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 and “An Hungarian Sextant via Bulgaria.”

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 nineteenth 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 such as the Sunderland flying boat, 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. I have seen an example from 1942 in a mahogany case with otherwise identical furniture and layout.

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 3 : Front (left hand) face of sextant.

The telescope has no rise and fall mechanism and is attached to the frame by a single screw that passes through a boss in the base of a shaped column. A dowel pin locates it so that it points in the correct direction. This pin can be seen above the boss in the close up photograph of the telescope column in Figure 4.

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.

In the view of the micrometer mechanism (Figure 7, below), note the fine pitch of the worm which allows a full-size drum to be used. The worm shaft has a tapered thrust bearing and a parallel portion, which run  in a single block of bearing, with preload applied by a U-shaped spring. The worm shaft is in two parts, to allow assembly. The release catch operates a cam which swings the assembly out of mesh with the rack.

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. In the 1942 sextant mentioned above, the shades mounting follows Hughes’ standard practice.

Figure 8 : Index shades mounting

The horizon shades are mounted on a single shoulder screw and are separated by red fibre washers that fit tightly on the screw. A Belleville washer, which behaves as a short, stiff spring, provides friction and the screw is locked by a shallow nut (Figure 9).

 

Figure 9 : Horizon shades mounting.

Figure 10 shows the bare frame of the instrument.

Figure 10 : Bare frame.

A kind French correspondent, whose name I have unfortunately lost, sent me a photograph of a similar sextant in his possession (Figure 11). It is identical except that it is named Heath and Co. and carries a telescope in the style of that company. It may be that the Ministry of Aircraft Production or the Air Ministry during the exigencies of World War II imposed some degree of cooperation between the two makers, or perhaps Heath and Co acquired some instruments as war surplus and added their own name and telescope.

Figure 11 : Heath and Co. seaplane sextant.