double sextant, for bringing two objects to the horizon-glass 

 at the same time and so measuring both angles. One of 

 these sextants, in box form, was invented by Captain George, 

 and another, in open form, by Captain Beechy. (For 

 description see Raper's " Navigation," early editions.) 



Repeating and Reflecting Circles. The first reflecting 

 circle was invented by Tobias Mayer, 1723-62 ; another was 

 by Chevalier de Borda, and known as Borda's Repeating 

 Circle ; another, by Mendoza Rios, is described in his treatise 

 on " An improved reflecting Circle," which was read before 

 the Royal Society in 1801 : a good illustration of it is given 

 in Norie's " Navigation " of 1810. Dollond and Troughton 

 also made excellent reflecting circles, the idea being that, 

 having the whole circle whereby observations could be 

 repeated and a mean taken, the centreing and other errors 

 were eliminated. In the Troughton Circle, three arms 

 were attached to the index, which gave readings on three 

 different parts of the circle from which the mean of the 

 three readings was obtained. (For description see Raper's 

 " Navigation," 1843, and Simms's " Mathematical Instru- 

 ments," 1849). 



Artificial Horizons. When the natural horizon is not 

 available, it is necessary to make use of an artificial one, 

 and many different instruments have been introduced for 

 this purpose. Some had mechanical devices which were 

 fixed to the sextant for obtaining a level for use on board 

 ship, but were never found very satisfactory. The only 

 form found to be accurate is a trough of some fluid, which 

 is obviously unsuitable for shipboard ; quicksilver is most 

 commonly used, with a plate-glass cover to protect it from 

 the wind ; but treacle, oil, or even water in a saucer, gives 

 an excellent result. Captain George invented one to get 

 rid of the vibration of quicksilver by using a disc of black 

 glass floating in a pan of mercury. Captain Beecher's instru- 

 ment is a mechanical contrivance with a counter-balance 

 working in oil to prevent rapid motion. Newey's has a 

 disc hung in gimbals with two sight vanes. Troughton 

 invented one with a black glass disc set in a frame, with 

 adjusting screws and spirit level. 



EDGAR TARRY ADAMS, 



Lieutenant, R.N.V.R. 

 April, 1918. 



