Sounding sextants 3

16 07 2009

Hughes and Son Double Sounding Sextant

A post on another  obsolescent navigation instrument seems to be apt, against a background of a newspaper report from Honolulu:  According to the report, in February this year a 9,600 guided missile cruiser, the Port Royal, ran aground at night near the entrance to Pearl Harbour. The causes were complex, but could perhaps be summarised as “When the modern navigational systems broke down or gave conflicting results, noone looked outside the ship in an attempt to fix its position.”

A double sextant, not to be confused with the reflecting circle, allows almost simultaneous horizontal angles to be measured between three objects. By applying the results to a station pointer (sometimes called a three-arm protractor), a fix can very rapidly be plotted on a chart. The speed with which this can be done is particularly useful when the vessel is travelling at speed. I have updated the account of fixing by simultaneous horizontal angles in the first post of this series, Sounding sextants 1.

Early double sextants from the first half of the nineteenth century had two index arms rotating about the same axis on a quintant, with one vernier reading on the inside of the limb and the other reading on the outside of the limb. By the third quarter of that century, Henry Hughes and Son were making double sextants in which the frame was circular. Double sextants from any period rarely come on the market and do not seem to be sought-after. Nevertheless, I was pleased to be almost the only bidder for a Hughes double sextant dating from 1951 or 1952.

The layout has not changed from the C19 version, but the 1950s instrument has an alloy rather than a bronze frame, is provided with micrometers, and has a telescope with an interrupted thread  to allow it to be deployed rapidly. The form of the mirror frames and the handle remain the same. This photograph gives the general arrangement:


The aluminium alloy frame of about 86 mm radius is in the form of a three spoked annulus with two lengths of rack on the periphery, and is held in the hand by means of an octagonal plastic handle that screws into the hub. The worms of two micrometers at the end of alloy index arms engage with the racks. The zeros of the scales of degrees are set 140 degrees apart. The narrow mirror of each index arm rotates independently about the centre of the instrument and the x2 by 30 mm Galilean telescope looks past their left hand edges at the middle mirror, which is about 35 mm square. This is unsilvered in a middle horizontal strip, so that the user has a direct view of the central object and reflected views of the right and left-hand objects. The mirrors are provided with adjusting screws which have screw-on protective caps.  The next photograph shows the layout, with the index mirrors set at micrometer readings of 60 degrees.


A feature of the mirrors not apparent from the photographs is that tiny discs of brass have been glued with shellac to the backs of the mirrors where the adjusting screws bear. The whole of the back has then been coated with a thick layer of varnish to seal the silvering. This is an approach that I imitate when restoring sextant mirrors, as the silvering often begins to deteriorate where the protective paint or varnish coating has been damaged by the point of a screw.

The next photograph shows a view of the underside of the instrument. The form of the handle is unchanged from 75 years previously.


As the sextant has a radius of only 86 mm it is possible to have micrometer drums of a useful size without having to have a conical worm in order to have the thread tangential to the rack while the worm axis is offset. In turn, having a cylindrical worm allowed Hughes to put into practice their 1929 patent (U K Patent number 309,648), in which the worm swings on a swing arm chassis out of the plane of the rack when the release catch is operated. The more usual arrangement in sextants is for the swing arm chassis to pivot in the plane of the rack. The next photograph may help to illustrate the Hughes arrangement:

Swing arm

The micrometer drums are pinned on to the worm shafts and the plastic of which they and the indexes are made is of a dark brown colour rather than a more pleasing black. Possibly they were originally black and have deteriorated over the years. They are divided into minutes and, despite the ample diameters of the drums, the scales seem inexplicably crowded.

The telescope can be very rapidly deployed. It has a relatively coarse interrupted thread. One has only to line up the white dots on the telescope and its mounting and rotate through less than an eighth of a turn to secure it in place. It is focused by sliding the eyepiece draw tube.

Scope thread

 The sextant came to me without a case, so my first step in its restoration was to make a new case for it. I used the tropical hardwood, kwila, for the carcase and sapele, sometimes called African mahogany, for the internal fittings. I followed Hughes’s practice of placing the handle on the side of the box, so that it can be set down without damaging the hinges, and of arranging the latch hooks both to point to the floor when the case is being carried. The joints are box dovetails, not a 1950’s practice, at which time comb joints were the norm.

I plan eventually to strip the paint off the sextant, deal with the small areas of corrosion and repaint it.






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