Sounding sextants 2

6 05 2009


The Survey Sextant Mark III, by Kelvin and Hughes, was designed specifically as a sounding sextant, rather than being an ordinary sextant without index or horizon shades. It has a cast aluminium alloy frame with a stainless steel worm running in the rack, which is one with the frame. The limb is divided to 140 degrees. The micrometer reads to single minutes. There are no shades nor is there anywhere to put them.

In the preceding post about sounding sextants, I pointed out that most of them were not provided with handles appropriate to their normal use with the frame horizontal, nor with legs placed so the instrument could easily be set down without changing hands. This sextant has a solid mahogany wooden handle attached only at the top by four very substantial screws, but it is still better adapted to vertical rather than horizontal use. The legs, however, can be unscrewed from the back and replaced in tapped holes in the front of the frame, circled in white in the photo above, so the instrument can easily be put down.


The Galilean telecope has a large objective lens of 45 mm diameter to give a large field of view, and a magnification of   x3.  It is very solidly attached to the frame via a spigot and three screws, and there is no rising piece nor provision for adjusting collimation. At the level of precision required of this class of sextants, the telescope mounting would have to have been very badly made indeed to have been significantly out of collimation, and Henry Hughes and Sons always produced instruments with fine workmanship.

The large, circular, sealed index mirror, 55 mm in diameter,  has no provision for adjustment either, but machining surfaces that are truly at right angles to each other is one of the more elementary operations in the engineering workshop. The  circular “horizon” mirror, 35 mm in diameter, is also sealed and has provision for adjusting out side and index error, more details of which will be found in my book, The Nautical Sextant.

Hughes and Son were granted a patent in 1928 (UK Patent 309,648) for a method of engaging and disengaging the worm from the rack, by swinging the worm out of the plane of the rack. They do not seem to have used it prior to WW II, but did make use of it in this sextant and in their handy little double sextant.

Its light weight certainly makes it easy to hold, but it has an annoying feature in use when taking angles greater than about 80 degrees: the horizon mirror “sees” past the edge of the circular index mirror over one’s right shoulder, as well as providing the image reflected from the index mirror, so that there are three images to sort out. With practice, though, one soon learns the position and form of the three images and to ignore the over-the-shoulder one.

This instrument’s former home was in the Solomon Islands and may indeed have been used by one of my uncles when he was Marine Superintendent in Honiara in the 1950s. It was not in good condition when it reached me and it had no case. It seemed to me fitting to make the latter out of the tropical hardwood kwila, which is found in the Solomons, and I followed Hughes’s style in putting the handle on the side and having the hook latches (cut by hand from 2.5 mm brass) arranged so that gravity keeps them both in place. The whole instrument has been repainted. The graduations and labellings were then cleaned out and re-filled with white paint.


If you love detail or simply would like to know more about the structure of your sextant, you will find much more along these lines in The Nautical Sextant . Next in this series will be an account of a double sextant by Henry Hughes and son.




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