A French Hydrographic Sextant

13 01 2019
2 a case inside

Figure 1: Sextant in its case.

I recently acquired for a relatively modest sum the three-circle vernier sextant shown in Figure 1. Attached at the front corner of the frame is a plate engraved with the letters “S.H.” or “Service Hydrographique (de la Marine)” or French Naval Hydrographical Service, formed in 1886 as successor to the “Dépôt des cartes et plans de la Marine”, founded in 1720. The plate seems to serve no other purpose that I can think of  than as an identifier.

3 a e bouty name

Figure 2: Front of the tangent screw mechanism.

Engraved on the front of the tangent screw mechanism is the name “E. Bouty”. Edmond Bouty (1845 – 1922) was a physicist in the Science Faculty at Paris, but I cannot find that he was an instrument maker, nor is there any other name on the sextant. It may be that his contribution was the design of the scale lighting system, about which more later. It is not even clear that the sextant is of  French manufacture, as at the left end of the limb are the letters “D.S.” indicating Deutsche Seewarte, the German Hydrographical Service, but the frame, of about 180 mm radius, differs in detail from that of C Plath’s Dreikreis sextant.

2 b frame turning marks

Figure 3: Turning marks on front of frame.

The bronze frame is of no particular interest except that when clearing old and perished paint from the frame during restoration I noticed marks (Figure 3) that showed that it had been faced in a lathe, giving a small clue to the manufacturing process.

3 c spring nut

Figure 4: Spring box detail.

Returning to the tangent screw mechanism, the spring box is shown exploded in Figure 4. A tongue on the sliding block is trapped between the end of the tangent screw and a long spring mounted on a guide and retained by a nut. The end of the guide can be seen on the right of Figure 2.

3 b clamp

Figure 5: Exploded view of index arm clamp.

The sliding block is retained in its slide in the lower end of the index arm by the retaining spring on the upper right of Figure 5, while the clamp screw and its leaf spring bears on the back of the limb. In use, the clamp is slackened and the index arm moved approximately into position, when the clamp is tightened, thus fixing the sliding block to the limb. Turning the tangent screw thus moves the index arm about the sliding block against the pre-load of the helical spring as a means of fine adjustment. In truth, it is the index arm that slides rather than the sliding block, but as no one else had given it a name, I decided to do so when writing “The Nautical Sextant.” This system of applying pre-load was used in many vernier instruments such as vernier theodolites and gun aiming systems. as well as in several makes of sextant.

4 perp adjust

Figure 6: Index mirror bracket.

The index mirror is held against a vertical bracket by means of a clip which is tightened against the bracket by means of a screw bearing on the back of the bracket. The mirror is made perpendicular to the arc of the sextant by a system that seems  to have been used only by French makers. Two screws attach the radiused feet of the bracket to the upper end of the index arm and the end of a screw held captive in the base of the bracket can then rock the bracket to bring the mirror square to the plane of the arc..

5 side error

Figure 7: Horizon mirror bracket.

Figure 7 shows a somewhat similar method of adjusting out side error of the horizon mirror, but in this case a deep slot cut nearly through the base of the bracket gives flexibility to the the adjustment by means of another captive screw.

7 horizon mirror

Figure 8: Horizon mirror detail.

The detail shown in Figure 8, as well as making clearer how the mirrors are held against their brackets, shows that the horizon mirror bracket can be adjustably rotated about an axis vertical to the plane of the sextant, in order to adjust out index error. Note that the mirror is fully silvered, which means that the direct view of the horizon does not pass through glass and that the edge of the silvering of the mirror can be given better protection against corrosion. It does however result in a smaller area of overlap of the direct image of the horizon and the  reflected  image of the observed body when using a Galilean telescope. Enter “Freiberger yacht sextant” in the search box at the top of the page for a discussion of why this is so.

6 index error

Figure 9: Detail of index error adjustment.

Figure 9 gives more detail on the index error adjustment. There is a boss as an axis on the underside of the horizon mirror bracket that passes through the frame and is held by a retaining screw. A further boss passes through a clearance hole in the frame  and has an internal thread tapped in it as a nut. The index error adjusting screw, held captive in the frame by a screw and clamp, engages with the “nut”, so that when the adjusting screw is turned, the whole mirror bracket rotates. When adjustment is complete, the bracket is locked in place by a  clamp screw..

This is a rather complex means of adjustment of the horizon mirror, which had long been achieved much more simply by means of   a pair of screws bearing against the back of the mirror, while lugs on the mirror clamp provided spring loading. Elegant though it may have seemed to its (?) French inventor, it is unnecessarily complex., though perhaps no more complex than the solution adopted by Brandis and its US successors.

9 battery handle

Figure 10: Interior of battery handle.

This sextant represents perhaps one of the earliest ones to light the scale in poor light. Scale lighting had to wait for the development of suitable dry batteries in the 1890s and of miniature flashlight bulbs with robust tungsten filaments in about 1904.

Figure 9 shows the interior of the Bakelite handle which accepts a 3 volt 2R10 battery.  A screw at the lower end holds the negative pole of the battery firmly in electrical contact with the frame of the sextant and at the upper end a spring loaded switch plunger makes contact with the positive pole. The top end of the lid is bevelled and the lid itself is slightly bowed, so that when rotated closed it remains in place.

10 b handle to bearing

Figure 11: Wire from handle to foot.

A wire passes from the body of the switch to the foot (Figure 11), inside which is a spring loaded brass plunger (Figure 12).

10 a switch to contact

Figure 12: Inside of foot.

The index arm journal is hollow and a wire passes up its centre to an insulated contact on the end, to make electrical contact with the contact inside the foot (Figure 13).

11 a journal contact

Figure 13: Insulated index arm contact.

The other end of the insulated wire passes down the index arm in a machined groove to a clip held on an insulator block (Figure 14).

12 lighting system

Figure 14: Lighting bulb holder.

The clip makes contact with the outside of the bulb holder and thence to the central contact on the bulb. The outside of the holder is insulated from the brass interior, which is threaded for the bulb. The brass interior fits snugly in the cylindrical shade which is attached to the index arm and hence the frame, thus completing the electrical circuit. Most subsequent makers contented themselves with a simple loop of insulated wire to conduct electricity to the bulb, but this more complex and no doubt more expensive system has the merit of not flexing any wire. Like most complex systems, however, there is more to go wrong.

13 rising piece in situ

Figure 15: Rising piece.

The telescope rising piece (Figure 15) is simpler than that of many of its early 20th century competitors and it has a rectangular mortice machined in its face to engage closely with a tenon on the telescope bracket, so that it can be slid up or down to vary the amount of light from the horizon entering the telescope. Collimation is standard, by means of a tilting telescope ring held in place by two screws.

8 index shades

Figure 16: Shades mounting.

The shades make none of the usual provisions to prevent movement of one being transmitted to its neighbours. Resistance to rotation is given by means of a Belleville washer, a conical washer with the characteristics of a short, stiff spring. Since these date from about 1870, they add no clues to the age of this sextant.

15 telescopes

Figure 17: Telescope kit.

The kit of telescopes shown in Figure 16 is for the most part standard, with a 4 x 24 mm Galilean “star” telescope for general use and a 6 x 16mm Keplerian “inverting”  telescope. By the twentieth century, this latter probably received little use except for artificial horizon sights to rate chronometers in out-of-the-way places of known longitude. The large 3 x 36mm Keplerian telescope is of interest as it has a wide angle eyepiece with an eye lens of 25 mm aperture. This gives an image nearly as bright as the 4 x 24mm telescope (the extra lens in the eyepiece causes some loss of light) and with a field of view about four to five times wider.

1 a case exterior

Figure 17: Case exterior.

The mahogany case was much battered and stained, and with several shrinkage cracks, so it was gratifying to be able to restore it to the state shown in Figure 17. It looks decidedly English and placing the handle on the side follows Henry Hughes and Son’s practice, but neither the sextant frame nor the mirror mountings  are consistent with this.

