The Nautical Sextant

The preceding posts covers “A C19 Sextant Restoration” , “Making a Keystone Sextant Case” and “Restoring a C. Plath Drei Kreis Sextant” .

Shortly after I completed the restoration of a Plath Drei Kreis (three circle) sextant I was given the opportunity to restore an English sextant from the same period, a Heath Curve-bar sextant, so called because the frame appears to be made up of thin, curved bars (but is in fact a bronze casting). This one received a certificate from Kew Observatory in 1898 and in addition to being named “Heath and Co., Ltd.,  Crayford, London S.E.” also carried the owner’s name in full engraved in on the limb. As was usual in 19th century instruments, the maker’s name was in copperplate script rather than in the later type.

Like the Drei Kreis, the arc is divided to read to 10 seconds with the vernier and the radius of the arc is much the same at about 165 mm.

Apart from there being widespread verdigris, this sextant posed no particular problems as everything was intact, with no missing parts, so I will not bore you with the details of restoration. It was a simple matter of dismantling it to its parts, cleaning off verdigris, polishing screw heads and re-lacquering. The case needed only filling of a large shrinkage crack in the bottom and cleaning up the handle and latches. Unfortunately, at some time the arc had been polished, so the divisions are in places rather hard to read. While some nineteenth century sextants were chemically blackened, I found traces of black lacquer on the little caps that cover the adjusting screws. The next photo shows the instrument cleaned and reassembled while I waited for the owner’s instructions about re-lacquering. 

This photo shows the finished article:

 I thought this an ideal opportunity to compare an English with a German vernier sextant of approximately the same period, at the turn of the twentieth century.

In the preceding post I mentioned the complication of the tangent screw mechanism in the German sextant. In the Heath instrument, it is perfectly conventional and typical of a mechanism that seems to have served makers and navigators well for over a hundred years.

In the photo above, the sliding block moves in a curved slide formed by a cut-out in the index arm expansion. The floor of the slide is formed, as in the German sextant,  by a plate that is screwed on to the front of the index arm. The end of the clamp screw bears on a small plate to which is attached the clamp leaf spring. The tangent screw is held captive on the index arm by a bearing. The tangent screw has the familiar squared end, and the knurled knob is attached over the square by a central screw which allows the bearing to be adjusted to remove end play. The squared end prevents rotational forces from being applied to loosen the screw. The sliding block is held against the floor of the slide by a leaf spring, seen in the next photo, of the front of the index arm expansion. The screw that passes through the leaf spring holds the nut on to the sliding block. This spring has the same function as the leaf spring on the rear of the sliding block of the German instrument.

The nut is split. This allows the nut to spring inwards and to grip the tangent screw thread flexibly and remove lost motion between the screw and the nut. When the clamp is tightened, the sliding block is held immobile on the limb and when the tangent screw is turned, the index arm is caused to move via the screw bearing.

While the German arrangement is free from backlash when in good adjustment, if there is any stickiness in the slide, it has the annoying feature of moving in one direction, but not returning by means of the spring pressure. In the other arrangement, if the fit between the tangent screw and the nut is not good, due, say, to wear in the screw, or if the bearing is out of adjustment, then there is backlash in both directions. In both types, if the slide is sticky, movement may be uncertain and jerky. In my view, each defect is as bad as the other, but the English mechanism has the merit of simplicity.

In every edition of his work “Wrinkles in Practical Navigation” (and it ran to 21 editions ) Captain Squire Thornton Lecky wrote “An indispensable condition in a sextant is regidity; flexure is fatal…”, so I thought to make a quick comparison of the two sextants, using the crude set up shown in the next photo. The middle 100 mm of the limb is held clamped firmly in a vice and a dial  indicator, reading in 100ths of a millimetre bears against the end of the index arm bearing. A force of one kilogramme is applied to the index mirror by means of an old  kitchen scale and the amount of deflection read off the dial indicator.

Though the German sextant at 1.5 kg weighed about 250 G  more than the English one, there was little difference in the amount of deflection, about 0.6 mm. Interestingly, two mid-nineteenth century English quadrants of radii 20 mm greater and of about the same weight, deflected significantly less at 0.5 mm for a Spencer, Browning and Co., and 0.4 mm for a Crichton. Late twentieth century die-cast aluminium alloy instruments typically deflected about 0.12 to 0.15 mm in this set-up and bronze-framed ones somewhat more. Allowance must of course be made for the smaller radii of the modern instruments, but even so, the superior rigidity of alloy instruments is clear. Neverthless, right to the end some buyers expressed a preference for the heavier, more expensive but less rigid bronze instruments.

