The Nautical Sextant

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.

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.

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 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 : 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.

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.”

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.

Hughes and Son made sextants and other navigational instruments from the middle of  the eighteenth 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, 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.

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 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.

Details of the micrometer mechanism, normally protected by a sheet metal cover, are shown in Figure 7.  The worm is kept in engagement with the rack by means of a leaf spring and is swung out of engagement by means of the release catch that bears a cam on its end. The cam presses on an extension of the chassis on which the worm is mounted and causes it to swing away from the rack in the plane of the rack. The worm bearing is tapered at the front and axial preload is applied by means of an inverted U-shaped spring that embraces the front bearing. The drum is divided to single minutes and there is no micrometer vernier. More details of this and other micrometer mechanisms will be found in my book The Nautical Sextant.

 

 

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.

 A much cheaper solution has been adopted for the horizon shades : the separating washers are of red fibre and fit tightly on the shaft, while the shades are an easy fit, so that to some extent at least, rotation is prevented from being transmitted from one shade to the next (Figure 9). The shaft is simply a shouldered screw that fits into a bracket and is prevented from rotation by a locking nut. A Belleville washer (a thin cupped washer with the characteristics of a short, stiff spring)provides some friction.

 

Figure 8 : Index shades mounting

Figure 9 : Horizon shades mounting.

 

This leaves only the bare frame for comment (Figure 10). It is a strongly ribbed bronze casting with brass index arm bearing attached by three screws from behind. A white metal arc is let into the limb of the frame and divided to degrees with numerals every ten degrees. Except for the limb, it is painted in thick, wrinkle-finish, black paint. The radius of the rack is about 91 mm or  3.6 inches.

 

 

Figure 10 : Bare frame.

 

Jean-Luc Fontaine was kind enought to send me pictures of a very similar sextant that he owns, shown in Figure 11 below.  Except for the telescope and handle, it is identical to the Hughes and Son instrument, but it is named Heath and Co.  It may well be that during the WWII some degree of cooperation was forced upon Hughes and Heath by the Ministry of Aircraft Production. Rather less likely is that Heath acquired war surplus sextants after the war and fitted them with their own telescope, which has a slightly larger aperture (30 mm) than the original.

 

Figure 11 : Heath and Co. seaplane sextant.

If you have enjoyed reading my account of this rare and interesting little sextant you will probably enjoy reading my book, The Nautical Sextant.

 

Description

As a guess, I would say that the sextant was made in the years following WWII, as the shades and micrometer mechanism are identical to those of C Plath sextants of the time, many parts of which seem to have reached Britain and the USSR as plunder and reparations. The frame, however, is of a most unusual form, cast in aluminium alloy with the rack cut directly into the frame (Figure 2). The lower case Greek letter gamma (γ) forms part of the frame and there is of course a star (though not a navigational one), gamma sextans to complete the allusion in the name. The frame is covered in a thick and tough coat of black gloss paint, so it is not possible to judge whether the frame was die cast. Very few must have been made , so that it would be hard to justify the cost of the die. More likely, it was sand cast as the front of the frame (except for the arc, which has been machined) is not flat.

Figure 2 : Sextant as received

Cutting the rack directly into the frame is not a problem in itself, but I was taken aback to see that the worm was made of steel rather than the more usual brass used on bronze racks or hard bronze used on aluminium racks. The threads bore a light patina of rust (Figure 3). Almost as undesirable, the brackets for the shades are cast as one with the frame (Figure 4), so that if part should get broken, and shades are very vulnerable to damage, it is practically impossible to make good the damage. The mounting of the shades in the brackets is standard for Plath sextants of the time, on a cylindrical pin prevented from rotating by a crossed taper pin and adjusted by closing up the cheeks of the bracket with a screw let into the end of the pin   A refinement is missing : that of a key way in the pin and keys in the washers that separate the shades, so that rotation of one shade is not transmitted to an adjacent one.

Figure 3 : Steel worm.

Figure 4 : Shades mountings.

The mirror brackets show signs of internal machining so that they are either die castings or  have been cast separately and machined afterwards. Of interest to lovers of detail are the clips for holding the mirrors in place and the method of locking the adjusting screws . The clips (Figure 5), which bear conical points, are fastened to bosses on the front of the brackets, whereas practically all other makers secure them to the edges, where there is little metal and a high risk of stripping threads. The adjusting screws (Figure 6) bear directly opposite the points, passing through a threaded hole in a separate brass piece and then through a clearance hole in the back of the mirror bracket. The brass piece is secured to the back of the mirror bracket and is split so that a pinch screw can close it up and lock the adjusting screw. This is a very practical and effective arrangement though of course it adds to the cost of the instrument.

Figure 5 : Mirror clips.

Figure 6 : Mirror adjusting acrews.

The index arm and its bearing are conventional. The taper of the bearing is typical of C Plath practice, rather steeper than in English sextants. and, again as in C Plath sextants, the index arm expansion that bears the micrometer mechanism is a separate piece, attached to the arm by four screws. Like war time Plaths and early US BuShips Mark II sextants, there is no micrometer vernier and the drum is divided to half minutes. The Galilean (star) telescope is 3 x 40 mm aperture and has  binocular-type eyepiece focussing.

Restoration

Apart from making two new pieces for the floor of the case, and reassembling it, there was relatively little for me to do in the way of restoration once I had taken the instrument apart and cleaned all its parts. I could not, however, leave a rusty steel worm in place and so I made and fitted a new one of brass (Figure7) .  I have given  an account of making a worm in a separate post (6 July, 2009, A worm turns). The micrometer drum had weathered to a dark nicotine brown, but careful cleaning and rubbing with 1000 grit emery paper converted it to a much more legible light orange colour. The telescope rising piece was also, surprisingly, made of steel, but I felt that it could be left, as it is a non-critical component. A thorough cleaning of the lenses of the ‘scope brought about a pleasing increase in clarity of view.

