10 HYDROGRAPHICAL SURVEYING [chap. i. 



merit as follows, and clamped permanently. The sextant being 

 mounted on its stand, observe the double altitude of the sun 

 in artificial horizon, with the telescope pointing as nearly as 

 possible to the centre of the artificial horizon. Make the suns 

 exactly cover each other, and bring the bubble of the level into 

 the centre of its run, clamping it securely. So long as the 

 clamp is not loosened, the sextant being set to the double alti- 

 tude of a star, if the sextant is turned truly vertically on its 

 axis until the bubble is in the centre of its run, and moved 

 bodily on its stand towards or from the artificial horizon until 

 the telescope points correctly to the centre, then both direct 

 and reflected images of the star will be seen in the field of the 

 telescope. 

 Electric A small electric light fitted on the arm carrying the magnify- 



Lignt. jj^g glass, with wires leading to a dry battery or a Bichromate 

 Battery, will be found a very great convenience. The light 

 coming from a fixed point, there is less liability to errors of 

 parallax in reading off the sextant, than if an ordinary lantern 

 is used. 

 Rack and This fitting for focussing the telescope enables the focus to 

 be obtained with greater nicety. 



SOUNDING SEXTANT. 



This useful form of sextant is made of various sizes. It 

 chiefly differs from the observing sextant in being generally 

 lighter and handier, in having the arc cut only to minutes, 

 and having a tube of a bell shape so as to include a larger 

 field in the telescope. 



All angles in the frame of the instrument should be 

 rounded off, especially that at the zero end of the arc. 

 Considerable injuries may result to the face of the observer 

 when using the sextant in a boat in a lively sea, if this is 

 not done. 



The graduation of the arc should be plain enough to read 

 without a magnifying glass. 



The measurable angle should be as large as possible, i.e. 

 about 140°. 



The index glass should be large, so as easily to pick up 

 objects. 



Pinion. 