If you enjoyed reading about this sextant, you may also enjoy reading my “The Mariner’s Chronometer“, also available via Amazon.com.

 

 

 

 





A C18 sextant named J. Watkins

17 09 2018

Previous posts in this category include: “An Old Wooden Quadrant Restored”, ” A turn-of-the-century French sextant”, “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”.

A few months ago I acquired an old sextant for a very modest price, as it was without telescopes or case. Restoration was straight forward, as all the parts were present. It was only necessary to clean and re-lacquer  parts and polish screw heads and then find a new home for the instrument. Figures 1 and 2 show the general arrangement of the restored sextant from the front and back, and I have labelled the main parts for those who have not yet had the wisdom to buy “The Nautical Sextant”.

Figure 1: The sextant and its parts.

 

GA rear better

Figure 2: Rear view of sextant.

I show the naked frame in Figure 3. It is slender by modern standards, but seems to be rigid enough for its purpose.

Frame bare

Figure 3: The naked frame restored.

I believe that this is a relatively early sextant, based not only on the name engraved on the limb but on several other factors. To take the name first, it is “J Watkins Charing Cross London”.  This must refer to Jeremiah Watkins who succeeded his uncle, Francis Watkins (1758 – 1810) in 1784 at the age of about 26 years. I have been able to find only one other sextant with J Watkins’ name on it (sold in London in 2014) and I suspect that he was the retailer rather than the maker. It was common practice for sextants, chronometers, clocks and the like to be sold un-named, for the retailer to add his name. The main activity of Watkins senior and junior, and later Watkins and Hill, was in making telescopes with achromatic lenses.

Name

Figure 4: The name.

Looking closely at the name (Figure 4a ), the second “s” in “Charing Cross” is the old English long s, with a nub on the left of the descender. This form of s  had fallen into disuse in printing by 1800 and its use here suggests the engraver had trained well before 1800.

Long s

Figure 4a.

The brass limb was attached to the hard bronze frame by countersunk screws whose heads were then filed off flush, and the ghost of one of these screw heads can be seen above the “n” of Charing Cross. Rather unusually, the divisions are made directly into the brass rather than into a band of silver let into the brass, a practice established well before 1800. The usual explanation for this practice is that brass of the time contained hard spots which could have diverted the scriber of the dividing engine from its true path and that lines on brass tended to be ragged. However, this may have applied only to English brass, as makers preferred if they could to import high quality “Dutch” brass from near Aachen in Germany.

Scale divisions

Figure 5: Magnified view of scale divisions.

A close up view of the divisions (Figure 5) shows them to be perfectly regular, so that the scale was, I am sure,  machine divided. Jesse Ramsden’s first dividing engine was finished in about 1766 . It surpassed all previous attempts at accurate machine division, but he was not satisfied with it, and completed an improved version in June 1774. Around about this time he produced a sextant shown in Figure 6, with a frame very similar, if not identical to my Watkins’. By 1789 there were three dividing engines in London, By Jesse Ramsden, John Troughton and John Stancliffe, and by 1808 there were perhaps a dozen.

Copy of Ramsden sextant

Figure 6: Sextant by Jesse Ramsden.

This suggests to me that the two sextant frames had a common source in a specialist foundry and we know that Ramsden used specialist  founders when he needed castings. The finish is very regular and there is no signs that it was sand cast. More likely is that it was cast in bell metal (a bronze high in tin) by the lost wax method.

The radius of the arc is nine inches (229 mm) and by 1800, because it had by then become possible to divide small radii more accurately, the more usual radius was around six and a half inches (165 mm). Thus, these several features lead me to suppose that the sextant was made some time after 1774 and before 1800.

The telescope ring and rising piece are of a form that remained common well into the twentieth century. The telescope is held in a ring that can be adjusted so that the axis of the telescope is parallel to the frame, while the whole can be raised or lowered on its square rising piece by means of an internal captive screw and knurled knob (Figure 7).

Telescope rising

Figure 7: Telescope ring.

Compared to later sextants the index mirror is unusual in that there is no provision made for adjusting it to be at right angles to the plane of the arc. In some sextants of this era, of the three large screws seen in Figure 8, only the two outer ones attached the bracket to the index arm, while the central screw was threaded into the bracket and its tip bore on the face of the index arm, so that it was possible to rock the bracket a little to adjust it. In this sextant, the bracket was simply made square in the first place.

Index rear

Figure 8: Index mirror bracket.

Figure 9 shows the front of the bracket and two of the three nubs or nipples on its face, against which the mirror was held by a clip with three tongues opposite the nubs. When the small central screw seen in Figure 8 is tightened, the mirror is held strain-free to the bracket by the clip.

Index structure

Figure 9: Index mirror clip and bracket.

The index arm bearing is shown in Figure 10.  It is of the form used by almost every maker until late in the twentieth century. The index mirror is attached to a disc to the underside of which is attached a tapered journal which runs in a corresponding hole in the index arm bearing. The fit is adjusted by means of an axial screw via a washer.  The washer has a square hole in it that fits over a square on the end of the journal. This prevents turning forces from being applied to the screw and loosening or tightening it.

In this sextant, the whole is enclosed in a cover, which also doubles as one of the feet of the instrument. Many makers copied this practice, though in the twentieth century, when two World Wars required quantity production, it was often omitted.

OLYMPUS DIGITAL CAMERA

Figure 10:  Index arm bearing.

The method of adjusting the position of the index arm is shown in Figure 11 and is typical of the practice used in practically all vernier sextants until the last were produced in the late 1930s. A block is able to slide in a guide fabricated on the rear of the index arm expansion and the block carries a bronze nut, which sometimes has an adjustable split in it to ensure a close fit upon the tangent screw. Running in the nut is the steel tangent screw, held captive in its bearing which is attached to the index arm. Rotating the screw can move the block in its slide, but in practice, once the position of the arm has been roughly set by sliding the index arm by hand, the clamp screw clamps the sliding block to the limb of the sextant. Then, when the screw is turned, it is the index arm that slides on the block.

Tangent screw

Figure 11: Tangent screw.

Figures 12 and 13 show the methods for adjusting the horizon mirror. The mirror bracket sits atop two circular tables. The top one may be tilted against a concealed spring using the adjusting screw, shown in Figure 13, to correct for side error.

Index adjust front

Figure 12: Horizon mirror adjustment, side view.

The bottom circular table may be rotated through a small angle to correct for index error, using two capstan headed screws that lock against each other. The screws bear on a tongue which is an extension of the frame and the table rotates about an axis which passes through the frame and is secured by the large screw underneath.

Index adjust rear

Figure 13: Horizon mirror adjustment, rear view.

This rather complex arrangement is used on several sextants of this era. The sextant described in my post of 10 November 2009 shows an identical system, while another , shown in the post of 10 June, 2010, has a different though equally complex system. Before long, the much simpler method came in, of screws bearing against the back of the mirror with springs opposing the movement against the front of the mirror. However, some makers persisted with unreasonably complex systems of adjusting the horizon mirror well in to the twentieth century. Brandis was one late C19 maker whose system was copied for the US Navy Mark II sextant of the 1940’s and I have illustrated it in Figure 9 of my post of 30 November 2010.

Like many sextants that I can afford to buy, this instrument lacked a case. For the last nine years, I had been hoarding a case that had housed a pillar sextant. Usually, sextant cases are of mahogany, not brass-bound rosewood, indeed, this is the only example I have ever seen for a proper sextant, rather than the decorative so-called  reproduction ones from the Indian sub-continent. Figure 14 shows how I have adapted the case for my Watkins sextant. Originally, it would probably have lived in a keystone case, which are difficult to make  and difficult to carry. I had two contemporary telescopes saved against the day when I would acquire an antique sextant without any and had only to do a little screw-cutting on the lathe for them to fit in the telescope ring.

Case 002

Figure 14: Sextant in new home.