Figure 7 : Two worms and their shafts.

Calibration

The instrument quality is generally rather good, so I was disappointed to find that it is the most inaccurate sextant that I have calibrated so far, though, paradoxically, it is quite precise. The reading of the sextant give values that are between 14 and 104 arcseconds (0.2 to 1.7 minutes) in error, but when the errors are plotted on a graph, the graph is very close to a straight line, so that the errors can be allowed for to give  readings that are very close to the correct ones. I will be giving more details of this and its probable cause in a separate post in the Chasing tenths of an arcminute category, but Figure 8 shows the table and graph of errors with a line of best fit plotted on the graph. Unfortunately, the errors are non-correctable, but an accurate estimate of the reading may be had by applying a correction from the graph.

Figure 8 : Calibration table and graph of errors.

The final photograph, Figure 9,  shows the completed sextant in its repaired and re-varnished case.

Figure 9 : Completed restoration.

Previous posts in this category cover:  “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” and “A fine sextant by Filotecnica Salmoiraghi.”

The Admiralty ordered sextants from Henry Hughes and Son and bought them provided that they met Admiralty specifications as determined by the National Physical Laboratory at Teddington. Pre-World War II they were given on Admiralty Form 575, but I have not been able to find a copy to discover exactly what the specifications were. Presumably they were exacting and conservative.

Figure 1 : Inspection certificate of sextant number 222**

In the late 1930′s  a batch of vernier sextants was ordered from Hughes and Son and in the early 1950′s another batch, was ordered, this time of micrometer sextants. Except for the rack and micrometer mechanism, they had many things in common: the form of the frame, the mirrors and their mountings , the shades and their mountings and their telescopes. The frame is the classical A type frame, of bronze, heavily ribbed and  the shades are conventionally mounted, but the mirrors are circular and sealed and the telescopes have coarse interupted threads that allow them to be rapidly mounted and dismounted. The sextant can be returned to its case with the index arm set in any position except at the extremes, said to be an  important practical point if a question about a reading occurs when calculating a position line (though most people would try to average several sights of the same body, when gross errors would be noticed).

A few weeks ago, I came upon an Admiralty pattern vernier sextant in good condition and enjoyed restoring it to a near-new condition, at the same time observing points of difference with other instrument types.  Figure 1 shows a general view of the instrument, with the star telescope fitted. In the background is the inverting telescope with an extra, high-power eyepiece ,  two eyepiece shades and an adjusting pick. Note the unusual mirror mountings and magnifier mounting.

Figure 1 : Admiralty Pattern sextant and accessories

Figure 3 : Galilean star telescope

Figure 4 : Inverting telescope

Figure 5 : Checking squareness of face of telescope ring.

Figure 6 : Rising piece mechanism.

 

Figure 7 : Attachment of rising piece mechanism to frame.

Figure 4 shows the external interupted thread on a telescope and the internal thread can be seen in the interior of the very substantial telescope ring in Figure 5 .  Alternate one-sixths of the thread are machined away, so that the telescope can be inserted with the white dots lined up and secured in place by rotating through less than one sixth of a turn. The Admiralty  may have decided on  this device by analogy with  the interupted threads of the breech closing mechanism of naval artillery, but Hughes also used it in their higher class sextants made for civilian use.

 

Index mirror mounting

The index mirror is sealed inside its bracket and is not adjustable. Like the telescope mounting, it was manufactured correctly in the first place and, short of breakage, would remain square to the plane of the arc throughout its life. The rear of the bracket bears the statement ” Pat. No 472814, which was accepted in 1935. Figure 8, extracted from the patent document, shows how the silvered rear of the mirror was protected from the effects of salt water by holding it against a “resilient washer” by means of a threaded ring with a lip that bears on the periphery of the mirror.

 

Figure 8 : Extract from patent on sealing mirrors

 

Horizon mirror mounting

 This is altogether more complex, since light has to pass through it. Figure 9 shows the mounting exploded. In the left half of the figure the mirror can be seen. A disc of optical glass is cemented to the back of the mirror to protect the silvering and the edges of the mirror and glass where they join is bevelled, so that there is a thin wedge of cement to prevent infiltration of water. There are four dots of silver on the rear of the mirror (Figure 10), presumably to ensure that the cement layer was of uniform thickness , to prevent any prismatic effect. The sandwich is contained in a cell and retained there by a threaded ring and washer.

 

Figure 9 : Horizon mirror mounting exploded.

Figure 10 : Rear of horizon mirror sandwich.

 

The right half of the photograph shows how the cell is attached flexibly to the bracket, by means of two slightly curved leaf springs, attached at their ends to the cell and in their middle to the bracket.  The usual two adjusting screws bear on the cell to adjust out side and index error, and are provided with protective screw-on covers. 

 

The bracket bears the caption “Patent number 30340/34″, but this is in fact a patent application number, one higher than the application number for the index mirror patent. I have not been able to trace the patent itself and assume that one was never granted, possibly because some important aspect had already been anticipated.

 

Magnifier mounting

 

Many manufacturers contented themselves with mounting a magnifier at the end of a swinging arm, centred about two thirds of the way down the index arm, with the centre of the lens approximately over the junction of the arc and vernier scales. Obviously, at each end of the vernier scale, the lens centre would be a little out of line with this junction.  When a Ramsden type of compound magnifier was used, this was of little consequence, as the Ramsden provides a relatively flat field, but many cheaper instruments were provided with a simple plano-convex lens, which suffers from quite severe off-axis distortion.

