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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.

Figure 2 : 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.

Figure 4 : Handle 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 : Sextant and case before restoration.

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 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.

Figure 10 : Front view of horizon mirror.

 

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 : Magnifier mounting.

 

 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. 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

 

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 thsi 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 : The sextant restored.

 

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

Freiberger Präzisionsmechanik of the then German Democratic Republic, produced three sextants over the years: the uncommon Skalensextant; their main line, the Trommelsextant, aimed at professional navigators; and in about 1980 they introduced their Yacht sextant. The latter was aimed at “Western” yachtsmen in competition with Plath and Tamaya at a price of around US$600, when a full-sized sized sextant by these makers could scarcely be had new for under US$1000. It had few features in common with its big brother, the Trommel or Drum sextant which retailed for about US$200 more, but presumably the pressure die cast aluminium alloy frame with rack cut directly into it, that they both had, was one feature that enabled Freiberger to undercut their Western rivals, or it may be that they were heavily subsidised.

In a contemporary handbook, written in a charming Engleutsch, the Yacht sextant was “destined for use on sporting and luxury ships… with conditions and requirements for accuracy …completely maintained though weight and volume of sextant have been reduced  down to approximately half their former values. In consideration of aspects of beautiful shape and representativity, graduated arc and arc webbing have been modified that dimensional stability at extreme temperature differences and mechanical stress can be expected at the same time.”

In my description that follows, I will comment mainly on points that distinguish this sextant from the conventional, touching only briefly on parts that follow standard practice.

General arrangement (Figure 1)

The frame is certainly austerely functional and the limb has a radius of 142 mm against that of the drum sextant, which is about 170 mm, while its weight with its permanently mounted telescope is 860 G, compared to, say, the 1300 G of an alloy-framed Tamaya with a star telescope mounted. There are only three index shades and two horizon shades, quite enough for most purposes. The mirrors, which are interchangeable, both measure 59 x 29 mm. This is not the advantage that it might seems, as the horizon mirror is fully silvered, that is to say, there is no plain glass half, the horizon being viewed directly rather than through a thickness of plain glass. The edge can be fully protected with paint, whereas the junction between the silvered and unsilvered parts of the traditional mirror was always a weak point for sea water to attack the silvering. However, each half of a Galilean telescope “sees” only its own field of view: if half of the objective lens is covered over, half of the field of view is lost. With the tradional arrangement of mirrors, some of the light from the index mirror is reflected off the front surface of the plain part of the horizon mirror into the telescope, so that the images of the body and of the horizon to a large extent overlap. With the fully silvered mirror and a Galilean telescope, there is very little overlap and this can make for difficulties for  an inexperienced or occasional observer.

Figure 1 : Front view of Yacht sextant.

 The arc is a  sliver of screen-printed aluminium secured to the frame with glue and two pins which, rather surprisingly are made of steel. Predictably, in my example which has seen a lot of sea use, they had rusted and the glue had also given way. The telescope is a 2.4 x 22mm Galilean that focusses by rotating the objective lens mounting rather than the eyepiece and is  less prone to go out of focus as the eye presses against the scope. The eyepiece is provided with a deep flexible rubber pad, a perhaps necessary adjunct for a lightweight telescope used on board a pitching and rolling yacht.

Figure 2 : Rear view of Yacht sextant

The rear view (Figure 2) shows that Freiberger retained the same arrangement for the index arm as in the Trommelsextant (and the Soviet SNO-T) siting it behind the frame out of harm’s way. To avoid having to have a bridging piece, the black anodised alloy handle is attached to the frame only at the top, by three screws. The release catch at the lower end of the index arm swings the worm out of engagement in the plane of the rack and, as will be seen, has a much simpler ( and undoubtedly cheaper) arrangement than in its larger brother.

Index arm bearing

Figure 3 : A cornucopia of screws.

Removal of the handle reveals a sextet of screw heads beneath. Two pass through the bearing (strictly speaking, the journal) and attach the index mirror mounting to the deep flange on the other side. Four attach the index arm to the journal (journal : the part of a shaft that is enclosed by a bearing). Removing these allows the journal to be withdrawn. The fit is necessarily a very close one and care has to be taken that it does not capsize and jam. Figure 4 shows the bearing dismantled and it is seen that the large parallel bronze journal runs directly in the frame. There is no provision for taking up wear as none is to be expected in a slow-moving part with such large bearing surfaces.

Worm and release catch

The Trommelsextant swung the worm out of engagement with the rack by having a relatively complex eccentric bearing that rotated the worm in its bearing out of the plane of the rack. Elegant though the method was, it must have been very expensive to produce. In the Yacht sextant, Freiberger have managed to give a large degree of protection to the worm by enclosing a substantial bronze swing arm with integral worm shaft bearing in a U-shaped alloy casting, that is attached to the index arm via a tongue and three screws (Figure 5). The shaft, immediately on emerging from its bearing, is enclosed by the micrometer drum and the knob for rotating it, so that a common consequence of dropping the sexant, a bent worm shaft, is avoided.  

Figure 7 : Swing arm details.

 

Shades

The horizon shades are mounted conventionally, but the index shades are mounted directly into the frame, as shown in Figure 8.

 

Figure 8 : Mounting of index shades

The three shades are separated by two washers that have an internal tongue which engages with a longitudinal keyway machined in the screw. The washers cannot rotate, so movement of one shade cannot be transmitted to the others. At one side of this club sandwich is a spacing washer and at the other a Belleville washer. This is a domed washer that has the characteristics of a short, stiff spring. The screw passes through the complete sandwich into the frame. One might expect that it could be used to adjust the resistance  of the shades to unwanted movement, but once inserted, it is locked against rotation by a transverse steel (!) pin. The pin of course could be left out after overhaul, supposing that it can be safely removed from the rather slender and emminently breakable projection of the frame through which it passes.

 

Adjusting screws

The screws themselves are standard M2 screws with squared heads. They pass through  collets that are threaded on the outside and which screw into bosses projecting from the back of the mirror mountings. Each collet has a tapered tip that fits into a matching female taper at the bottom of the boss, so that when the collet is tightened, it closes up around the screw, thus increasing the resistance to the screw’s rotation or locking it if required (Figure 9). There is a clearance hole through the mirror bracket for the screw.

 

Figure 9 : Adjusting screw and tool.

 

Though the adjusting screws for the SNO-T and the other Freiberger sextants are superficially similar, anyone who treats them the same is in for an unpleasant surprise; and jammed, broken or stripped screws are a common occurence. In these sextants,  the collets which have coarse threads on the outside are not split, and the adjusting screws pass through threaded holes both in the collet and in the back of the bracket. The screws are locked by unscrewing the collet a little.  The screw can be withdrawn by rotating the collet a little to find the position at which the screw is unlocked. Any attempt to fully unscrew the collet before the screw has left the thread in the mirror mounting inevitably causes damage.

 

If you have enjoyed reading this account, you will I am sure enjoy reading my book, The Nautical Sextant, available through booksellers, from Amazon or direct from the publishers, Paradise Cay and Celestaire. Intending buyers in Australia and New Zealand may find it interesting to Contact me, as I am able to offer them a discount on the published price.

 

 

 

 

 

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