The preceding posts covers “A C19 Sextant Restoration” and “Making a Keystone Sextant Case”

On Christmas Eve of 1974, The small city of Darwin in Northern Australia was struck by Cyclone Tracy, a very compact and intense cyclone with winds gusting to well over 220 kph (140 mph). Eighty percent of the houses in Darwin were totally destroyed. One of the casualties of the destruction was a sextant that in the aftermath found its way into a trunk with several other sextants and theodolites. The trunk was sold at auction for $20, the sextant was passed to a neighbour, a retired merchant sea captain, and he in turn recently passed it on to me. Such generosity must be very rare.

 The sextant had escaped major damage. Though some screws were bent or missing, they could easily be repaired or replaced. Mild corrrosion with verdigris was  present, but the frame and index arm had been carefully cleaned without polishing the arc, which was in good condition. Fortunately, in the bottom of the trunk were a sighting tube and  two telescopes. The inverting ‘scope was missing  the objective lens, but my friend in Darwin went back to his neighbour’s trunk and found it in a corner. It was intact, but the balsam between the two parts of the achromatic lens had perished and the lens was unusable in that state. Apparently, it was this that had saved it from being converted into an air-rifle sight.

 The next picture shows the sextant as I received it. The bottom screw that attaches the handle was missing and the clamp screw was bent. The lacquer on the telescope tubes had deteriorated to a powdery and patchy brown and the draw tube of the inverting telescope had seized completely. The clamp spring was absent and one of the dowel pins that held it in place was damaged. One of the legs had been replaced by an odd leg. The ground glass diffuser screen above the vernier scale had disappeared. There was, of course, no case, so I felt very lucky to have a complete kit of telescopes and sighting tube.

The famous name “C Plath, Hamburg” was clearly visible on the middle of the limb, and after a little cleaning of the bronze at the right hand end I found  the original tiny Plath logo of a little stick man with a sextant.

Less easy to see was the serial number 5499 on the silver of the arc. The arc is divided from  – 5 to +157 degrees, but the scale is usable up to only 138 degrees 30 minutes,  so that although it was sold as the Drei Kreis (“three circle”) quintant, it falls a few degrees short of 144. The serial number places it a little before 1910 in date, on the basis that Plath made an average of about 320 sextants a year between 1910 and 1925.  The arc of about 175 mm radius is divided to 10 minutes, with the vernier allowing reading to 10 seconds, in principle if not perhaps in practice. Apart from the rather complex tangent screw mechanism, the construction of the sextant is perfectly conventional.  The next photo shows the rear view of the sextant as received.

The next step was to dismantle the instrument to its component parts, clean them in a solution of 50 percent household ammonia with washing up liquid added, rinse, dry and apply masking tape as in the next photo, prior to spraying them with black lacquer.  Although antique sextants are often found with all paint removed and the “brass”  (in fact, bronze) polished to a shine, they were never sold this way. Most often they were painted or lacquered black, or, with earlier ones, chemically browned or blackened. If the silver scale is unreadable owing to tarnish, it usually yields to the diluted ammonia and gentle rubbing with the pad of a thumb. On no account should it be polished, as the markings are very shallow and can be obliterated completely by vigorous polishing. I have one 1920s sextant in which only the numerals remain and am currently restoring one in which the markings over about 20 degrees of the scale are very faint.

Here are the parts looking distinctly refreshed.

I had to make a new leg and this slightly un-sharp photo shows the turning in progress to make the rather slender, tapered leg. Once parted off, a thread then had to be cut on the wider end. Fortunately, all the threads are metric ones.

Although the threads are standard, some of the screw heads are not, so I had to make a new screw to attach the handle. This photo shows it being parted off from the parent 12 mm brass bar.

The clamp screw was bent, but I was able to persuade it back to straightness. The tangent screw mechanism is rather complicated when compared to the standard nineteenth and early twentieth century arrangement, and I hope the next two photographs will help to make its workings clearer. It came into my hands just too late to be included in the print edition of my book, now in preparation.

The clamp is a thin slab of brass that is retained in the sliding block by two dowel pins that allow it some up and down movement, opposed by the leaf spring (ground to shape from a piece of clock spring and soft soldered to the clamp). The clamp screw locks it and the sliding block to the lower edge of the limb. The sliding block is hollow and contains a strong helical spring and its guide, the latter projecting from one end of the block. The other end is split and tapped for the tangent screw. A tongue, which is attached to the underside of the index arm expansion by a screw, is sandwiched between the tip of the tangent screw and a bronze block on the end of the guide. Two screws can be tightened to close the split in the sliding block a little, to ensure a snug fit of the tangent screw.  The sliding block is guided by two curved edges machined in the index arm expansion and is prevented from lifting away from these edges and the underlying surface by the sliding block spring, itself attached to an elbow bracket.