 

While Hughes provided a Ramsden type magnifier in their top-of-the line pre-WWII vernier sextants, in this Admiralty pattern sextant, there is only a simple magnifier, but it is carried in a substantial mounting that allows the lens to follow the curve of the scales (Figure 11).

 

Figure 11 : Magnifier mounting

 A curved bed is mounted on two pillars and a short dovetail slide with keeper underneath carries the magnifying lens mounting. An elegant little brass knob completes the assembly.

 

Tangent screw and clamp

 The tangent screw mechanism is one that was (and is) commonly used in theodolites for obtaining slow motion. I touched on it briefly in my description of a Troughton and Simms sextant and I repeat the description here (see Figures 12 and 13).

 

A block that can slide in guides on the back of the lower end of the index arm can be clamped to the limb. A tongue or lug projects from the sliding block and is sandwiched between the end of the tangent screw and an opposing spring, both of which are contained in a tubular frame that is secured to the lower end of the index arm. When the clamp is released, the index arm is free to move over the arc. When it is secured to the limb by the clamp, the tangent screw can be used to make fine adjustments, and the spring inside the spring box provides motion in the opposite direction. It also takes care of backlash, which can be an annoyance in a vernier instrument, even though it does not affect the accuracy of the reading.  Other makers were content to leave the ends of the dovetail slide open, but in this instrument they are closed off with end pieces.

 

Figure 12 : Tangent screw mechanism in situ.

 

Figure 13 : Tangent screw mechanism exploded.

 

 

Figure 14 : Sextant in case

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

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

Figure 1 : Exterior of case

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

Figure 2 : Name plate.

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

Figure 3 : Contents of case

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

Figure 4 ; Frame of sextant.

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

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

Figure 5 : General arrangement, front view.

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

Figure 6 : Micrometer mechanism

Figure 7 : Swing arm detail

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

Figure 8 : Micrometer worm shaft bearings.

Figure 9 : Chart of non-adjustable errors

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

Figure 10 : Telescope attachment

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

Figure 11 : Exploded telescope mounting

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

Figure 12 : Monocular and its rising piece.

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

Figure 13 : Battery handle

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

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

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

It is seldom that I can afford distinctive instruments except by buying homeless wrecks and restoring them, but recently I was able to acquire a genuinely antique sextant at a price that left both me and the seller happy. I had seen it on the local auction site, TradeMe. It attracted no bids. It was relisted the following week and attracted only one bid, mine, that did not reach the reserve price. A few weeks later, I invited the seller to relist with a fixed price offer. Happily, I was able to afford the asking price, but the matter did not end there, as I was suspicious from the rather poor photographs, that the sextant might turn out to be from M’bai (Bombay), rather than from the putative maker, Henry Hughes. I did not want to part with my money without seeing the sextant “in the metal” and the seller did not want to part with the sextant without seeing the colour of my money. We had a slightly one-sided compromise: she travelled 80 km north and I travelled 200 km south to a meeting.

It turned out that the sextant was not by Henry Hughes, though it had a bedraggled Husun certificate in the case, suggesting that it was still in use sometime after about 1920, when the Husun trade mark came into use. Engraved on the limb was the worn and barely visible inscription “King’s Patent. GILBERT & Co. Tower Hill. LONDON.”  John Gilbert was a noted C18 instrument maker who at the time of his death in 1791 was working from 8 Postern Row, Tower Hill, in the shadow of the infamous Tower of London. The firm continued as Gilbert and Son and merged with another noted instrument maker in 1808, when the firms became Gilbert and Gilkerson. An identical sextant named “Gilbert and Son” is illustrated in Figure 48 of Peter Ifland’s book “Taking the Stars” and has been given an approximate date of 1790. I think mine is probably dated between 1791 and 1808, and I would be glad to hear from anyone who can date it more closely. Here are front and rear views as the instrument came to me (most of the photos will withstand a lot of enlargement. Click on the picture to enlarge. Right click outside the edge of the picture and select <Back> to return to the post)

Fig 1 Front view

Fig 2 Rear of sextant

It had lost nearly all trace of whatever paint it once had, or perhaps an owner had followed the all-too-common practice of  stripping the paint and polishing all the “brass”, something that I think makes a once-powerful instrument look forlorn, rather like a woman without her false teeth or a man without his trousers. Happily, the arc had not been polished, and the divisions looked sharp and clear:

Fig 3 Detail of arc and limb

As there was no trace of tarnish, nor of polishing of the arc or adjacent brass, I briefly wondered whether it might have been divided on platinum, but there is no sign of an inscription “platina”. If it is of platinum, and only time and perhaps the application of a little mustard will tell (it will turn silver black), it would make its date no earlier than 1804, when William Wollaston first isolated it in commercial quantities and probably not earlier than 1808, when makers like Edward Troughton applied it to  sextants. (See Postscript no. 1)

This venerable sextant has many interesting features that the owner of a modern sextant will never see outside a museum. I certainly felt a little like an industrial archeologist, as I uncovered little clues that gave me insight into the mind of the eighteenth century instrument maker.

Starting with the frame, it is a bronze casting, rather slender by modern standards, while the limb, of brass 1.92 mm (0.075 in) thick, appears to have been attached to the frame by nine rivets, the head of one of which can be seen just to the right of the figure 50 in Figure 3, above. More likely, it was first sweated on, a process in which each surface is “tinned” with soft solder and then reheated with the surfaces pressed in contact. This would have been followed by drilling and tapping for countersink-headed screws, whose heads would then have been filed off flush. The next photo, which shows how relatively slender is the bronze frame, gives us an idea of the small diameter of the screws. With modern taps, made of high speed steel, tapping would be a task needing great care to avoid the disaster of a broken tap. One wonders how many eighteenth century apprentices earned a whipping for breaking a crude tap of the era. Compared to a modern tap, the steel would have been of uncertain quality, with the thread and flutes ill-formed. Looking at the screws used, every one made by hand, they have very fine  and shallow threads (48 threads per inch, sometimes more), as a coarser or deeper thread would need more effort to cut it. In fact, forming a thread in brass was more of a moulding than a cutting process.