This completes my one hundred and third blog post on sextants and, while I have material for two more, I will then have run out of subjects. I am open to suggestions  about further subjects. If you have a particularly interesting sextant I would be happy to consider a guest post, though I would reserve the right to edit it if necessary. If you are interested in things navigational, don’t forget to have a look at my other site: http://www.chronometerbook.com





Carl Plath’s earliest sextant.

20 04 2017

This post was preceded by “C Plath Yachting sextant“ “Making a shades adjusting tool” and “Eighty years of Carl Plath Sextants”. Other posts on C Plath sextants may be found by entering “C Plath” in the search box on the right.

C Plath bought the business of David Filby, Hamburg’s member of Parliament, in 1862 and shortly after he disposed of the book and chart side of the business while retaining the nautical instrument side. Around about the time that he moved into the address known as Stubbenhuk 25, he acquired a dividing machine from Repsold, his former apprentice master, and began to make his own instruments, including sextants. A year or so ago I acquired an early C Plath sextant for a very modest sum on e-bay. It and its case were not in good condition and so I restored them, but other commitments have until now prevented me from writing about it.

Case as received

Figure 1: The case

Figure 1 shows the case as received, with shrinkage cracks in the top and signs of water damage. Much of the varnish had crumbled, so I stripped off all the old varnish from the outside, filled cracks, made good loose joints, re-lacquered the hardware, re-stained the mahogany and applied several coats of modern varnish to give the result shown in Figure 2. The corner joints are rebate joints, reinforced with steel pins at a time when most makers were using hand-cut corner dovetails. The top, however, is attached with brass screws, while the bottom is glued and pinned on, following the practice of nearly every maker throughout history. The pins tend to rust, being “out of sight, out of mind”.

Case restored

Figure 2: Case restored.

The interior  as received is shown in Figure 3. Apart from dust and many flakes of paint the green felt lining of the floor and roof of the box had decayed. Fortunately for the instrument, the pocket for the handle and the pads for the legs were attached by screws and glue which had not given way.

Interior as received

Figure 3: Interior as received

Cleaning the interior and fitting new felt  completed the restoration of the case, and restoring the instrument itself presented few problems as everything was present and intact. Figure 4 shows the back of the frame, which is a heavily ridged bronze casting. In the nineteenth and early twentieth century, a very large range of frame patterns was offered by the major makers, but it was not until the early part of the twentieth century that Plath left behind this initial pattern in favour of the ladder pattern, with the occasional three circle or “Dreikreis” pattern being offered (see my post for January 2010). Note the heavy re-inforcement in the areas where the telescope and horizon mirror brackets are mounted.

Rear as received

Figure 4: Frame before re-painting.

The traces of paint that remained were a pale green, but the sextant illustrated on page 50 of Friedrich Jerchow’s history of C Plath, From Sextant to Satellite Navigation, is painted black, so I surmise that the green represents perhaps a primer coat. At any event, I stripped it all off and re-coated the parts black, taking the illustration as a pattern. Figure 5 shows the sextant in its case after restoration.

Interior restored

Figure 5: After restoration

Figure 6 shows the kit of telescopes and other parts supplied. Well into the twentieth century, sextants were often supplied with several telescopes and supplementary eye-pieces. I doubt that there was ever a time when most of them were used at sea. The 6 x 16 Galilean telescope has a tiny field of view and the 10 x 17 inverting telescope is no better. There may have been a time when they were used on land with an artificial horizon to check on the rate of chronometers in distant ports of known longitude, but at sea the 3 x 28 Galilean was probably the one used most, with the ‘zero magnification” sighting tube being substituted in rough weather. The eyepiece shade may have been used to check index error using the sun, but again, most people would probably have used the horizon, as it is easier on the neck to do so.

Telescope kit

Figure 6: Telescope kit

The shades and mirror brackets are perfectly conventional and avoid the complications used by French makers in particular and also by Brandis and successors. The tangent screw is of some interest and is shown complete in Figure 7. With slight modification it was also used in the Dreikreis sextant before the micrometer sextant was developed by C Plath around about 1907.

Tangent screw

Figure 7: Tangent screw

The spring box, which I have called a “sliding block” in different designs, slides in a close-fitting pocket on the rear of the index arm expansion and can be clamped to the limb using the clamp. A leaf spring keeps the box in place when un-clamped. The end of the tangent screw bears on a tongue projecting from the back of the index arm against the pressure of a coil spring within the spring box. thus, as the screw is turned, the index arm moves along the arc in slow motion. On releasing the clamp, the index arm can be swung rapidly by hand. Figure 8, which shows the mechanism exploded, may help to make this clearer.

Tangent screw exploded

Figure 8: Tangent screw mechanism exploded.

The mechanism for raising and lowering the axis of the telescope so that more or less light from the horizon can enter it is shown in Figure 9. It represents an intermediate stage of complexity on its way to the simplicity of the second half of the twentieth century.

Rising piece

Figure 9: Telescope rise and fall.

A telescope bracket having a vee groove and flat machined into it is attached to the frame of the sextant. The rising piece of the telescope has a matching vee and flat to guide it up and down. The lower end of the rising piece has a threaded hole for a screw that is held captive in the telescope bracket, to that when the screw is rotated, the rising piece rises or descends. A clamp holds the rising piece at the selected height.

Arc and name

Figure 10: The arc.

The silver arc, let into the bronze limb, is divided to 10 minutes (Figure 10) and the silver vernier allows readings to ten seconds, though, as with many similar verniers, it is usually impossible to decide which particular pair of lines coincide. It is easier to decide which two pairs of lines just do not coincide and to choose the middle value between them. It bears the C Plath name in flowing copper plate script, a feature of Plath’s earlier sextants.

Serial and S

Figure 11: Serial number and inspection mark.

 Dr Andreas Philipp has kindly provided me with the date of early 1899 for the instrument, or at least, its certification by Deutsche Seewarte. He tells me that both “S” and “D S” were used irrespective of date. He also sent me an illustration from Plath’s number V catalogue of  1906, which I reproduce below (click on this image to enlarge it). Based on the D.S. records, it seems that between 1876 and 1901, Plath produced an average of only 27 sextants per year.

Cat photo 001.JPG

Catalogue entry 1906 (Courtesy of Dr Andreas Philipp)





A turn-of-the-century French Sextant

11 08 2015

Previous posts in this category include: “A Half-size Sextant by Lefebvre-Poulin”,

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.





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.





USN BuShips Mark II sextant: some design oddities.

30 11 2010

The United States Navy Bureau of Ships was founded in June 1940, tasked, among other things with the …”design, construction, procurement and maintenance …” of naval ships and their equipment. One item of equipment that was needed in large numbers was the nautical sextant. The BuShips Mark II seems to have been founded on a pre-Second World War model, which in turn owed much of its design to Brandis and Sons. The latter traded under various Brandis names from 1871. The Pioneer Instrument Company gained a controlling interest in 1922 and was itself bought out by the Bendix Aviation Corporation in 1928, though sextants continued to be made under the Brandis name until 1932.

At the onset of the War, contracts were let to at least three manufacturers to produce tens of thousands of sextants in a relatively short time, using modern mass-production methods. The Pioneer Instrument Division of the Bendix Aviation Corporation was probably the design leader, with the David White Instrument Company of Milwaukee and Ajax Engineering of Chicago also producing instruments that showed minor variations on the main design.

Figure 1 : Ajax Engineering Mark II Sextant

All had pressure die-cast aluminium alloy frames with bronze rack attached in a variety of ways: Ajax managed to cast the rack integrally with the frame, Pioneer-Bendix attached it with radial screws and dowels, necessitating a wider rack casting, while David White attached it with screws inserted from the back of the frame, a practice continued with the Mark III successor (see my post on Evolution of the Sextant Frame). There seems to be no special advantage to the adoption of a composite frame, as a bronze worm running in the alloy of the frame itself seems to have worn very well in post war sextants. There may have been production advantages, as an annular rack blank could have been mounted on a hobbing machine to cut the teeth and then cut into four racks, though this does not seem to have been the method used to produce racks for the Mark III, which were fitted to the frame before cutting the teeth. The alloy frame had the advantage of rapid production, high strength, similar to that of mild steel, good stability, cheapness and light weight.

