precisely the claim they would have made. Figure 14 shows the restored instrument in its case. If you have enjoyed reading this post, you may enjoy reading my book “The Nautical Sextant”, available through good booksellers, from Amazon and direct from the pjoint publishers, Paradise Cay Publications and Celestaire.

Figure 14 : Restoration completed.





Restoring an Observator classic sextant

26 02 2015

This post is preceded by one on “The Observator Mark 4 Sextant”.

I was recently asked if I would restore a sextant which had been given to Laura Dekker prior to her setting out from the Netherlands in August, 2010 to circumnavigate the world in a yacht. She had finally come to rest where she had been born, on her parents’ yacht, in the New Zealand city of Whangarei, some 200 km south east of my home in Pukenui. It was my pleasure to undertake the task and later to return the instrument to this remarkable young person. She describes her battles with the Dutch authorities, who refused to allow her to set out at the age of 14 years,  and her subsequent voyaging in her book: One Girl, One Dream (Harper Collins, 2014).

I had previously described the interesting Observator Mark IV sextant (April 2014). Laura’s instrument was plainly of an earlier era, probably from the 1950’s. Figure 1 shows the sextant as received. It bears no particular inscription, so I have chosen to call it the “Observator Classic”. The frame is of bronze and the whole sextant is exceptionally heavy at 2.32 kg (5 lbs 2 oz), compared to a Hughes and Son three ring micrometer sextant at about 1.5 kg and a W Ludolph sextant of similar robust build at 2.05 kg. It has a lighting system and as Figure 1 shows, corrosion had caused the lighting fixture to part company with its stem (All figures may be enlarged by clicking on them. Return to the text by using the back arrow.).

Figure 1: Sextant as received.

Figure 1: Sextant as received.

Figure 2 shows the condition of the rack and the entry to the battery compartment, complete with long-dead batteries. One can also see the exceptionally heavy ribbing of the frame, suggesting that it was perhaps a sand casting. Patient wire brushing restored the battery compartment to a conducting state while, once the stem of the lighting fixture had been persuaded away from the index arm fitting, it was the work of a few minutes to silver solder the stem back into place.

Figure 2: Rack and battery compartment.

Figure 2: Rack and battery compartment.

Figure 3 shows the structure of the micrometer worm and its shaft, perhaps somewhat over-engineered, but robust and adequate to purpose. The shaft has a taper on to which the worm is forced by a nut to ensure concentricity of the worm. At the other end of the shaft , a combined thimble and drum screws on to the shaft. It is located by a parallel register ahead of the threaded portion and secured in place by a locknut. The shaft rotates in two bearings in a bronze swing arm and axial preload is provided by a spring which is held in place by a cap that screws into the swing arm.

Figure 3: Micrometer exploded.

Figure 3: Micrometer exploded.

Figure 4 shows more detail of the micrometer mechanism. The cone ends of two screws, one each side of the swing arm engage with conical holes at each end of the arm and these screws are adjusted so that there is no end play but the arm can rock freely between the two screws, which are then locked. A leaf spring (not seen) between the swing arm and the lower end of the index arm holds the worm in engagement with the rack and the worm can be disengaged by squeezing the two together. Extensions of the trunions  that house the cone-ended screws act as keepers to prevent the lower end of the index arm from lifting off the frame and keep the worm aligned with the rack.

Figure 4: Detail of micrometer mechanism.

Figure 4: Detail of micrometer mechanism.

The telescope rising piece design lies somewhere between the more-or-less elaborate nineteenth and early twentieth century arrangements for adjusting the height of the telescope above the frame (see, e.g. https://sextantbook.com/?s=fine+C+Plath), and the very simple later vee and flat used by most makers after about 1950. The telescope rising piece is square and fits closely into a socket  screwed to the frame. A thumb screw locks it into position (Figure 5).

Figure 5: Telescope mounting.

Figure 5: Telescope mounting.

After cleaning up, making and replacing the mirrors, it remained only to repaint the instrument to give the pleasing result shown in Figure 6 and to clean up and re-varnish the case (Figure 7).

Observator 010

Figure 6: Finished instrument.

 

Figure 7: Restoration complete.

Figure 7: Restoration complete.