Fig 4 Structure of frame

 As close examination of Figure 1 will show, the silver arc does not extend right to the ends of the limb. This is a most unusual feature which suggests that the undercut slot for the arc was formed before the limb was attached to the frame. It could not otherwise have been cut, as dovetail milling cutters, still less milling machines, had not by then been invented and it would have been nearly impossible to form with chisel and files. Both earlier and later sextants, where the arc extends to the ends of the limb, would have cut the slot by turning the part on the lathe, but as the radius of the arc is 192 mm (7.56 in) the intermittent cut would have presented a challenge to the lathes of the day. Possibly, the ends of the slot were filled in with a sliver of brass after setting in the arc. (See Postscript no. 2)

The arc itself, nearly always of silver, would start as a strip of metal, which would then be rolled into a semi-circular section and be hammered into the slot before being filed off flush. Silver is used because until the middle of the nineteenth century sheet brass was an uncertain material, produced by hammering with a broad faced hammer and by rolling, followed by rubbing on or with an abrasive stone, so that it would have many hard spots that could divert the scriber of the dividing engine from its true path (Figure 5, showing  the underside of the index turntable shows the chatter marks as a hand-held turning tool argues with a tenacious material). Unfortunately, because silver is soft, polishing rapidly wears away the divisions, and it also tarnishes rapidly in a sulphur rich atmosphere like  coal-fired industrial areas.  The arc should never see silver polish. Tarnish can be removed by careful wiping with dilute ammonia solution.

Fig 5 Chattering under the table

The flat surface for the attachment of  the horizon mirror and the index shades is again of 1.92 mm brass sheet, attached to the frame by seven screws, whose heads reside in counterbores. The next photograph, of the front of the naked frame,  shows that the region of the horizon mirror is also supported by an extension of the frame, to the side of which the horizon shades are attached by two screws. A similar extension supports the telescope mounting.

Fig 6 Front of frame

 Finally for the frame, there is an unusual triangular brace, again of sheet brass, attached to the back of the frame by five brass screws (two of which had lost their heads), reinforced at the corners by the screw-in legs and the middle screw of the unusual handle (Figure 2). The latter is attached at the middle as well as the usual top and bottom, and has a threaded socket about 10 mm diameter for an extra handle, which must have made taking horizontal angles and lunar distances a good deal more comfortable. A sextant named  Gilbert and Wright in the National Maritime Museum in Greenwich (F5183) has a similar bracing that forms a diagonal cross, but the frame of this sextant appears to be much more slender and to be lacking the middle of the transverse bars. It is not clear to me whether the bracing served any useful purpose in my sextant, and other makers do not seem to have copied the practice.

The threads of one leg had all but stripped and the other one was bald and the leg seemed to be held uncertainly in place with something like Araldite. Thus, jobs for the frame included machining off the threaded portion of the legs, fitting in a new piece and cutting a thread on it, and drilling and tapping the holes to fit the new thread. There was no question of copying the old threads, as there was no standardisation much before the middle of the nineteenth century. I used 4 BA threads for the legs. There were also two headless screws that had once secured the brace to the frame. The remains of the screws had to be filed flush, centre popped, drilled out and retapped, and I had to make new screws to match the remaining ones. Number 9 BA seemed to be about the right size. I was very thankful that a friend had recently given me a full set of BA taps and dies, as the odd sizes are rather hard to come by.

The index arm and its bearing was next for attention.  A tapered shaft attached to the upper expanded end of the index arm runs in a bearing that fits closely in a hole at the apex of the frame. Bearing play is taken up by moving the shaft axially into the bearing by means of an adjusting screw on the end of the shaft operating through a washer. The end of the shaft is squared to fit a square hole in the washer so that latter rotates with the shaft and cannot loosen the screw. This design did not change at all during the next two centuries.

In many sextants, including this one, the end of the shaft is enclosed in a cover. The index arm between the top and the expanded bottom is stiffened by a central rib which is attached by means of six small screws. A pillar for the arm of a Ramsden scale magnifier is mounted about two thirds of the way down. At the lower expanded end of the index arm is a standard extended vernier scale reading to 30 seconds and a conventional tangent screw and clamp. The latter was missing its spring, though one of the original attaching scews was still present.

As so far described, the index arm showed no change for as long as vernier sextants were produced. This sextant is slightly unusual (but not unique for the period) in that the vernier scale is attached to the upper side of the window in the expansion. At the upper end, however, the index mirror is not attached directly to the index arm. Instead there is an intermediate turntable which rotates on an extension of the bearing shaft and is secured to it by a screw that sits in a counterbore in the turntable. At the top end of the turntable is a lug which allows small adjustments to be made to the position of the turntable to which the index mirror is attached. In short, it is a rather elaborate method of adjusting out index error. The next three figures may make this clearer.

Fig 7 Turntable bearing

Fig 8 Turntable adjustment

Fig 9 Index mirror mounting

The index mirror is mounted in a way that became pefectly standard and is probably due to Peter Dolland. In a letter that he wrote to Neil Maskelyne, the Astronomer Royal in 1772, he wrote “…I have contrived the frame, so that the glass lies on three points, and the part that presses against the front of the glass has also three points exactly opposite the former.”The method of rotating the index mirror on a turntable may be what is referred to by the words “Kings patent” on the limb, granted to Peter Dollond in May 1772 for ” …adjusting and improving the glasses of Hadley’s…sextant…“    . It is possible that it refers as well to the method of adjusting the horizon mirror for side error, about which more later.

