Figure 1 : To illustrate dip and its measurement.

For those not familiar with the concept of dip, Figure 1 shows how the height of the eye affects the distance to the horizon and in turn the angle of dip. If the angle between the horizon in front of the observer and the horizon behind him can be measured, then dip can be deduced directly, as it is half the value of that angle.This is not a new problem. In 1900, John Blish of the United States Navy applied for a patent for an attachment to add to a normal sextant. The patent can be viewed on Google Patents by searching for Patent number 714,276. Figure 2 shows one of the patent drawings.

Figure 2 : Blish prism attached to nautical sextant.

Essentially, the device is a prism that diverts light rays through 180 degrees so that the horizon directly behind the observer can be viewed at the same time as the horizon in front and the amount of dip read out directly from the sextant’s scale.

Several other inventors devised instruments or attachments to do the same thing. Among the more complex dedicated instruments was one patented by Boris Gavrisheff in 1961 (US Patent number 2,981,143). A telescope views via two prisms light coming from one horizon behind the observer at the same time as the light from the horizon in front of the observer. One of the prisms is rotatable so that the deviation from 180 degrees, i.e. the dip, can be directly read off a micrometer drum. A third prism, labelled 12 in the diagram, diverts the rays into a telescope (Figure 3).

Figure 3 : Gabrisheff's dip meter.

It occurred to me that expensive prisms are not needed. They are not used in nautical sextants though for some reason most American bubble sextants used them. I had a box of wreckage from three Hughes and Son Mark III survey sextants that a kind friend gave me, so I set about building a dip meter as an exercise to illustrate the principle rather than as a serious sea-going device, though it could certainly be made more robust for sea-going use. An index arm extension carries a mirror that receives light from the rear horizon over the top of the observer’s head and diverts it through 45 degrees into another mirror that diverts it another 45 degrees into the telescope. The telescope receives light from the front horizon over the top of the second mirror so that, provided the mirrors are correctly aligned, both horizons can be viewed at once and the deviation from a straight line be measured using the micrometer drum. Figure 4 shows the layout of parts and Figure 5 the path of the light rays from front and back.

Figure 4 : Parts of the dip meter

Figure 5 : Ray path of dip meter.

Figure 6 shows how the dip meter is adjusted. Two autocollimators are set up facing each other and their axes adjusted so that they coincide in the vertical and horizontal planes and are parallel to the surface of the surface table. One is then “shuffled” sideways using a mirror, so that its axis is displaced from but remains parallel to the axis of the other. The plane of the sextant’s arc is set parallel to the table using a dial indicator and shims under the feet of the sextant. The sextant is set to zero and the two mirrors adjusted until the images of the crosswires of the autocollimators coincide when viewed through the telescope. The dip meter is then ready to measure the angle of dip directly.

Figure 5 : Light paths from the autocollimators

Figure 6 shows how the mirrors are adjusted, the same way that an horizon mirror in a standard sextant is adjusted, with screws bearing on the back of the mirror opposite spring clips bearing on the front.

Figure 6 : Mirror adjusting screws.

If you enjoyed reading this post, you will enjoy reading my book The Nautical Sextant, available from the joint publishers, Paradise Cay and Celestaire and via Amazon. Readers in Australia and New Zealand may Contact me, as I am able to offer them a discount on the published price.

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.

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.

Bob Hauser asks a question in a different category that I think best deserves an answer in the form of a short blog. He asks

Recently acquired Bendix AN 5851 had stalled averager that could be wound up to stop as per directions but would then simply remain in that state when the release lever (“no. 3″) was pressed—-for lack of any better solvent/lubricant, I lavished Reel-X on the bull gears and down under those gears into the inner chronometer mechanism and gently turned the pawl driver clockwise by hand and repeated this 4 or 5 X manually until the averager ran on its own for the required 2 minutes …yes, it worked but for how long with that stuff in there before it gums up even worse? Reel-X is a solvent/lubricant that has about the viscosity of sewing machine oil and may wind up being the worst thing to admit in the chronometer like that…can you advise?

Generally, watches and clocks do not respond well to being flooded with lubricant for a variety of reasons: there is very little power at the end of a watch gear train, at the point where the rate at which the machine runs down is regulated by the escapement and balance wheel, so that even the surface tension of oil between the gear teeth or in the coils of the balance spring can bring the mechanism to a halt; the pivots, or bearings about which gears and other parts rotate, are provided with tiny oil wells (“sinks”) and the shafts are shaped at the end to keep the oil where it belongs. If the oil strays on to the plates of the mechanism, the oil that should be confined to the bearing tends to follow it; and  excess oil combines with fine dust and grit so that the bearings and pinions (those gears with relatively few teeth) eventually grind themselves to a halt.

