NavList:

John Howard you wrote: 

Following a thread on the Douglas protractor I came upon a refrence to the Douglas-Appleyard Arcless Sextant.  I have found a pix and a short discripsion of it but very little else.  It is mentioned in a NACA report from 1922 but no pix.

The sextant seems to be used in aircraft and surveying. Does anyone know of or have any more info on this sextant?

Admiral Percy Douglas and Rollo Appleyard were both clever inventors, and Douglas spent most of his time in the RN engaged in hydrographic survey at a succession of increasing levels including planning for the Dardanelles landings and the Ostend and Zebrugge Raids .  Put the two together and anything could happen.  The patent is UK 13689/16 HO203, so the sextant might originally have emerged during WW1.  It used a screw thread to turn the index mirror rather than an arm and arc.  Tens of degrees were indicated on counters and the micrometer turning wheel was apparently divided into intervals of 1/6th of a degree.  Whether there was a futher method of reading angles to intervals of one minute is unclear.  The only advantage I can see for use in aircraft of the period is that the sextant was relatively compact fitting into a space of 178x140x140mm.  Compare this with the space taken up by the box containing Brown’s arc sextant in the cockpit of the Atlantic Vickers Vimy.  There was no kind of artificial horizon.  It might also have been easier to hold for taking horizontal angles without a great arc to balance, but you’d have to hold one to assess that.  Sale adverts appear for these sextants made by both Adam Hilger Ltd and ER Watts & Son optical instrument manufacturers, and these firms themselves amalgamated in 1948 to make Hilger Watts.  DaveP