Figure 10 Broken journal

As I have already mentioned, the index arm bearing was entirely conventional, but on removing the cover, I found some “repairs” had been carried out:  someone had broken the head off the original screw and crudely replaced it with a modern self-tapping screw. On further dismantling , the Araldite  that had been used in the “repair” to attach the shaft to the flange that marries it to the index arm gave way. You see the result in Figure 10. I had to attach a new piece of brass to the flange (Figure 11), taking great care to maintain squareness and then turn the  journal to match the existing taper of an included angle of about 0.8 degrees (Figure 12). Those interested can Contact me to find out how I set up the lathe to cut this rather fine taper. I then milled the square on the end of the shaft (Figure 13). This is easily and precisely done with modern machinery, but the eighteenth century instrument mechanic would have used a file and filing rest to do the same job and probably in much less time than it takes me to set up the machine.

Fig 11 Journal repair 1

Fig 12 Journal repair 2

Fig 13 Journal repair 3

Fig 14 Tangent screw and clamp

There is nothing unusual about the mounting of the shades, except that all the washers are free to rotate on the tapered shafts, while later designs adopt some means to stop them rotating, so that when one shade is rotated into place, the movement is not transmitted to the adjacent ones. The design of the rising piece is also unremarkable for the time and was followed with various more-or-less complicated minor modifications until the mid 1950s.

This leaves the horizon mirror and its mounting. Peter Dollond’s patent of 1772 (number 1017) mentions improvements to mirror mountings. In a letter of the same year to Nevil Maskelyne he writes: “To adjust the horizon glasses in the perpendicular plane of the instrument, I have contrived to move each of them (there was also a “back horizon glass” that soon fell into disuse with the increasing popularity of the sextant) by a single screw, that goes through the frame of the quadrant, and is turned by means of a milled head at the back, which may be done by the observer while he is looking at the object.” His letter also makes plain that horizon shades were an innovation devised by Maskelyne.

The next photo (Figure 15) shows the arrangement in place. The screw passes through the frame into a tapped hole in the base of the mirror mounting and is used to tilt the horizon mirror in such a way as to remove side error, as index error has been taken care of by the index turntable, but note also that there is a capstan headed screw which can also deal with index error. Perhaps it was a later addition, the index turntable having been found wanting.

Fig 15 Dollond's adjusting screw 1

Although Dollond gives no details, the hole through the frame of the sextant is much larger than required for the screw alone, and I surmise that there must have been a helical spring to take up backlash at each end of the screw. However, the screw is threaded almost to the milled head and it is possible there may have been some sort of locknut arrangement to hold the screw captive, since lost. While the spring seems to be obvious solution, there are other possible simplifications to the design of this sextant that also seem obvious to us in the light of two centuries of hindsight, but which plainly were not so to the instrument makers of the late eighteenth century. Figures 16 and 17 make clear the design of this curiously complicated solution.

Fig 16 Dollond's adjusting screw 2

Fig 17 Dollond's adjusting screw 3

 While some museologists might regard it as heretical, I try to restore my instruments to the way they might have been when fairly new, perhaps just out of the instrument maker’s shop after an overhaul. This usually includes repainting with something as close to pre-modern paint as possible. Modern paint seems to be much too thick and I use a black lacquer, which gives a very pleasing finish. Most of the screw heads would have been polished and their slots cleaned out, following clock-making practice. I will allow that the telescopes were probably polished rather than painted. The next two pictures show the finished instrument from front and back. These imges will withstand considerable enlargement if you wish to see more detail.

Fig 18 Finished, front

Fig 19 Finished, rear

Both Figs 18 and 19 show the outside of the case, which was in quite good condition and responded well to fine sanding and a coat of wax. One of the hooks and its eye were missing, and their holes had been closed off with pink car body filler. I made fairly close copies of the hook and eye by sawing and filing them out of plate brass, and think they will pass at least casual inspection. I made the octagonal handle out of a piece of unidentified New Zealand hardwood, stained and polished, and let in a piece of brass rod, threaded to fit the hole in the back of the handle, I secured it in the handle with a transverse piece of brass rod. Used as we are to hexagonal nuts and bolts, we might wonder why handles of this type were octagonal. I suggest that it is firstly easier to make, by planing off the corners of a square, and secondly, while a hexagon is easily held in a three jaw self-centring chuck, such chucks had not come into use in the eighteenth century and if a turner possessed a chuck at all, it was likely to have four independent jaws, better adapted to holding an octagon than a hexagon.

 The final picture shows the sextant in its case. The telescope is not without interest. It is a Galilean or “star scope” of about 3 power and with an objective lens of only 14 mm in diameter. Newton’s rings are clearly visible when looking at the objective, showing that it is an air-spaced achromatic lens, as lenses were not cemented until well into the nineteenth century. Its small size is probably because large pieces of good flint glass were hard to obtain. The eyepiece is threaded for an eyepiece shade, but there is no accommodation for one in the case.

The name of Dollond arises again in relation to achromatic objectives. John Dollond is often cited as the inventor of this type of lens, even though it is well established that Chester Moor Hall was making them for himself in 1729 and it was well known to the optical trade by the middle of the century. Nevertheless, John’s son Peter urged him to take out a patent and in this he was successful, against staunch opposition from about thirty of the leading London opticians. There is a link to Jesse Ramsden, the inventor of the first precision circular dividing engine. Ramsden is thought to have tried to put the record straight and he was in a good position to know it, as his wife was Peter Dollond’s sister. There is a space for an inverting telescope, which would have been about 140 mm long and of probably about 6 power.