The design of the index arm bearing is conventional, in the form of a tapered hard bronze shaft in a brass bearing. This self-centring bearing arrangement is unchanged from the form originated, probably by Jesse Ramsden, in the eighteenth century and used by nearly every sextant maker since. At the other end of the index arm, however, the micrometer worm has for some reason a left hand thread, so that when turned clockwise by the user, the reading descends (Figure 2). It is hard to see any advantage in this and cutting left hand threads is slightly more difficult to achieve.

Figure 2: Mk II Micrometer drum

Pioneer-Bendix and Ajax bury the complex little release catch mechanism between the index arm and the swing arm chassis, making its construction hard-to-fathom. For the un-initiated, I have covered the details in The Nautical Sextant. The axial pre-load spring is L shaped and the upright of the L is forked to surround the worm shaft and press on a shoulder in front of the worm. A conventional leaf-spring is used to keep the worm in contact with the rack (Figure 3). David White, by contrast, bury the latter spring between the index arm and the swing arm chassis, while the cam of the release catch is clear to see. Axial pre-load is applied by a leaf spring bearing on the end of the shaft and White introduced the complication of an adjustable split bearing for the swing arm chassis (Figure 4). White used two conventional keepers to keep the index arm in place while the other makers took the short cut of using a single shouldered screw.

Figure 3 : Pioneer-Bendix micrometer mechanism

Figure 4; Layout of David White micrometer mechanism

There are two striking  features of difference  when the sextant is compared to most of its contemporaries, particularly German ones and their Japanese followers:  the clear aperture of the telescope objective lens is a mere 15 mm, recalling the rather inadequate Galilean telescopes of eighteenth and early nineteeth century sextants, and the mirrors are small to match. The magnification was a nominal x 3.  The aperture of the silvered part of the horizon mirror is only18 x 16 mm or 288 square mm, exactly the same as its Brandis ancestor’s, compared to its Carl Plath contemporary,which had a silvered area more than four times as great. There is of course no point in having the light gathering power of large mirrors unless the telescope aperture is large to match.

As navigators use for the most part the sun, moon and navigational stars, only one of which has a magnitude greater than 3 (Acamar), light loss through the telescope may seem unimportant. However, accuracy of sights depends quite heavily on getting a clear view of the extended object of the  horizon, and in marginal conditions, light grasp is important. While the human eye-brain combination can only  just notice a halving in brightness of light, ability to perceive contrast is very much influenced by the relative light levels of the horizon and sky. Why then was the telescope designed as it was (Figure 5)?

Figure 5: Mark II sextant telescope exterior

The specification may have called for a telescope giving an erect, wide field image of x3 powers and may not have emphasised brightness of image. A Galilean telescope can easily be designed to give a bright, erect image magnified three times, but the field of view is constrained by the diameter of the objective lens. Unprepared as the USA was for war, it may be that all suitable large diameter objective lenses were reserved for binocular production, even though here there were bottlenecks in the production of good quality prisms. In Germany and Japan at the time, Plath and Tamaya were producing sextants having 3 power Galilean telescopes with 40 mm diameter objectives. Even so, the usable field of view was only about 5.7°, compared to the 8.6° f.o.v. of the Mark II telescope. Inexperienced navigators sometimes struggle to find and keep in sight heavenly objects, particularly stars, when in any sort of seaway and the wide field of view may well have been regarded as a good trade for some brightness.

The lens plan shown in Figure 6, which gives approximate focal lengths and separations, illustrates that there was plenty of scope for light loss, as there were seven lenses, each with two air-glass interfaces at which light could be lost by reflection (about 8 percent per surface) and contrast lost by scattering of light.

mk-ii-telescope-layout1

Figure 6: Telescope lens plan

Figure 7 shows some of the elements of the ‘scope’s construction. A two-lens objective group screws into one end of the front tube and the reverting lens group into the other end. A thread on the other end of the reverting group joins the front and rear tubes. All the threads are locked by radial screws.

 

Figure 7 : General construction of Mark II sextant telescope

Would-be restorers of these venerable instruments may find that the focussing eyepiece is seized or has received the attention of water-pump pliers or a vice in an attempt to get it moving. In the first instance, I suggest that some releasing compound is used and left to work overnight in a warm place. If the eyepiece will still not rotate, forcing it is liable to shear the slender screw that passes through a clearance hole in the thimble and through a tapped hole in the slide into the eyepiece assembly (Figure 8). It is best to remove this screw if at all possible before donning rubber household gloves to enhance one’s grip and pulling while at the same time twisting. The eyepiece lenses may then be removed, if necessary, in order to clean them by wiping from side to side with lens paper moistened with a little alcohol.

See Update at end of post.

Figure 8: Mark II sextant telescope eyepiece assembly

A final quirk of the Mark II design is in the method of adjusting the horizon mirror for side and index error (Figure 9). Nearly every other maker had adopted the simple mechanism for adjusting mirrors devised, or, at least, first described by Peter Dollond in 1772, of applying adjustable screw pressure to the back of the mirror at two out of three  points, opposed by springs. Brandis, however had early adopted a relatively complicated method of levers adjusted by opposing screws, that was not only difficult to adjust, but also easy to knock out of adjustment. This method was carried over into the Mark II.

Figure 9 : Mark II horizon mirror adjustment.

The horizon mirror is carried on a bracket that pivots as a second class lever  in the region of the mirror’s base, while the adjusting force is provided by two screws, one of which, as it were, pushes the lever, while the other pulls. When both are tight, the mirror is locked, albeit uncertainly, in position, but just to make sure of it, a screw passes through the pivot into the mirror bracket.

The mirror bracket itself is carried on another bracket that I have labelled “index error bracket,” as this tilts the mirror bracket in the plane of the sextant frame. It uses the same potentially unstable lever system, except that the lever arm is four or five times longer and much less stable, again requiring a screw through the pivot . The whole is mounted on a cast bracket that is screwed to the front edge of the sextant frame and perhaps the best that can be said of the set up is that it simplifies the attachment of the index and horizon shades in the Pioneer Bendix model. In this sextant there is an upstand at each end of the base bracket for attachment of the shades, though the Ajax model discards half of even this slight advantage by having a standard bracket attaching directly to the sextant frame for the index shades.

In sum, the Mark II sextant design, compared to German and Japanese instruments of the same period, was far from the pinnacle of perfection. Perhaps, like the camel, it was designed by a committee.

Addendum In response to a request by a reader I have added some pictures of the cases. Only Ajax used solid wood. The others used plywood, which has not stood the test of time, since it was not of marine grade. The White case has almost disintegrated and the bottom of the Bendix case has had to be replaced.

From top to bottom, cases by Bendix, Ajax and White, front view

 

 

From top to bottom cases by Bendix, Ajax and White, three-quarter view

Update 16 July 2017: There are at least two different approaches to the focusing assembly of the telescope, though both have a sliding piece in a part helix. In one pattern of telescope, the second lens from the left replaces an achromatic lens with a single, symmetrical biconvex lens. There were a couple of errors in the original drawing, with two of the lenses back to front. I have now corrected these.





Evolution of the Sextant Frame

28 09 2010
This post is followed by one about the sextant micrometer’s evolution. See also Eighty years of C Plath Sextants (November 2012)
MATERIALS
Edmund Halley used a form of sextant during  a voyage of 1698 to 1699. It was made by or for Isaac Newton when he was Master of the Royal Mint, but it has not survived. Though we know about its general form, we know little else about it. We can be fairly sure that it had a large radius and that its frame was probably made of wood with an ivory or boxwood scale. The large radius was necessary because instrument scales were at that time divided by hand and the larger the radius, the smaller were the errors. While we cannot be sure about the material of the frame, because a typical radius was about 380 mm (15 inches), a brass frame of this radius would have been too heavy to handle conveniently.