So what is Bob to do? One could say “Sufficient unto the day is the evil thereof,” but ideally, the clockwork mechanism should be removed from the sextant, stripped down, cleaned, re-assembled and oiled in the correct places. This of course needs clock-repairing  knowledge and practice. While the official manual explains how to remove the mechanism, it involves removing substantial bits of the sextant first, with all the risk of introducing new problems or damaging or disturbing the optical system. I suggest that he simply expose the clockwork and try to do a bit of dry cleaning, removing visible Reel-X from the plates where they are accessible and from the gears that he can reach. He should pay special attention to the balance mechanism, removing any lubricant from between the coils of the hairspring and excess oil from the pivots. For the larger parts like the plates and large gears, small pieces of old cotton handkerchiefs applied with fine forceps are ideal, though he should take care not to leave stray threads behind. For smaller, more delicate parts, I suggest scraps of lens paper which, though it is not all that absorbent, it less likely than paper towels or handkerchieves to leave fibres behind.

How to access?

Figure 1 : Remove two screws and nut and bolt.

Figure 2 : Remove two screws.

 

Figure 3 : Remove one screw.

 

Figure 4 : Remove one screw. Note washer.

 

Once the lighting unit is out of the way you can get access to all the twelve round-headed screws that hold the sheet metal cover around three sides of the clockwork mechanism. Remove the screws and cover (Figure 5).

 

Figure 5 : Remove twelve screws and cover.

 

This partially exposes the mechanism. Concentrate on getting as much excess oil from the low power end of the gear train. The stars in Figure 6 show the important areas. There is plenty of power further up the gear train, but it will still pay patiently to remove as much oil as you can see and get at.

 

Figure 6 : Important areas from which to remove excess oil.

 

Re-assembly is the reverse of dis-assembly. Good luck!

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.

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.

 

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” , “A fine sextant by Filotecnica Salmoiraghi” and “A British Admiralty Vernier Sextant.”

 A few weeks ago on e-bay, I bought a sextant that was said to be Bulgarian. The seller was Malaysian and I seemed to have been the only bidder for this interesting-looking instrument so that my finances stretched to its purchase. The pleasant and communicative seller initially sold it without a case, as it was broken (Figure 1), but I contacted him to ask him to keep the case and send it with the sextant, which he very happily did. When it arrived, the brass placard on the ruined pinewood box read “Gamma Budapest”, but there was a stencilled Cyrillic word on the top which, if it were Russian, would transliterate as something like shtuomluskhii, which does not appear in my rather small Russian-English dictionary. The Hungarian language uses Roman letters and its nearest neighbour with a sea coast, onto the Black Sea, and that also uses Cyrillic letters, is Bulgaria. Hungary itself is totally landlocked, but it has not always been so. However, the Treaty of Trianon following the First World War removed huge fragments from the Kingdom of Hungary, and gave them to Austria, Czechoslovakia, Romania, Bulgaria and the former Yugoslavia. Hungary had the misfortune to be on the losing side in the Second World War too and lived under the heavy hand of the Soviet Union for many years after.

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.

A comment in February this year on NavList about the Tamya Regulus sextant set me wondering, as the Tamaya sextants I have examined seem to be well-constructed. The writer commented on problems people had with the Regulus pattern at a nautical training establishment in the 1970s, so when a Tamya Regulus II recently came into my hands for overhaul and restoration work, I looked at it with unusual care. It seems to have been well-constructed, following the pattern set by C. Plath many years ago. It has an aluminium alloy frame with bronze rack, large mirrors and shades to match,  an adequate 3 x 40 Galilean telescope, a very good scale illumination system and a switch that is accessible and easy to overhaul. I was as puzzled by the adverse comments about the instrument by the time I had put it together again – until I came to align the telescope.

The telescopes of many sextants can be collimated, that is to say, the axis of the telescope can be adjusted so that it lies parallel to the plane of the arc. A lot of modern sextants do not have this feature, as the effects of mis-collimation have relatively little effect on the accuracy of observations, unless the observed angle is high or the angle of misalignment is great. For example, if the observed angle is 60 degrees and the misalignment is 55 minute, the error will be only half a minute. In fact, the error is proportional to the tangent of half the angle of observation and to the square of  the angle of misalignment in minutes.