The case itself is of mahogany and it is interesting to note that the bowed front is not a segment of a circle but its radius increases towards the right, so that the sextant will fit in the case only if the index arm is parked close to zero. This appears to be so of many early sextant cases and was presumably deliberate.

A question which will occur to some might be “How accurate is it?” This will have to wait for another time… (See Postscript no. 3)

Fig 20 Restored sextant in restored case

I hope this rather lengthy description of an old sextant will be of interest to lovers of fine instruments as well as contributing to the record of instrument making. I cover in detail the structure of sextants from about 1850 onwards in my book “The Nautical Sextant” , jointly published by Paradise Cay and Celestaire. Contact me if you would like to buy a copy directly from me (probably only worthwhile for New Zealand and Australian readers).

Postscript no. 1: It later occurred to me that platinum is harder than silver. On the Vickers scale of hardness, it is about twice as hard. I made a new business end for an automatic centre punch to give it an included angle of 135 degrees and set up the punch so that it delivered a constant blow when actuated. The size of the tiny punch mark made in the end of the scale was of the same size as that made in an arc known to be of silver. The conclusion is that the Gilbert arc is the same hardness as silver. If it looks like a duck…

Postscript no. 2: On careful examination  with magnification and the right light, a rivetted screw head is just visible in the brass at the left hand end of the arc. My surmise that a slip of brass was rivetted in after machining the groove for the silver arc seems to be correct.

Postscript no. 3: The errors are as follows:

Degrees                                Error, arcseconds

15                                                  +6

30                                                 +2

45                                                  -3

60                                               +10

75                                                 +6

90                                                 +3

105                                             +24

120                                             +25

See more details at http://www.fer3.com/arc/m2.aspx?i=113271

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

I recently acquired a Troughton and Simms sextant, with all its attachments, but without a case. I find I can sometimes afford such homeless sextants and, after having made ten cases, I can now make them so that they look reasonably in period. It was described in a later catalogue as a “Surveying Sextant, 7 ins., divided on silver and reading by vernier (with magnifier) to 10 secs. ..” In fact, it would also pass as a nautical sextant, as it is provided with the usual seven shades and kit of telescopes and sighting tube. The inverting scope, provided with four wires in the eyepiece,  is of 6 power with an effective aperture of 17.5 mm. The star telescopes are 2 power x 17 mm and 3 power x 15.5 mm. The latter may have belonged to another instrument, but both are in the same style.The Troughton brothers were active in London around about 1800. Edward Troughton (b.1765) carried on the business of instrument making after the death of his equally famous older brother, John, in 1788.  By 1824, getting on in years, he took into partnership William Simms (b. 1793), describing him to an acquaintance as “the best craftsman he knew.” Troughton died in 1835 and Simms in 1860, but the business carried on as Troughton and Simms until 1916, when it became a Limited Company. It merged with T Cooke and Sons of York in 1922, to become the  famous firm of instrument makers, Cooke, Troughton and Simms, with manufacturing being carried out in York, and offices and service centres in London and throughout the then British Empire.

Thus, my sextant is unlikely to be later than 1916, but is otherwise rather difficult to date. There are features like the handle and the index mirror mounting that would place it in the nineteenth century, while the horizon mirror frame is possibly a twentieth century replacement. The cast bronze frame is perhaps a little old-fashioned, while the maker’s name on the limb is in an elegant script, typical of the nineteenth century rather than later stamped typeface. There is little information to be had about serial numbers of this maker. A sextant numbered 1920 is reliably dated to 1836 and if we take forty instruments a year as a reasonable estimate of number made, my sextant would find itself in the last quarter of the nineteenth century. Troughton and Simms made very large numbers of surveying and other instruments during the expansion of the British Empire, and it is likely that sextants formed only a very small part of their output. The next picture shows it as I received it. 

The rising piece had a bend in it at the junction with the ring and the telescope ring was heavily corroded with more than a century of verdigris lurking in the crevices:

Plainly, the rising piece had broken the fall of a dropped instrument. In the next photograph, which also shows the handle, it can be seen that the lead screw has punched off the end of the rising piece body. This normally appears to be solid with the body, but is in fact a disc of brass soldered on to the end of the body, as it would not otherwise be possible to form the square hole. The leadscrew is normally held captive in the body, the disc being sandwiched between a flange on the leadscrew and the thumb screw.

In this instance, it was fortunate that the joint gave way, as the rising piece was jammed solidly in the body, and it would not otherwise have been possible to extract the leadscrew. Once it was removed, I could press out the rising piece in the vice:

Once I had the rising piece out of the body and separated from the telescope ring, I could straighten it using a straight edge as a datum, so that it could once again slide smoothly up and down without any shake. Note that there are two recesses machined into the stem of the rising piece for a pair of leaf springs. These I imagine were fitted to help reduce shake, but the mechanism functions perfectly well without them. The thumb screw had still to be parted from the leadscrew. This required only a well-fitting screwdriver and some brute force. I could then clean up the disc and resolder it to the body.

The rest of the restoration simply involved dismantling and cleaning all the parts, followed by fresh coats of black lacquer to the frame and other parts. Then I had to make a case with dovetailed corners and pockets for the various ‘scopes and tools:

I carved the hook latches out sheet brass and had to make the round-headed screws with which they engage, as none of  the round headed screws I could find on sale had plain slots. The handle is a drawer pull, which I have provided with a finger plate. I hope the handle  does not look too out of place:

The next photos show the maker’s  name on the limb…

The structure of the handle is practically unchanged from one from the 1820s. Note the tablet shaped wooden handle, the elegant lower bracket and the way the handle is attached to it with a pin screw. These screws, with two pin holes for a pin wrench were found aplenty in the Troughton Brothers’ famous pillar sextants of the late eigtheenth and early nineteenth century. In these sextants, the frame was formed from two plates about 1.5 mm thick, held apart by about a dozen pillars, to which they were secured by pin screws. Imitating clock-making practice, this gives a relatively rigid and light-weight structure, but was very expensive to make.