Wood

Wooden frames continued to be used for another 150 years for cheaper instruments or those where a high degree of accuracy was not required, for example, in finding latitiude by measuring the angular height of the sun above the horizon at local noon.  Strictly speaking, these were octants, measuring up to 90 degrees, because no body has an altitude greater than this. By the middle years of the eighteenth century there was in England a push for greater accuracy of observations. Nevil Maskelyne, the Astronomer Royal had published in 1763 The British Mariner’s Guide, which contained an English translation of Tobias Mayer’s tables of predictions of the moon’s position in the sky, together with instructions on how to use them to fix longitude at sea. Since the moon moves across the sky at a different rate to the movement of the Sun and stars, it can be used as a sort of celestial clock by measuring the angle between the moon and another heavenly body. By comparing one’s local time with that derived from the lunar distance, say, at Greenwich, the longitude could be deduced. Unfortunately, the difference in rates of the movement between the moon and the other bodies is relatively small, so that a small error in measuring the angle results in a large error in the deduced longitude.

Wooden instruments were not capable of measuring the angles with the required degree of accuracy nor, for that matter, were hand divided instruments. Jesse Ramsden described his dividing engine in 1774 (see my post under Chasing tenths of an arc minute, September 2010) after which practically all sextants were machine divided. From about this time also, sextants, able to measure up to 120 degrees of an arc, started to be made of metal, as they could now be made smaller, while wood continued to be used for many of the lowly octants, more usually called Hadley’s quadrant or simply “a Hadley” .  The wood used was usually heart ebony, a very hard, stable and dense wood  (Figure 1), though mahogany was also sometimes used. The index arm (alidade), mirror and shade mountings were made of sheet brass, using techniques well established by the clock-making industry.

Figure 1: Mid 19th century octant in ebony and brass

Bronze

Though museum cataloguers often describe sextants as being made of brass, an alloy of copper and zinc, the metal used was bronze, an alloy of copper and tin. Sound castings could not be made in brass with the techniques and alloys of the time, whereas it is possible to make relatively large and intricate castings out of bronze. As the proportion of tin increases, the metal becomes harder and more rigid until, at a maximum of around 20 percent of tin it becomes known as bell metal. This is somewhat brittle and the bronze used for sextants contains about 8 to 10 percent of tin. Early metal sextants commonly had a limb of brass screwed on to the front of the frame and heads rivetted and filed flush (the heads are visible in Figure 2). Brass of the time was made by hammering and rolling into sheets and often had hard spots that could divert a scriber from its true course, so almost from the beginning, the divisions were made onto an arc of silver let into the limb. For a short period in the early 19th century, when platinum was cheaper than silver, platinum was used sometimes and, rarely, gold for the vernier. Ivory was used for cheaper instruments.

Figure 2: late 18th century bronze sextant frame

  Brass

An exception to the use of brass for frames was the pillar sextant, produced almost entirely by Jesse Ramsden and by the Troughton brothers. Copying the techniques used for clock plates, two thin brass plates were held apart by numerous pillars of brass to give a relatively light and stiff frame.

Aluminium alloy

An electrolytic process for the smelting of aluminium was devised in 1866, converting the metal from one of great rarity and expense to something much cheaper, but the pure metal is soft and not suitable as a structural material. Nevertheless, sextants were occasionally made with aluminium frames in the late 19th century, usually as an experiment or for a special purposes.

Early in the 20th century, hard and strong alloys of aluminium were developed. This, combined with the pressure die casting process, allowed manufacturers to mass produce aluminium alloy frames of about a third of the weight of those of bronze, with a strength and stiffness approaching or exceeding that of mild steel. Despite the availability of aluminium frames from the mid 1930s onwards, mariners on the whole continued to prefer bronze frames, regarding sextants with aluminium frames as cheap “knock offs”. The exigencies of the Second World War, however, required a much increased rate of production and, especially in Germany, non-ferrous metals were scarce. The US BuShips Mark II sextant in the USA and C Plath wartime sextant in Germany were produced in very large numbers and Tamaya in Japan also used aluminium in the later stages of the war. In the US and Japan, its seems that makers could not bring themselves to abandon bronze entirely for the rack, either attaching it with screws to the machined frame or casting it in place (Figure 3). C Plath, however, machined the rack directly into the aluminium frame, as did all post-WW II Russian instruments (Figure 4), with no consequences for accuracy and durability.

Figure 3: Cast-in rack, BuShips Mk II by Atlas Engineering
Figure 4: Rack and limb of Soviet SNO-T sextant

After the end of  WWII, most makers reverted to making the frames either entirely of bronze (despite its inferior stiffness)  or of aluminium alloy with a bronze rack. The USSR, however, made the frames for their SNO-M and their later SNO-T sextants entirely from aluminium alloy, with a bronze micrometer screw running in a rack cut directly into the frame (Figure 4, above). Both sextants performed well and the SNO-T was possibly the best sextant ever made.

Plastic
During the Second World War, very many ships were sunk by submarine and surface raiders. The US Maritime Commision equipped lifeboats with a basic navigating kit that included a plastic vernier sextant. For a sailing lifeboat, unable to sail upwind, this was no doubt adequate to make a landfall. With the increased popularity of recreational sailing in the sixties and  seventies, the East Berks Boat Company in the UK and the Davis Instrument Corporation in the US each produced  basic vernier sextants made entirely of plastic which were very useful for taking horizontal angles and for establishing an approximate position. Davis later increased the complexity and produced instruments, again entirely of plastic, that had the form of traditional sextants (Figure 5). Its performance, however, was very far from traditional. Its frame lacked rigidity and index error could scarcely be relied upon to stay the same from one observation to the next; and the index arm bearing of all makes suffered very badly from stick-slip, a problem that had been previously eliminated in the eighteenth century by Ramsden’s application of a tapered bronze shaft running in a brass bush .

Figure 5 : Davis Mk 15 plastic sextant

FORM AND CONSTRUCTION

The requirements of the frame are to unite the centre bearing with the arc or rack in a fixed relationship, to make provision for the attachment of shades, mirrors, handle and telescope and to do so without flexing or twisting. The axis of the index arm bearing must be square to the plane of the arc or rack and stay that way in use. The horizon mirror must also stay perpendicular. Nearly every sextant joined the bearing to the curved limb with two radii about 60 degrees apart and had one or more transverse bars that joined the two radii. At first there was no handle by which to hold the large frame, but as frames became smaller, the transverse bar was used as one of the points of attachment for the handle. The way the various subsidiary parts were attached to the frame varied greatly from maker to maker and they usually filled in the space between the radii and limb with sometimes fanciful frame work. There was probably little thought or experiment to determine whether one form of frame work was more rigid than another, though as knowledge of engineering structures improved it was probably appreciated that depth of the framework was an important contributor to stiffness.

Wooden frames

Figure 6 shows the rear of a typical wooden-framed octant of about 380 mm (15 inches) radius. It has the basic form described above and the position of the joints are shown by white lines.

Figure 6: Ebony and brass octant, ca. 1850, Norie and Wilson.

The wood is heartwood ebony, a hard, dense, black and very strong African wood, resistant to rot. The index arm is of brass and the arc and vernier of ivory. Through tenon joints unite the radii to the limb and “blind” tenon joints are used elsewhere.

Figure 7: “Blind” mortice and tenon joint

Figure 8: Through-mortice

The joints of the curved transversal that unites the two radii are also blind. The part-radius between the transversal  and the limb probably adds little to its rigidity and served as a handhold. It may come as a surprise to some readers that this instrument by Norie and Wilson dates from after 1843, the year of Norie’s death. It has shown essentially no advance, even in detail, from instruments dated a hundred years earlier. We may note in passing the unsatisfactory index arm bearing, which was very prone to stick-slip.

Brass frames

As noted above, brass as a framing material was used only in pillar sextants. In the mid-19th century pillar sextant shown in Figure 9, the plates are only 1 mm thick, the pillars are 5 mm thick and they are united for the most part by screws with 10 mm diameter heads. A special screw driver was required for these screws, presumably to discourage “over-handy gentlemen” from dismantling the frame. The limb was about 3 mm thick and to unite it with the pillars that attach it to the rear frame, cheese head screws are passed through flanges on the pillars into the back of the limb, as screw heads on the front would interfere with the movement of the index arm.

