A turn-of-the-century French Sextant

11 08 2015

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

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

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

Figure 1 : Label in lid of case.

Figure 1 : Label in lid of case.

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

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

Figure 2: Sextant as received.

Figure 2: Sextant as received.

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

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

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

Figure 3: Front of index mirror clamp.

Figure 3: Front of index mirror clamp.

Figure 4: Index mirror in place.

Figure 4: Index mirror in place.

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

Figure:

Figure 5: Index mirror bracket with screw in place.

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

Figure 7: Horizon mirror bracket and clamp.

Figure 6: Horizon mirror bracket and clamp.

Figure 8: Horizon mirror locking screws.

Figure 7: Horizon mirror locking screws.

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

Figure 9: General view of horizon mirror adjustments.

Figure 8: General view of horizon mirror adjustments.

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

Figure 9:mounting of telescope rising piece.

Figure 9: Mounting of telescope rising piece.

Figure 10 shows the mounting exploded.

Figure 10: Exploded view of telescope mounting.

Figure 10: Exploded view of telescope mounting.

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

Figure 11: General view of tangent screw mechanism.

Figure 11: General view of tangent screw mechanism.

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

Figure 12: Tangent screw bearing.

Figure 12: Tangent screw bearing.

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

Figure 13: Underside of sliding block

Figure 13: Underside of sliding block and clamp.

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

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

Figure 14: Sighting devices.

Figure 14: Sighting devices.

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

Figure 14: Case exterior.

Figure 15: Case exterior.

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

Figure 17: Restored instrument in its case.

Figure 16: Restored instrument and case.

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





A Half-size Sextant by Lefebvre-Poulin

29 06 2014

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

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

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

 

Figure 1: General arrangement, front view.

Figure 1: General arrangement, front view.

Figure 2: General arrangement, rear view.

Figure 2: General arrangement, rear view.

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

Figure 3: Main parts of the sextant.

Figure 3: Main parts of the sextant.

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

Figure 4: To show handle.

Figure 4: To show handle.

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

Figure 5: Index arm bearing, and handle attachment.

Figure 5: Index arm bearing, and handle attachment.

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

Figure 6: Exploded index arm bearing.

Figure 6: Exploded index arm bearing.

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

Figure 7: To show the rack.

Figure 7: To show the rack.

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

Figure 8: Micrometer mechanism.

Figure 8: Micrometer mechanism.

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

Figure 9: Micrometer drum divisions.

Figure 9: Micrometer drum divisions.

Figure 10: Main scale close-up view.

Figure 10: Main scale close-up view.

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

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

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

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

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

 

 

 





A Fine Sextant from Spencer, Browning and Co.

23 01 2014

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

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

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

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

Figure 1: Restored sextant and case.

Figure 1: Restored sextant and case.

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

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

Figure 2: Face of limb and arc.

Figure 2: Face of limb and arc.

Figure 3: End view of limb and arc.

Figure 3: End view of limb and arc.

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

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

Figure 4: Close-up view of graduations.

Figure 4: Close-up view of graduations.

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

Figure 5: Index mirror bracket.

Figure 5: Index mirror bracket.

Figure 6: Index mirror clip.

Figure 6: Index mirror clip.

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

Figure 7: Index mirror and shades.

Figure 7: Index mirror and shades.

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

Figure 8: Scale magnifier.

Figure 8: Scale magnifier.

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

Figure 9: Bent index arm.

Figure 9: Bent index arm.

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

Figure 10: Bent index arm corrected.

Figure 10: Bent index arm corrected.

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

Figure 11: Unrestored case.

Figure 11: Un-restored case.

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

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

Figure 12: Interior of case with telescopes.

Figure 12: Interior of case with telescopes.

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

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





An Early C19 Ebony Quadrant Restored

11 03 2012

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

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

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

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

Figure 2 : Leadenhall Street in mid C19

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

Figure 3 : Loose parts

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

Figure 4 : Frame and index arm.