When the clamp screw is tightened, the sliding block is locked to the limb and when the tangent screw is tightened, it moves the tongue and the index arm to which it is attached. When it is slackened, the helical spring moves the index arm in the opposite direction. The two screws that are intended to remove backlash (lost motion) from the tangent screw are unnecessary because the opposition from the helical spring does this. They were a nuisance as far as I was concerned because they both lost their heads when I was trying to remove them and needed about half an hour’s work to drill them out and re-tap forslightly larger screws, which I  had to make from scratch. When a spring-opposed mechanism like this works, it is a pleasure to use, but dirt or a breakdown in lubrication can easily cause it to fail to return in one direction, and this is very annoying.

The rise and fall mechanism for the telescope is conventional:

Having overcome these various obstacles, it was time to reassemble the instrument and adjust the mirrors to ensure they were square to the plane of the arc and, when the scale reads zero, that they were parallel to each other. The paintwork is not quite as shiny as this flash photograph seems to suggest.

This still left the inverting telescope, with its seized draw tube and damaged objective lens. I tried various combinations of spirituous chemicals, releasing compound, heat and cold, but could not persuade the rather thin-walled tubes to release their grip on each other. Vigorous twisting was of course out of the question. In the end, I removed all the optics, pressed out the various light stops and machined a steel slug that fitted snuggly in the wider tube. I then used my larger lathe as a press jack to push out the narrower tube. In the next photograph, the tube is held loosely in the chuck against the shoulder and the steel rod held in the drill chuck is being pressed against the slug, which is out of sight inside the tube. It worked like magic.

The objective lens was even more of a problem, as it was swaged into a little brass cell. Swaging means that a lip of metal is bent over the lens to hold it in a recess in the mounting, and often the only way to get the lens out is to somehow hold the mounting in the lathe and carefully turn away the swaged metal. I managed to do this with only minimal damage to the edge of the lens, and still had enough of the mounting left to make replacement possible. Replacement lenses of the correct focal length and diameter are very hard to come by, so this was a relief.  Once the lens was out, I heated it slowly in a pan of water and, just short of boiling point, was able to slip the two components apart. A little xylene, in which Canada balsam is soluble, cleaned up the two part-lenses and I recemented them with Ultra Clear Araldite, rather than the now-obsolete Canada balsam or one of its very expensive modern replacements. I also used a tiny smear of the same adhesive to remount the lens in its cell. The telescope now gives a very clear view. The next photo shows the rear view of the sextant with restored telescopes, new adjusting pick and a tiny screw driver made from scraps of silver steel, brass and mahogany. The brush is C19 English, but does not seem out of place with this German instrument.

Some woodwork came next. Although I am slow at making dovetails, I can now do so without disaster. The sextant was supplied originally in a box with machine-made comb corner joints, but dovetails with narrow pins are also in period. These can just be seen in the next photo, which shows the interior fittings too. The wood is Sapele (Entandrophragma cylindricum), an African hardwood that closely resembles mahogany.

The photo following shows the instrument and its accessories in place:

The final photograph shows the exterior of the case. I don’t know what contemporary latches and handles would have looked like, so have copied English hook latches of the same period and made a handle that I hope looks slightly Germanic. It is made of five pieces of 6 mm  brass rod silver soldered into a whole. I used French polish for the final finish. I can recommend Zinsser Bulls Eye French polish. It is easy and quick to apply, and an amateur like me can get a good finish with it.

Do let me know if you would like more details of this or any other sextant I have covered on this blog.

If you have enjoyed this post, you might enjoy my book, now in print and available from the publishers or through amazon.com

The preceding post covers “A C19 Sextant Restoration”

Looking around me at my collection of nautical sextants I see that about forty percent of them came to me as homeless refugees from the internet. Eight now have cases with dovetailed corners and two with rebated and pinned corners. Recently, I attempted to reproduce a keystone case, that is to say, one that looks as if it has been cut as a wedge from a round cheese. Two problems distinguished the task from the making of a square case: making the bowed front of the case and cutting dovetails with narrow pins at angles other than 90 degrees.