Figure 9 : C19 Pillar frame sextant by Troughton and Simms

  Bronze frames

While maintaining the basic form with two radii uniting the centre to the limb, there was a great variety of ways that the space in between was filled in. A sextant is generally used with the frame in a vertical position and has to resist distortion mainly due to its own weight. However, resistance to bending in the plane of the frame is relatively easy to measure, so that I have used that as a surrogate for general resistance to distortion. I illustrate the set up for comparing resistance to flexion  in Figure 10. A 1 kg force is applied to the end of the index arm and the deflection measured by means of a dial indicator that reads to 0.01 mm.

Figure 10: Jig measuring flexion of Heath Curve Bar sextant frame

 A beam gets more resistant to bending as its depth increases and a substantial increase in bending resistance can be achieved with a relatively narrow vertical web. The frame of a mid- 19th century octant of about 200 mm radius shown in Figure 11 follows the pattern of its wooden predecessor with the addition of 10 mm deep ribs to radii and transversal of a mere 1 mm thickness.

Figure 11 : Mid-19th century octant

  This was not very resistant to bending when compared to some later 20th century instruments. Changing the shape slightly, as in the tulip frame octant of the same period shown in Figure 12 may have increased sales, but it did not increase stiffness.

Figure 12: Mid 19th century “tulip” framed octant

  In spite of the fairly heavy webs, it was no more resistant to bending than the sextant shown in Figure 13, which was sixty or seventy years its senior. Apart from the composite limb of bronze and brass, this sextant of 210 mm radius is, as it were, all webs of substantial 10 x 2 mm section, with the addition of screwed-on triangular bracing (the bare frame is shown in Figure 2).

Figure 13: Late 18th century sextant by Gilbert and Co

 Troughton and Simms, a noted maker of surveying equipment, obviously appreciated the benefits of triangulation in making a structure stiff, and the sextant of 200 mm radius shown in Figure 14 flexed about 40 percent less than the ladder framed sextant by Hughes and Son from the same period, shown in Figure 15. In both instruments, the webs are about 10 x 2 mm in section, but in the Troughton and Simms sextant, the “beams” are also 2 mm thick and average about 15 mm wide, so the contribution to resistance to flexing may not come simply from the pattern of the webs.

Figure 14: Early 20th century sextant by Troughton and Simms

 By the end of the nineteenth century, the method of lunar distances had well and truly fallen into disuse and by 1910, lunar distances were no longer included in the Nautical Almanac. The drive towards high accuracy was no longer necessary and the stage was set for the introduction of the micrometer sextant by C Plath in about 1907.  By the mid 1930’s, all major manufacturers  were making micrometer instruments, which were not necessarily as accurate as their vernier predecessors but which were much easier and quicker to read. A further consequence of the introduction of the micrometer was that the radius had to fit the pitch of the micrometer worm. English manufacturers settled on a pitch of 18 threads per inch with a pitch circle radius for the rack of 6.366 inches (161.70 mm), while most other manufacturers had a pitch of 1.4 mm and a pitch circle radius of 160.43 mm (6.316 inches). This reduction of radius from about 180 to 160 mm had a disproportionately beneficial effect on the frame stiffness. As flexion varies as the cube of the length of a beam, flexion reduced in a ratio of about 5.8 : 4.1.
The design of the frame had to be altered so that there was enough “meat” in which to cut the rack, and most makers placed the rack on the edge of the limb, with a slot for the keepers that hold the index arm in contact with the limb(Figure 15). The exception was Heath and Co, who placed the rack on the back of the limb (Figure 16), arguing somewhat spuriously that wear on the index arm bearing would thereby be reduced.

Figure 15 : Typical rack layout (Plath, Tamaya, Hughes etc)

Figure 16 : Heath and Co rack layout.

 A popular pattern for the frame, which seems to have given great rigidity, was the three circle, used briefly by C Plath on their Drekreis vernier sextant, by the two major English makers, Hughes and Son and Heath and Co, and by La Filotecnica, an Italian company, little known for making sextants (Figure 17). Plath, Tamaya and the major US maker of sextants prior to WW II, Brandis and Sons, settled on various patterns of ladder-like lattices for their bronze-framed instruments (Figure 18).

Fig 17 : Rear of three ring sextant by La Filotecnica of Milan

Fig 18 : Ladder frame clone of C Plath sextant by Tamaya

 Aluminium alloy frames

As noted above, by the mid 1930’s all the major makers had introduced aluminium alloy frame, made by the pressure die-casting method. In this process, molten metal is injected under high pressure at several points into a closed die or mould and the pressure maintained until the metal freezes. This produces dense, uniform, castings, free of hard spots or bubbles and having a surface finish as good as that of the interior of the die, but as importantly, the material used is about  twice as resistant to bending as bronze and about its equal in resisting corrosion.

Thanks to the generosity of Ken Gebhart, I am able to illustrate the manufacturing steps in the production of an alloy frame with bronze rack, for the fabulously costly US Navy Mark III sextant, made in Milwaukee under licence from C Plath.  Figure 19 shows the frame fresh from the die except that the risers for the escape of air and gases and the injection points have been “fettled” off. Also shown is the part-machined bronze rack. Flash, the thin slivers of metal where it has seeped between the two halves of the die is then cleaned off and both faces of the casting machined flat. The hole for the index arm bearing is then reamed out to finished size and this hole is used to locate the frame for further machining operations. For example, to turn the seat for the rack (Figure 20) the hole is located on a well-centred and close-fitting spigot on the turning machine.

Figure 19 : Rear of alloy frame casting and bronze arc

Figure 20 : Seat for rack

 The rack casting is then attached to the alloy frame with screws (Figure 21) and finish-turned to size before being transferred to a hobbing machine to have the teeth of the rack generated. Again, the central hole is used to locate the composite  frame on the machine to ensure that the teeth of the rack will be truly centred. Figure 22 shows the finished frame, ready to have the index arm bearing and other parts fitted.

Figure 21 : Frame and rack casting united

Figure 22 : Rear of finished MK III frame

Most makers of alloy framed instruments  copied the forms  of bronze predecessors, mainly ladder patterns as in the Mark III above, though during the Second World War C Plath settled on a standard triangulated pattern (Figure 23). This was copied in the Soviet SNO-M sextant and the Chinese  GLH 130-40  sextant, while Plath reverted to the ladder pattern when sextant production resumed in about 1950.

Figure 23 : Wartime C Plath sextant, triangulated frame

Freiberger Prazisionsmechanik, who as far as I know had never made sextants prior to WW II, applied their expertise in surveying instruments to produce  their Trommelsextant (Figure 24) and the radically different Skalen sextant (Figure 25). The Freiberger sextants , together with the  Soviet SNO-T, closely similar to the Trommelsextant, had exceptionally rigid frames, with material concentrated around the edges, a stiffening central web and the handle attached directly to the edges of the frame via a sub-frame.

Figure 24 : Freiberger Trommelsextant

Figure 25 : Freiberger Skalensextant

The WW II Plath instruments and the post-war SNO-M, SNO-T and Freiberger instruments all had the rack cut directly into the frame with a bronze worm. This combination seems to have stood the test of time, as a well-used 1982 SNO-T sextant I once calibrated had no non-adjustable error of greater than 11 seconds. Nevertheless, many makers, including Plath and Tamaya, continued to fit their sextants with bronze racks, presumably because their customers preferred them that way, despite the extra expense involved in making them. A demand continued for wholly bronze frames. Despite their theoretically inferior properties, they perform just as well.

One professional mariner of my acquaintance, a retired deep-sea trawler captain, says that he preferred a bronze framed instrument because of its greater weight, which he felt gave it extra stability when taking sights in rough weather. A bronze rack adds about 150 G to the weight of the frame, while a typical bronze frame, at 600 G is about 250 G (abut half a pound) heavier than a typical alloy frame.





