The Nautical Sextant

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A few months ago, I acquired an unusual little sextant, but was only recently able to collect it in Europe and bring it home to New Zealand for cleaning and overhaul. According to the seller, it had been found in the attic of a merchant seaman who had been active in the 1950’s, but there was no other information about its origins. It bears the name “Lefebvre-Poulin, Montrouge, but the naming of French sextants is rather confusing. Poulin made sextants in the second half of the 20th century in Montrouge in the south-west suburbs of Paris, but Jules Lefbvre was active in central Paris in the latter half of the nineteenth century, so it is unlikely that there was an association between the two makers, and the name of Poulin is sometimes associated with Blanchet.

Apart from its small size, there are one or two other unusual features such as its handle and its micrometer drum, which spans two degrees. Figure 1 shows a front view of the un-restored instrument and Figure 2 gives a rear view. You can get a magnified view of all figures by clicking on them. Return to normal by using the back arrow.

Figure 1: General arrangement, front view.

Figure 2: General arrangement, rear view.

To help readers who have not yet had the wisdom to buy my book, “The Nautical Sextant“, in Figure 3 I show the restored sextant with its main parts labelled.

Figure 3: Main parts of the sextant.

At the heart of any sextant is the frame and its bearing for the index arm. In this case, the frame is of an aluminium alloy with a cast-in bronze rack for the micrometer and a bronze bearing for the index arm. The latter also serves as a point of attachment for the unusual handle (Figure 4).

Figure 4: To show handle.

The hexagonal alloy handle screws over the index arm bearing (Figure 5) and is locked in place by two Allen grub screws. The bearing itself is attached to the frame by three brass screws.

Figure 5: Index arm bearing, and handle attachment.

The vast majority of sextants ever made had tapered index arm bearings, but as micro-finishing of plain parallel bearings advanced in the second half of the twentieth century, C Plath and Observator made use of the new technology in their sextants. As Figure 6 shows, Poulin followed suit in this sextant. The parallel steel journal rotates in the bronze bearing (strictly speaking, the bearing is the enclosure in which the shaft or journal rotates), and is secured against axial movement with a phosphor-bronze spring washer and a brass screw. No provision is made for adjustment as no wear is to be expected in the slow-moving, lightly-loaded bearing.

Figure 6: Exploded index arm bearing.

Figure 7 shows the rack with a view of the micrometer mechanism. The bronze rack appears to be cast in to the frame and its pitch is relatively large for its radius of about 80 mm, so that one turn of the micrometer worm advances the sextant reading through two degrees. Henry Hughes and Son in their WWII half-size seaplane sextant reduced the pitch of the rack  so that they could use most of the components of the full-sized micrometer mechanism, needing to modify only the pitch of the worm to match the rack. Their micrometer advances the sextant reading through one degrees per rotation of the worm (see my post of 26 September 2011).

Figure 7: To show the rack.

The micrometer mechanism is of conventional design (Figure 8). The lower end of the index arm carries a post for the bearing of a swing-arm chassis to which the plain parallel bearing of the micrometer worm shaft are attached. A large leaf spring (radial preload spring) holds the worm in contact with the rack and a simple cam allows the worm to be swung in the plane of the frame out of contact with the rack against the spring pressure. This allows the index arm to be moved rapidly into position before releasing the catch to re-engage the worm for final adjustment. A smaller leaf spring (axial preload spring) bears on the end of the worm shaft to take up any axial play in the bearings. Also shown in the figure are the two keepers, which prevent the index arm from lifting away from the front of the frame.

Figure 8: Micrometer mechanism.

Since one rotation of the worm with its attached micrometer drum advances the reading through 2 degrees, the drum is divided 0 to 60 minutes twice with subdivision to minutes (Figure 9). The main scale on the limb and the micrometer drum are divided to single degrees and minutes respectively, with alternate tick marks being long (Figure 10). At first sight, this gives the impression that half-degrees and minutes are being shown and it might have been easier to read had the normal practice been followed of making the 5  tick mark longer than the others.

Figure 9: Micrometer drum divisions.

Figure 10: Main scale close-up view.

The shades are unremarkable except that there are only two index shades instead of the more usual three or four and one horizon shade instead of two or three. No provision is made to prevent the index shades rotating together, but as there are only two of them this does not pose much of a problem. The two shades together give adequate reduction for viewing full sun and the single horizon shade also gives adequate reduction of glare beneath the sun.

The mirrors and their brackets are conventional. The horizon mirror is fully silvered, but there is adequate overlap of the direct and reflected images when viewed through the 4 x 25 mm Galilean telescope; though the field of view is somewhat restricted, I found no difficulty in finding the sun, though with star sights the story might be very different.

I have not been able to discover for whom this instrument was intended. Its small size and simplicity suggests it might have been aimed at yachtsmen, for whom storage space may be at a premium. It may also have found the occasional sale among surveyors and explorers. A merchant marine officer is unlikely to have wished to be seen using anything other than a full-sized instrument. It came to me without a case. Monsieur Hervé Le Bot has kindly provided me with a photograph of the case interior, which is shown in Figure 11. Unusually, the sextant is stowed in its case upright, in a socket which accepts the hexagonal handle and which has cheeks to prevent it from rotating.

Figure 11: Interior of case (courtesy of M. H Le Bot)