The Nautical Sextant

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

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.