Usually, the telescope screws into a flanged ring, and two screws allow the ring to be rocked about the rising piece, with two cone-ended screws for an axis, as shown in Figure 1.

Figure 2 : top - faulty collimation; beneath - corrected collimation.

Readers looking for a manual that helps with maintenance and repair of the Freiberger Trommelsextant (drum sextant) will find my SNO-T Sextant Manual very useful, as the design of the one is based on the other. While the manual describes the SNO-T, it also gives an account of the Freiberger drum sextant where its design details differ. See under “The USSR SNO-T sextant” and “Buy”

The firm of Freiberger Präzisionsmechanik has been in existence since  late in the eighteenth century and by the 1870s had a large workshop employing over eighty people in the manufacture of surveying instruments. At the end of WW II it was overrun by  Soviet forces and dismantled, leaving only fifteen workers to carry out maintenance on surveying equipment. It was refounded in 1950 and in that decade the trommel sextant was developed. As far as I know, the firm had never previously made sextants.

The sextant is unusual in several respects. While the over-all shape and placement of shades and mirrors is conventional, it has a die-cast aluminium alloy frame, which combines lightness with a strength and hardness near to that of mild steel. The ladder or three circle patterns of its main competitors were ignored. Material is concentrated around the edges and the whole stiffened by a central web (Figure 1). The worm runs in a rack machined directly into the edge of the limb, thus avoiding the complication of attaching a bronze rack to an aluminium frame.  The substantial but unseen bronze worm seems to run very well against the alloy rack.

Figure 2 : Freiberger drum sextant, rear view.

 The micrometer mechanism is concealed within an alloy casting attached to the lower end of the index arm. The cylindrical worm runs in eccentric bearings in a bronze casting that itself rotates against the force of a helical spring in bearings machined in the alloy casting. Thus, when the bronze casting is rotated, the worm swings out of engagement with the rack. The closeness of this engagement can be adjusted by means of a tangential screw whose head is just visible in Figure 2 to the left of the drum. While Freiberger chose to swing the worm out of the plane of the rack, nearly every other maker swung it out of engagement in the plane of the rack, following the pattern devised by C Plath in about 1907.  The latter method must certainly have been cheaper to manufacture, even allowing for the unecessary complexity of the worm shaft bearings in some pre-war marques. However, Freiberger’s method totally encloses the worm and solidly supports the shaft at both ends, so it is hard to imagine the shaft getting bent by an accidental knock, as had happened to at least three conventional instruments that have passed through my hands.

The sextant was usually provided with a 3½ x 40 Galilean telescope only. My own instrument has a 7 x 35 monocular, which gives a superior field of view as well as making the point of coincidence of the body with the horizon easier to determine. The vee and flat of the mounting are the reverse of all other makers, so their telescopes cannot be interchanged.

The USSR imitated Freiberger’s design in their SNO-T sextant, albeit in an instrument of slightly smaller radius and one provided with an unusually full complement of tools and spares. The edge of the SNO-T frame is  8 mm thick (compared to 3 mm), making it an even more rigid and robust instrument than the Freiberger. The bare sextants weigh 1300 and 1200 grams respectively.

Nearly all bearings in the nautical sextant move slowly, are lightly loaded and are somewhat inaccessible, so these need grease, which is essentially soap and oil. The mixture is thixotropic, which is to say it is thick and sticky until sheared, when the oil is released and lubricates the bearing. There is some concern, perhaps largely theoretical, that additives of phosphorous and sulphur compounds may attack bronze and brass, so I suggest a waterproof lithium-based marine grease for everything except the rack and worm.

Grease here would mix with grit and dust to form a moderately effective grinding paste, so all manufacturers who took the trouble to mention it recommended brushing of the rack followed by the application of a few drops of oil. This is then distributed by winding the worm from one end of the rack to the other, followed by brushing off the excess.

While clock oil is sometimes recommended, perhaps because it does not evaporate to form gums, my experience is that it seems to be too thin, and gives an impression of metal to metal contact when rotating the worm. If the oil is too thick, the oil film may vary in thickness, depending on loading and speed of rotation. After some experimentation, I have settled for a monograde SAE 30 “spindle oil”, the sort of stuff that is sold for use on rotating agricutural machinery and in lathe bearings. This gives a silky smooth feeling, with none of the fine crepitus a sensitive hand may detect when there is no lubrication or the lubricant is too thin.

This advice may well have to be modified if the sextant is to be used at low temperatures. I have no experience of this, so your own experience with low temperature lubricants will have to be your guide