Seven cases with four corners each amounts to a lot of practice at cutting dovetails, so I felt that I could overcome the second problem relatively easily. Everything hinged on making the bowed front, so I tried that first. I had a ninetenth century case as a model to guide me and the thickness of its walls is about 8 mm. A plank of that thickness, 150 mm wide and 350 mm long cannot be bent easily, but some guidance from the internet suggested I had only to steam it and it would be as putty in my hands. It wasn’t. Half an hour steaming in an improvised steamer resulted in warping of the wood, but bend it would not.

Having a 50 mm plank of Sapele (an African wood similar to mahogany) reduced to three 10 mm planks had left me with some of only 2.6 mm thick, so I tried steaming these and then clamping them in a mould while they cooled down. The mould is made from slices of thick tri-board board cut to (nearly) the same shape and glued together, with aluminium foil glued to the surfaces to protect them from the steam and water. It took quite a while to smooth the large concave surface to shape with a spokeshave, so long in fact that I contemplated buying a compass plane – until I discovered the price. The convex surface was large enough to make attack with a bench plane quite feasible.

The thinner wood was much more amenable to steaming and bending, but it didn’t stay very bent on removal from the mould, though it did acquire interesting three dimensional shapes, combined with splits. My third attempt was much more successful. I simply applied glue to the boards, laminated them and squeezed them to shape in the mould. Once the glue had dried, there was only minimal “spring back”. Once I had planed the edges, it was not possible to say that the proto-front had begun as three pieces.

I did not anticipate much trouble with the front dovetails as, except for the spring-back, the angles were about 90 degrees. However, I had never made any with the traditional narrow pins (the bits that fit between the tails). I don’t know why the pins were made narrow, rather than about equal in width to the tails. They do seem to be more elegant, but perhaps that is just a matter of taste.

This isn’t a woodwork site so I won’t describe how to make dovetails. There are several blogs that will give you a little video on how to cut dovetails. Usually, the demonstrator shows how to mark out and then makes rapid saw cuts that miraculously result in joints that instantly fit together perfectly and without any need to pare with a chisel. I do need to use a chisel, sometimes quite a lot, and sometimes I need a putting-on tool, something that does not exist in the engineering field but which in the woodworking  field is called mahogany paste. I find that it does help when marking out to use a marking knife for all cuts and then to cut out little 90 degree triangles into the waste wood using a chisel. They seem to help guide the saw and also provide a clear guide for the paring chisel that follows.

The rear corners are at an obtuse angle and I relied on my model case to give me the angles to mark out the dovetails. Sometimes my saw went a cut too far, but I was able to disguise this with mahogany paste after gluing the bits together. Clamping them cannot be done with any clamp that I possess, so I resorted to the old trick of using webbing. The blocks of scrap wood seen in the next photo serve the dual purpose of tightening the webbing as they are slid towards the corners as well as concentrating the clamping force near the corners. It is best to sit the carcase on a flat surface and “persuade ” it to sit flat before the glue dries. A sextant can be seen in the background, watching its new home being built.

I didn’t have any wood wide enough for the top and bottom of the case, so I glued pieces together edge to edge. When doing this, it is of no use to hope that glue will do the job without proper preparation. It is essential that the edges be both straight and square before rubbing them together with a layer of glue between and clamping them. It is I think  fair to say that the joint I made is invisible. Once formed and cut to shape I attached the top and bottom to the walls with glue and brass pins that were then punched below the surface, and the heads buried under mahogany paste. The next photo shows the new case in the rough, sitting atop its model.

The whole was now rigid enough to be put in the vice and the edges and corners finished by planing and sanding. It then had to be cut into two parts. I now use a hand saw for this and a Japanese dozuki saw is hard to beat. Those who have a bench saw might use this, but I have had one or two near disasters and now rely on the Mark I eyeball and hand. After dovetails, fitting hinges and producing the simple cut outs for the various pockets inside the case feels very easy. The sextant is looking very interested…

so in he jumps…

and closes the lid.

I finished the case by careful sanding, followed by a coat of red mahogany stain and wax. Given another hundred years of waxing and polishing, it should have acquired a beautiful patina. Now, if I ever sell this sextant, should I say that the case is not original? It is not exactly a fake, nor is it an exact copy. Is reproduction the correct word?

My comprehensive book on the nautical sextant has a chapter on cases, Christmas is coming and, as my grandchildren would say, I really, really need to buy a new sextant. Won’t you help me towards this goal by buying a copy of my e-book?*

* Now available only as a revised and extended print version (see Buy the book)

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

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

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

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

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

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

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

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

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

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

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

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

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

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