The Nautical Sextant

Large numbers of sextants were required for the officers of these ships and contracts were let to several manufacturers who had not previously been involved in sextant production. Among them were Leupold and Stevens, who were noted pre-war for making water flow gauges and telescopes. One of their sextants, bearing an inspection certificate for April 1945, recently came into my hands.

Unlike the BuShips Mark II instrument, the merchant service sextant was conventional and relatively up to date, with a gesture to wartime austerity, in that it was fitted with only three index and two horizon shades, giving six and three possible combinations of shade respectively. The paintwork seems not to have stood the test of time as in the two examples I have seen, it had disappeared almost completely from the instrument (Figure 1). Traces of  paint were present here and there, revealing that the frame had been painted with a smooth semi-matt finish and the rest in a wrinkle finish.

Figure 1 : Paint-free sextant as found

Figure 1a : Rear (RHS) of sextant as found

The frame, of ladder pattern, at first sight looks very much like the Pioneer-Bendix Mark II’s, except that it made conventional provision of tabs for mounting the shades, horizon mirror and telescope, rather than the rather antiquated attachments of the Mark II. However, the frame is a high quality sand casting in bronze rather than a composite of aluminium alloy with bronze rack. The placing of the rack is conventional and the micrometer mechanism copies closely the design of Henry Hughes and Son. The standard tapered index arm bearing  is concealed beneath an alloy cover that serves as a third leg while the elegant black Bakelite handle is swept back at an ergonomic thirty degrees, unlike that of most sextants of the period (Figure 2). There is an interesting little rest for the thumb.

Figure 2 : Rear (RHS) view of restored sextant

 The index arm is a stout aluminium alloy die casting, with an integral stiffening rib. The words “U.S. Maritime Commission” and “Leupold and Stevens Instruments Portland Oregon” appear in cast-in raised letters. The front end of the index arm expansion has a bronze bush that forms the bearing of the very simple Hughes-pattern swing arm. The latter is also an aluminium casting with bronze bushes for the micrometer shaft. The cam of the very simple release catch bears on a spring loaded extension of the swing arm, and an L-shaped spring bearing on the tip of the worm provides axial preload to the micrometer shaft (Figure 3). The micrometer drum is of brass and the micrometer vernier is divided to 10 seconds.

Figure 3 : Micrometer mechanism

Figure 4 : Keeper detail

Of interest perhaps only to the true lover of detail is the form of the index arm keepers. Attached by a single screw and steadied by a dowel, the tongue that engages with the slot in the edge of the limb, to keep the index arm from lifting, is slit, so that by spreading the tongue slightly, a very close fit indeed can be obtained (Figure 4).

Figure 5 : Index and horizon mirror mounting detail

The shades and mirrors are mounted conventionally, but the latter show another interesting thought for detail. Each mirror is provided with a backing of thin, springy sheet brass that is interposed between any adjusting screw and the mirror back, thus preventing the paint film on the back of the mirror from damage that might allow access of sea water to the silvering (Figure 5), this at a time when it was more usual to insert a sheet of thin card or nothing at all.

The 2 1/2 power telescope has a field of view of 6.5 degrees and, at 120 mm long, is unusually long for a Galilean sextant  ‘scope, but is none the worse for that. The 30 mm aperture objective lens gives a bright and clear image, as might be expected from a specialist telescope maker. It  does with two lenses nearly what the Mark II telescope did with seven. Provision is made for rise and fall with a conventional vee and flat slide.

Figure 6 : Case inside

Figure 7 : Front of case

 The sextant was provided with a well-made pine wood box, stained reddish brown, with drawer dovetails front and back, brass piano hinge, locking latches and a very substantial brass handle. The sextant was retained in the case by an aluminium pocket with a cross latch, and the legs sat on alumnium pads to protect the bases of the box. The pocket has been sprayed with a felt-like green paint which, unlike the rest of the paint work, did survive sixty-five years (Figures 6 and 7).

The final photograph shows the instrument restored to a near-new condition (Figure 8). We may surmise that at least 2000 were made by the end of the War.

Figure 8 : Fully restored instrument