C19 sextant restoration

10 11 2009

Recently I returned home after a trip around the world during which I visited relatives in Britain, France and Texas. Shortly before setting out, I had secured a BU Ships Mark II sextant by Atlas Engineering of Chicago and while in Texas, I bought two more, a US Navy octant by Brandis and Sons, and a sextant from the nineteenth century. I am now busy restoring the instruments to good order, starting with the oldest.

 It has a bronze tulip pattern frame and, although it has no name, Spencer, Browning and Co (formerly Spencer Browning and Rust) made very similar or identical sextants in about 1840 to 50. As in the twentieth century, many of the component parts were standard and appear on sextants by various makers. The real heart of the sextant is its frame and divided scale and we know that these too were made by only a few makers, perhaps no more than ten in the whole of nineteenth century Britain. Thus, it is certain that many of the dozens of instrument makers whose names appears on sextants were in fact assemblers and finishers of parts made by others. We know that some makers actually did make most of the instrument and also that they were prepared to sell finished instruments for others to add their names.

As with so many old sextants, a previous owner had thought that polished bronze and brass looked better than whatever the maker had clothed it in, often black lacquer, but sometimes the bronze was chemically browned or blackened. In stripping it, it had probably been dipped in a bath of solvent. This treatment did not agree with the ivory main and vernier scales which had shrunk so that the main scale was loose and the vernier scale had cracked around the rivets which attached it to the index arm. Both had taken on a green tint.The ebony pear-shaped handle is intact apart from a fine crack, but at some stage the index arm clamp had been lost and replaced by a makeshift one fabricated from a 5/32 inch Whitworth screw and a disc of bronze. The scale magnifier had been similarly bodged together. The top part of the horizon mirror mounting was absent. The instrument was without a case.

I made a start with the angle base for the horizon mirror mount, which simply involved cutting a piece of heavy brass angle, filing it to size and shape, drilling holes in the right places and finishing the front of the angle to leave three tiny platforms opposite the tabs of the clip, yet to be made.

Copy of 100_3153

For the clip I first marked out what the finished object would look like unfolded and then cut it out of thin brass, using a jeweller’s piercing saw. The next picture shows the cut out piece in the rough state, before filing to size, bending into shape and soldering. A hole had first to be drilled for the threaded bush for the fixing screw, as it is easier to drill a small hole in thin brass when it is flat. Once the clip was bent into shape, the bush was rivetted on the inside.

100_3135

The magnifier called for some more work with the piercing saw, harder work this time, as the brass was thicker. To stay with the spirit of things, I used the front plate of a scrapped table clock. The next picture shows the cutting completed. As you can see, it is not the first time this plate has provided metal for a replacement part.

100_3147

As with any sawing, the closer you can keep to the line the less work there is to do to finish the part, but you also need to remember that putting-on tools are in short supply! With a little practice (quite a lot, really), it becomes easy to file to the lines. The secret with brass is to keep a set of files  that are not used on anything else. Use them on steel and they tend afterwards to skid uselessly over brass unless you use a lot a pressure, and then they tend to go where you don’t want them to go.

Once I had the outside filed to shape, I could put the part in the lathe to drill and bore the large hole to a size that fitted the outside diameter of a piece of thin walled tube from a scrapped Victorian something-or-other. If you haven’t got a lathe, this could at a pinch be done with piercing saw and files.

100_3148

I then glued the tubing in place. It would probably have been soft soldered in place in the nineteenth century, but I am not above using modern aids to fabrication. The lens was a scrapped field lens from an old microscope eyepiece. I had to make a piece of tubing for it and cut the 40 threads per inch internal thread using the lathe. The post also needed some attention, as the screw had been replaced by a soldered-in stud with a nut. I had to make a new washer, filing the square hole with a needle file. The washer fits over a squared section of the post and its purpose is to prevent rotational forces being transmitted to the screw and loosening it. The next picture shows the finished article with alongside it the monstrosity that it replaced.

100_3155

The new clamp screw was a fairly straightforward bit of turning and knurling, except for the thread which had to be 5/32 inch Whitworth. I went metric over twenty years ago and my odds and ends of Imperial screwing tackle do not include 5/32 x 32 tpi, so I had to screwcut it in the lathe. The clamp itself also needed attention, as it did not have a spring (and was the wrong size and shape anyway). I filed it to a somewhat better shape and made a new spring by hammering a sliver of sheet brass until it work-hardened and became springy, a trick that would have been well-known to C18 and C19 clockmakers. You can just see in the next picture traces of the solder that hold the spring to the clamp .

Copy of 100_3154

I was able successfully to glue one of the splits in the ivory of the vernier scale, but first I had to remove it. Traditionally, they seem to have always been rivetted into place, not a good practice, as the rivet inevitably expands a little in the hole and ivory tends to shrink as it dries out. Add a little corrosion and the stage is set for splits to develop. The rivets had rounded heads, so I made a little jig out of a stub of steel bar with a hole one end that just fitted over the head of the rivet with a through hole for a drill of the same (carefully measured) size as the shank of the rivet. Using the jig with a block of wood sawn to an angle of 20 degrees to support the index arm, I could drill through, confident that the drill would go through the centre of the rivet and cut off its head, without wandering. I elected to tap the resulting holes in the index arm 10 BA and to use screws to re-attach the repaired scale, partly because I have no tiny rivets, but mainly to avoid the very problem I was trying to cure. The next photo shows the finished result. You can see that the repair has been quite successful at the zero end.

Copy of 100_3160

I finished by stripping down the instrument to the last tiny screw, cleaning everything with an old toothbrush in a 50% solution of ammonia and washing up liquid, polishing the screw heads,  and painting the individual parts.

In the past, I have not been entirely happy with the appearance given by modern paints on antique instruments. Modern spray paints give a result that is almost too good and the paint film seems to be too thick. I mentioned this to an engineer friend when I called into his workshop to pick his brains about drilling out the rivets and he recommended a spray-on protective lacquer called CRC Black Zinc (also available in a variety of other colours). It sticks to bare metal without a primer, once cured it is tough and resists scratching and, best of all, gives an effect that pleases me. Take a look at the final appearance of the sextant and, as the TV shows say, you be the judge.

Copy of 100_3159

Once I have finished restoring the other sextants, I plan to try my hand at making  a wedge-shaped case out of my precious stock of African mahogany. If I make a mess of the bowed front, I will still have two sides to use in a square box.





Update on Byrd Aircraft Sextant

11 08 2009

This post is preceded by “Bubble illumination of Mk V and AN 5851 bubble sextants” ,  “Refilling Mark V/AN5851 bubble  chambers” ,  “Overhaul of MkV/An5851 bubble chamber” ,  “AN5851-1 : jammed shades carrousel” and “A Byrd sextant restored”

Since writing the previous post about the Byrd sextant (or should it be the de Florez sextant?) I became disatistfied with the restoration and, for those interested in air navigation intruments I decided to add to the photographs. You will need to read the previous post first.

As noone followed my hint to donate me a set of index shades, I have had to make them myself. For the outline, it was simple enough to scribe around a genuine shade on to a sheet of 2.5 mm brass four times, drill and ream the mounting holes and saw out the blanks using a piercing saw. This tool is a little like a small fret saw and takes very fine blades having as many as 40 teeth per inch. It cuts on the down stroke and is used with the blade held vertically. Here is a picture of me cutting out a clock wheel blank. Note that it helps to have lots of light and vision…

Copy of 100_0066

I could then bolt the four blanks together with a close-fitting screw through the mounting holes and files the outlines to shape. It is actually easier and quicker to do the straight bits using a vertical milling machine – if it is already set up – and just to file the rounded corners by hand. The block of four could then be mounted and centred in the four jaw chuck of a lathe and all four drilled through and bored to 22 mm diameter. The outermost blank was then counterbored to 24 mm to a depth of 2 mm and removed, then the next counterbored and so on for all four, taking care to loosen and tighten the same pair of chuck jaws each time. This is not good practice, but it saves time when accurate centring is not vital.