Figure 5 : Detail of arc.

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

Figure 6 : Trying clamp for size.

Figure 7 : Clamp screw being checked.

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

Figure 8 : Tangent screw mechanism.

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

Figure 9 : Taper-turning a leg.

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

Figure 10 : Front of index mirror clip.

Figure 11 : Rear of index mirror clip.

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

Figure 12 : Perpendicularity adjustment of index mirror.

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

Figure 13 : Horizon mirror clip.

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

Figure 14 ; Horizon mirror adjustment 1.

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

Figure 15 : Horizon mirror adjustment 2.

Figure 16 : Horizon mirror adjustment 3.

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

Figure 17 : Screw-cutting telescope ring.

Figure 18 : Completed telescope mounting.

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

Figure 19 : Cheaters in place and coarse file.

Figure 20 : Fine filing close to completion.

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

Figure 21: Mounts ready for drilling and boring.

Figure 22 : Shade mounts ready to receive glass.

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

Figure 23 : Base for pillar and cheeks.

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

Figure 24 : Shades aseembled in mounting.

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

Figure 25 : Index arm bearing and journal.

Figure 26 : Fitting of index arm bearing.

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

Figure 27 : Front (left-hand) view.

Figure 28 : Rear (right hand) view.

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





Heath and Company’s best vernier sextant

10 12 2011

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

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

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

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

Figure 1 : Inspection certificate.

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

Figure 2 : Exterior of case as found.

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

Figure 3 : Safety hook latches.

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

Figure 4 : Release knob.

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

Figure 5 : Interior of case as found.

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

Figure 6 : Telescope kit.

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

Figure 7 : Telescope mounting.

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

Figure 8 : Index arm bearing.

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

Figure 9 : Horizon mirror mounting.

 

Figure 10 : Horizon mirror clips.

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

Figure 11 : Horizon shades

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

Figure 12 : Scale magnifier.

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

Figure 13 : Release catch mechanism.

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

Figure 14 : Restoration completed.





A Fine C. Plath Vernier Sextant

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

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

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

Figure 1 : Exterior of case

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

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

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

Figure 4 : Mirror adjusting device

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

Figure 5 : Rising piece.

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

Figure 6 : Rising piece exploded.

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

Figure 8 : Rack and worm.

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




A Half-size Sextant by Hughes and Son

29 09 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 and “An Hungarian Sextant via Bulgaria.”

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Hughes and Son made sextants and other navigational instruments from the middle of  the nineteenth century until 1947, when they merged to become Kelvin and Hughes. Prior to and during the Second World War they made a wide variety of aircraft instruments, among which was a small sextant intended for use in seaplanes such as the Sunderland flying boat, perhaps not so much for celestial navigation as for taking anchor bearings and amplitudes for checking the magnetic compass. Quite why an ordinary nautical sextant was not issued is unclear, as the small sextant in its case weighs only 600 G less than a full-size Hughes sextant of the same period that weighs in at 3.9 kg. There was scant advantage in volume either : the smaller instrument’s case is 200 x 200x 140 mm against the full-size case of 275 x 260 x 145 mm. The sextants were made under an Air Ministry contract and, like the Mark IX series of aircraft bubble sextants, were issued in a case made of a heavy dark brown plastic material similar to paxolin. It is probable that in a period when imported timber was at a premium and skilled woodworkers were engaged on aircraft production, the plastic cases, made of 5 mm sheets pinned together with brass nails, were seen as a satisfactory solution. The examples in Figure 1 show a full-sized Hughes and Son nautical sextant and its little brother  along side it. The smaller one was made in 1943 and eventually made its way to Australia, where it was sold by T.M.Burroughs of Flinders Street, Melbourne  to the Third Officer  of a ship (whose name I cannot decipher) in May 1948 for the sum of twenty pounds. This was about the going rate for a full-sized sextant: one in my possession was sold new  in 1945 for eighteen pounds, with an extra four pounds for a large aperture telescope. There are very many so-called reproduction or “replica sextants” of similar size on the market, but this is a fully functional and accurate instrument able to perform at nearly the same level as a full-sized instrument.