As one might expect from a maker whose principal products were surveying instruments, the tangent screw mechanism follows theodolite practice. A block that can slide in guides on the back of the lower end of the index arm can be clamped to the limb. A tongue projects from the sliding block and is sandwiched between the end of the tangent screw and an opposing spring, both of which are contained in a tubular frame that is secured to the lower end of the index arm. When the clamp is released, the index arm is free to move over the arc. When it is secured to the limb by the clamp, the tangent screw can be used to make fine adjustments and the spring inside the spring box takes care of backlash, which can be an annoyance in a vernier instrument, even though it does not affect the accuracy of the reading. The next photo shows these details in the un-restored instrument:

Edward Troughton originated a dividing engine to rival that of his famous near-contemporary, Jesse Ramsden and described it in the Proceedings of the Royal Society for 1805. With a sextant coming from this background I  expected the dividing of the arc and vernier to be of a hight standard and I was not disappointed, as the next photo shows:

(In this and other pictures, you can enlarge them to see more detail by clicking on the picture. Return to the post by clicking on the back arrow.)

Finally, a picture to illustrate the archaic index mirror adjustment. The mirror is held against an angle bracket by a keeper and screw. The bracket is held to the top end of the index arm by two screws and can pivot about two pins adjacent to the screws. A third, adjusting  screw rocks the whole bracket.

If you enjoy reading about details of fine instruments, you will enjoy my forthcoming book The Nautical Sextant. You will find some details about it in “About the Book.” Contact me via this site if you would like to be informed when the book goes on sale.

The preceding posts covers “A C19 Sextant Restoration” , “Making a Keystone Sextant Case” , “Restoring a C. Plath Drei Kreis Sextant” and ”Heath Curve-bar sextant compared with Plath” .

Some good friends came for the weekend about ten days ago and one of them presented me with a wooden box that rattled ominously. An old uncle had been clearing out his house and had asked her to find a good home for the box and its contents. I recognised the box as belonging to a Hughes and Son sextant from the placing of the hook latches so that gravity keeps them both in place when the box is being carried, and on opening it, this is more or less what I found inside:

Figure 1 : Sextant as received.

As you can see, the key hole of the case is plugged with mud and some of the smaller parts inside were also encrusted with it. Most of the paint had disappeared from the frame and index arm and all the woodwork was loose, but there was no obvious major damage to the individual parts. In particular, the arc was all in one plane, the index arm was not bent and the index arm bearing was in good condition. All the screws, both great and small were present, but the lamp and its mounting for illuminating the scale were not and the electrical parts of the battery handle were the worse for wear. Of the wiring there was no sign.

This is the third valuable sextant I have been presented with as a gift and I count myself very fortunate to have such friends. I foresaw hours of pleasure ahead of me, though with some tedium involved in polishing screw heads and cleaning parts.

I began with the frame and index arm. The release catch was still attached to the latter, so I removed the parts and also removed the legs from the frame to make the clean-up easier. I left the index arm bearing in place and in general, this should never be removed, though in this instance, as there was no inspection certificate, there would have been no great harm in removing the bearing and re-calibrating the sextant. However, it would have required a special ring spanner and it seemed pointless to make one for an unnecessary task.  There is no harm in removing the tapered “journal” from the index arm, but it is seldom necessary to do so unless there is heavy corrosion around it.

I gave the parts a prelimary soak in diluted household ammonia, rinsed and then stripped off the remaining paint with a proprietary paint stripper. I then brushed all the mud and verdigris from the teeth of the rack, dried everything off and hung the frame and index arm up for repainting.

Hughes and Son at this time (1938) used a wrinkle finish paint on these parts, with a black lacquer for most of the others. The advantage of such paint is that it hides any minor blemishes in the finish of the underlying metal, it is not distractingly glossy and, having been baked on, it is reasonably tough. However, it had not withstood immersion in sea water very well. Happily, modern paint technology allows us to get the same finish without baking. I use PJH brand “VHT Wrinkle Plus” spray paint and find it gives a very good result. The wrinkles can be made tighter by heating with a hot air gun as the paint begins to dry and before the wrinkles have started to form. The next picture shows the finish I obtained, and it is quite close to the original (I have other Hughes sextants with which to compare it).

Figure 2 : Wrinkle finish paint.

Although the wrinkle paint appears to dry quickly, it is very easy to damage it, and, as a full cure takes five days, I moved on to some parts that I knew would take care and time to restore: the shades assemblies. I started with the easier horizon shades. These are mounted on to a bracket by means of a shouldered screw and locknut and separated from each other by washers. The washers have integral keys that engage with a keyway on the screw so that they cannot rotate. This ensures that when one shade is rotated, the others do not follow. Some resistance is applied by means of a Belleville washer, a dome-shaped washer that behaves as a short, stiff spring. You will see from the next photograph that the Belleville washer had not survived.

Figure 3 : Unrestored horizon shades

The index shades assembly uses a much older system. The shades and their separating washers together form a tapered hole. A tapered pin passes through the mounting bracket and  through the shades and washers, which must therefore be assembled the correct way around and in the correct order. An adjusting screw forces the pin further into the composite hole to adjust resistance to rotation. The tapered pin is prevented from rotating in the bracket by means of a cross pin. Any attempts to remove the tapered pin without first removing this cross pin are doomed to failure, so look for it carefully and punch it out. There are other ways to prevent rotation of the pin and I cover these in my book. Once the cross pin has been removed, brute force in the shape of a wooden drift and press or hammer may be needed to force the tapered pin free. The next photo shows the result.