It is hard to discover sources of neutral density glass, so I made a trephine out of mild steel and cut out discs of plastic Cokin filter material. This is used in photography and seems to be flat and parallel enough for a sextant likely to have observational errors of the order of minutes. I made the fit so the discs just popped into the brass frames and saved myself the trouble of having to swage them into place, though they might have looked more “genuine” if I had. Making the shouldered screw that holds them all in place was staightforward turning and I made the Belleville spring washer by making a thin brass washer, sitting it on the end grain of a block of hard wood and hitting it hard with a ball bearing. The finished set of shades shows well in the next photograph, as does the semi-circular lens that allows the level to be seen in focus.

Copy of 100_2948

I wasn’t happy with the fiducial line in my original restoration. It was simply a piece of fine thread wrapped around the level vial and secured in place with clear varnish. Since it is wrapped around a curved surface, it can be viewed only from one angle and still be seen as a straight line. In any case, it was rather too thick, so I scribed a thin line on some perspex and then cut and filed and drilled a tiny piece to size, securing it to the vial carrier with two 12 BA screws. These are only 1.3 mm in diameter and I was greatly relieved to have tapped the two blind holes without breaking my only tap of this size. The next photograph shows this small but important part. The flash has made it look dustier than it was in reality. I have no idea what was used in the original models. The Smithsonian Museum has two examples, but both are incomplete and I have only web photographs to look at.

100_2958

Making the case from African mahogany needed only normally careful woodworking. Dovetail joints for the corners had by the 1920s given way to comb (finger) joints, but as some later American aircraft  cases used corner rebates, which are much easier to make without special machinery, this is what I used, with brass pins across the joints to prevent disaster if the very strong glue should fail:

Corner rebate

I copied the hook latches from a Hughes and Son sextant case and the handle is a very close copy of the handle used for a Brandis Aeronautical Sextant Mark 1 Mod 4 of 1931. I am not good at sheet metal work, so will gloss over the battery box, with its belt loop. The pick for the two capstan headed screws was simple to make and the mirror-adjusting wrench required only the ability to convert a small round hole to a small square hole using a file. It remained  to dismantle the instrument to its component parts and spray-paint them using a satin finish paint that, while not perfectly imitating the original finish, at least has the merit of pleasing its owner.

The final photograph shows the sextant in its case with its furniture and fittings. It is certainly not an easy sextant to use on land, but since these latest retoration efforts, there has been little clear sky around for me to make a serious assessment. If there is some particular aspect of this instrument that you would like discuss or to see illustrated, do contact me.

GA





A Byrd Sextant Restored

30 05 2009

This post is preceded by “Bubble illumination of Mk V and AN 5851 bubble sextants” ,  “Refilling Mark V/AN5851 bubble  chambers” ,  “Overhaul of MkV/An5851 bubble chamber” and “AN5851-1 : jammed shades carrousel”

I recently acquired  a Brandis nautical vernier sextant without case, telescope, or any shades. It appeared to have an extra mirror in front of the horizon mirror and I recognised it as an early bubble sextant of the type used by the then Commander Richard Byrd on his claimed flight to the North Pole in 1926. There are several magazine photographs extant that show Byrd in posed pictures, using a similar sextant, this one, for example:

Almirante_Richard_E__Byrd

K Hilding Beij, writing in the Bureau of Standards Report 198, Astronomical Methods in Aerial Navigation in about 1926, refers to the sextant as a “Byrd sextant”, though Luis de Florez, a prolific  inventor, claimed priority. He had filed for a patent for exactly this type of bubble sextant in March 1919 and he was granted US Patent number 1,536,286 in May 1925. My sextant looked like this when I received it:

100_2772

The one Byrd is using is a full-size Brandis vernier quintant with an arc of 180 mm radius reading to 30 seconds, whereas my example is unusually small for a vernier quintant, having an arc radius of only 140 mm, also reading to 30 seconds, so my hopes of possessing an historic instrument were disappointed (but see postscript). Nevertheless, it is a rare and early instrument dating from about 1920 and I felt it was worthwhile  to restore it to working order. As I have no access to original instruments, I did not set out to make exact copies of the attachments that make a nautical quintant into an aeronautical one, but I did follow the same principles, while retaining all the original parts. Needless to say, I dismantled it completely to begin with, and cleaned all the individual parts. A photograph of the restored instrument will perhaps best help to explain its workings.

100_2796

The optical path for the heavenly body is as usual, via the index mirror and silvered half of the horizon mirror. An ordinary spirit level vial is held in a carrier and viewed via an auxiliary mirror set at 45 degrees above it, through the plain half of the horizon mirror. The image of the bubble would be out of focus viewed directly through the x 2 Galilean telescope and so an extra, semicircular, lens is interposed in the light path to bring it into sharp focus. The auxiliary mirror may be swung downwards to allow direct view of the natural horizon by pressing a spring-loaded catch.

The sensitivity of the vial has to be carefully chosen. If too sensitive, it is never at rest when the instrument is held in the hand and if not sensitive enough it is not possible to get meaningful results. I settled for one where the bubble moves 2 mm for 6 minutes change in level. This is of the same order of sensitivity as most other bubble sextants.  The bubble is illuminated from one end and, to try to get even illumination, the vial is painted white over most of its surface, including the end distant from the lamp. This has parallels in the lighting of some circular bubble cells, where an attempt is often made to conduct the light around the periphery of the cell with some sort of light guide. The next photograph shows a view of the lighted vial through the telescope (the view is somewhat more extensive than this in reality).

100_2792

In sextants with circular bubble cells, one usually aims either to centre the body in the bubble or to align it with its equator, but this cannot be done with a linear cell, so a more-or-less central datum line is used and the sextant adjusted so that when the bubble is centred, the datum line lies on the horizontal. As is usual for a Galilean telescope, each half of the objective lens “sees”  its own half of the field of view (compare this with the inverting or astronomical telescope, where obscuring half the objective simply cuts out half the total light). Thus, the auxiliary, semicircular lens in the telescope attachment sees the left half-field and  is used to bring the bubble into focus by sliding back or forth.

The heavenly body is seen on the other half-field and, as is usual, there is a narrow band of overlap where both the sky and the horizon or bubble may be seen together. I have not made any significant trials as yet, but it is relatively easy on dry land to align a star, the moon or the sun with the datum line while trying to keep the bubble centred. However, the instrument gives no indication of lateral tilt and in an aircraft the results must have been very uncertain.

The scale lighting is particularly good. A shaded lamp shines via a standard diffusing screen on to the scales, which are viewed through a simple magnifier. Unlike many such systems it gives a very even illumination so that the main and vernier scales are seen with equal contrast, making reading relatively easy and rapid.

100_2802

The handle seems to be a modified early Brandis battery handle. Power is now from an external source. Scale lighting is via the original press switch, while the bubble unit is switched by a rather crude rotary switch on the front of the handle, seen in the general arrangement photo above. The following photo shows the wiring layout, pretty well as found except for the decayed silk covered wire.

Byrd handle

I have coated the parts that I have made – telescope and attachment, and vial carrier – with modern paint. The original paintwork on the rest of the instrument is careworn and I cannot help but feel that it would look better for a fresh coat of paint. I would repaint a more modern instrument in the same condition and wonder how readers might feel about that. Is it sufficiently “historic” to preserve it as found? Would repainting it devalue it in some way.”? After all, noone is likely to mistake it for a modern fake, repainted or not.

The index shades, by the way, I borrowed from another Brandis sextant, so that if anyone has a spare set of Brandis or US Navy Mark II index shades that I can beg or buy, I should be glad to hear from them.

Post script, 11 September 2015.

When I wrote this post, I thought Byrd was holding a larger sextant than mine, but an enquiry led me to look more closely at the structure of the sextant he is shown holding and I now believe that he is holding the same type of sextant that I describe, viz.a survey sextant of 140 mm radius.