Figure 1 : Two Hughes and Son sextants

Figure 2 shows the instrument in its case. The handle is the same as for the larger instrument, on the side of the case, and the latches are very similar to those used for Hughes cases of the 1930’s. The sextant’s legs sit in mahogany pockets and it is further restrained by two pads in the lids, all typical of Hughes’s full-size practice.  There is a Husun (Hughes and Son) calibration certificate in the lid and pockets for the oil botttle, adjusting pick and a key for the box lock. I have seen an example from 1942 in a mahogany case with otherwise identical furniture and layout.

Figure 2 : Interior of case.

The front view of the sextants, seen in more detail in Figure 3, reveals that the shades, mirrors and micrometer mechanism are all  full sized, while the x2 fixed focus Galilean telescope has an aperture of 20 mm against that of 30 mm for the larger sextant.

Figure 3 : Front (left hand) face of sextant.

The telescope has no rise and fall mechanism and is attached to the frame by a single screw that passes through a boss in the base of a shaped column. A dowel pin locates it so that it points in the correct direction. This pin can be seen above the boss in the close up photograph of the telescope column in Figure 4.

Figure 4 : Location of telescope in frame.

The rear (right hand) view is shown in Figure 5.

Figure 5 : Rear (right hand) view of sextant.

  The handle, like the telescope,  is mounted on a single column, but instead of being restrained from rotation by dowels, the column has squares on each end that fit into sockets in the handle and sextant frame, being held there by single large screws (Figure 6). Notice too that the index arm bearing is concealed by a stout brass cover that screws over it and doubles as a third leg.

Figure 6 : Method of locating handle.

In the view of the micrometer mechanism (Figure 7, below), note the fine pitch of the worm which allows a full-size drum to be used. The worm shaft has a tapered thrust bearing and a parallel portion, which run  in a single block of bearing, with preload applied by a U-shaped spring. The worm shaft is in two parts, to allow assembly. The release catch operates a cam which swings the assembly out of mesh with the rack.

Figure 7 : Micrometer mechanism.

 The mounting of the shades, particularly the index shades, is a little unusual (Figure 8). Normally, the shades are mounted on a shaft that is prevented from rotating, and the shades are separated by washers that are also prevented from rotating, so that when one shade is rotated into position rotation is not transmitted to adjacent shades, and they do not follow. This is very convenient when taking sights, as it is easier to find the sun with a relatively light shade in position, when a darker shade can then be swung into place without one having to take one’s eye off the quarry.  In this little sextant, there are no washers. Instead, slots have been milled in the mounting for each shade. These can be closed up by means of  nut on the end of the mounting pin or shaft, and the latter is prevented from rotating by a crossed taper pin through its head. In the 1942 sextant mentioned above, the shades mounting follows Hughes’ standard practice.

Figure 8 : Index shades mounting

The horizon shades are mounted on a single shoulder screw and are separated by red fibre washers that fit tightly on the screw. A Belleville washer, which behaves as a short, stiff spring, provides friction and the screw is locked by a shallow nut (Figure 9).

 

Figure 9 : Horizon shades mounting.

Figure 10 shows the bare frame of the instrument.

Figure 10 : Bare frame.

A kind French correspondent, whose name I have unfortunately lost, sent me a photograph of a similar sextant in his possession (Figure 11). It is identical except that it is named Heath and Co. and carries a telescope in the style of that company. It may be that the Ministry of Aircraft Production or the Air Ministry during the exigencies of World War II imposed some degree of cooperation between the two makers, or perhaps Heath and Co acquired some instruments as war surplus and added their own name and telescope.

Figure 11 : Heath and Co. seaplane sextant.