Figure 4 : Unrestored index shades

If you cannot obtain a replacement Belleville washer, and living where I do, I cannot obtain most things, one can be made. I turned a washer from a scrap of phosphor-bronze bar, sat a scrap ball bearing in the hole, placed the two over a larger hole in a steel block and hit the ball with a hammer. It may sound crude, but it works.

As most of the paint had deteriorated to a mixture of paint and verdigris, I completed the destruction with wet  emery paper prior to repainting. In the past, I have done the painting by hand, but as spray painting gives a much better appearance, I worked out a simple way to mask the glass, one that does not attempt the near-impossible of punching out large discs of masking tape. The next photo shows the steps. The knife has to be sharp and have a fine point to be able to follow the curves.

Figure 5 : Masking shades.

If the shades are then suspended from a bar that just fits through the mounting holes, they can be sprayed with black lacquer without too much of it getting into the holes.

There are some tricks to remounting the index shades. It is best to have a trial mounting on the pin, to make sure that you have the shades and washers in the correct order and the right way around. The cross pin is not usually exactly on a diameter and will usually fit only one way round, so make some sort of mark on the head of the tapered pin and bracket to indicate the correct orientation. Then lay the parts in order  on the bench while you grease the tapered pin and place a tiny dab of grease on each face of the washers. You can then reassemble them on the pin and remove the pin without everything dropping apart. With the shades and washers on the pin, wangle them between the cheeks of the bracket (the right way around, of course). Once they are partially engaged with the bracket, you can withdraw the pin and carefully engage them further until the narrow end of the pin can be re-entered through the bracket, waggling the shades a little so that the pin passes deeper and deeper into the composite tapered hole. Make sure the cross hole lines up correctly and then press the head of the tapered pin home. Insert the cross pin, adjust the adjusting screw and you are home.

Much of the rest of restoring this sextant consisted simply in cleaning and repaint parts, so for much of the rest of this post I will illustrate, usually without commentary, parts that differ in Husun sextants from other makes. A lot of the ground is traversed in my book, together with details of other manufacturers’ approaches.

For example, the telescope mounting:

Figure 6 : Telescope in situ

Figure 7 : Detail of telescope mounting.

The micrometer mechanism:

Figure 8 : Micrometer mechanism and rack.

The battery switch:

Figure 9a : Switch button.

Figure 9b : Switch exploded.

The wire passes through an internal passage drilled in the handle and leaves it via a plastic grommet on the left-hand side of the handle. Thence it passes to the central contact of the lamp bulb. The plunger of the switch makes contact with a brass disc which forms the top of the battery compartment. Both sides of the disc need to be clean for the system to work. Earth return is from the lamp mounting to the frame. The spring in the bottom of the battery compartment is attached to the  lid of the compartment. The lid is connected through its hinge to the bottom handle mounting and thence to the frame. A slender spring-latch, not electrically connected to anything, holds the lid closed.

Figure 10 : Battery compartment lid.

The next photo shows the connections at the lamp. I have only partly followed Hughes’ design. Mine is simpler, as it omits an insulating bush which is not really needed, as the wire itself is well insulated.

Figure 11 : Lamp mount exploded.

But first I had to make a new lamp-holder, shade and mounting. A hollow pillar is attached to the index arm expansion by means of a 4 BA countersunk screw. The other end of the pillar is drilled to accept a spigot on one end of the mounting arm and split so that it grips the spigot. The lamp shade is mounted at the other end of the arm. The shade is threaded to take a standard torch bulb at one end and, at the other, the threaded cap through which the wire passes. To change the bulb, the shade and its mounting arm is simply withdrawn from the pillar to allow access to the underside of the shade.

Making the pillar is a fairly simple turning , drilling and tapping exercise:

Figure 12 : Turning lamp pillar.

So is the outside of the shade:

Figure 13 : Turning outside of shade.

Boring the inside of the shade is a little more difficult. As for  all boring operations, it is harder to see and to measure what you are doing:

Figure 14 : Boring inside of shade

Do not expect to be able to buy a tap to cut the internal thread, at least, not for any reasonable price. It is 14 threads per inch x about 0.38 inches major diameter, with a rounded thread form. Most people will attempt to cut it on a lathe, which is how I did it. I sometimes help out new friends who extravagantly praise my work.

Figure 15 : Internal thread of lamp shade

I have covered making things like cutting out connecting arms in  previous posts. Here is a reminder:

Figure 16 :Tools for cutting out arm.

The completed lamp mounting, together with wire and new retaining clips shows up well in this picture of the completed sextant:

Figure 17 : Sextant restoration complete.

After its near-death experience, I felt that the sextant should have a decent home, and so I gave the box  some attention. The base had a very large shrinkage crack in it, so I removed it, whereupon the rest of the bottom half of the box began to fall apart and had to be re-glued. I planed the edges of the crack straight and square, and glued in a strip of mahogany, which can just be seen in the final photograph. The front had also shrunk a little so that the lock no longer fitted, and this required some attention with a wood chisel. The lock had at some stage been forced, leaving a large chip and the ends of no fewer than three broken screws in one hole, in the rebate in the lid where the hasp had been. I made a fresh start at the damaged area and glued in a new block of wood before re-fitting the hasp.

Like many households, we have a large bunch of keys that fit nothing in particular, but I found one that nearly fitted the keyhole, so after washing out the lock and oiling it, I filed away at the key  with the aid of marking blue to guide me, until it operated the lock. The various mounting blocks had to be screwed and glued into place and the felts of course had to be replaced. Aftere cleaning of the brass “furniture” and making a new adjusting pick, here is the final result:

Figure 18 : Restored case.

If you would like information or advice about some detail that I have not fully covered, contact me and I will try to help.