TERRESTRIAL MAGNETISZf. (APP. B.} 335 



is kept turned (arrow-head down) with its 

 mounting-stopper against the framework. The 

 observer turns the instrument round its vertical 

 axis till the prism and lens are fairly opposite to 

 the object, then places his eye so as to see the 

 object over the prism and reads the bearing of 

 the object from the compass-card as seen re- 

 flected in the prism. 



The pointer is used merely as an aid in 

 directing the instrument towards the object, but 

 the bearing is read directly from the object as seen 

 on the compass card. It is not necessary that 

 the pointer should be pointing exactly towards 

 the object unless the altitude be very high. For 

 objects from horizon up to 38 of altitude the error 

 on the bearing, as in the first described method, 

 would be less than half a degree, even if the pointer 

 were pointing 4 away from the object. 



Another advantage of the azimuth mirror 

 particularly important for taking bearings at sea 

 when there is much motion, is that with it it is 

 not necessary to look through a small aper- 

 ture in an instrument moving with the compass- 

 bowl, as in the ordinary prism compass, or in the 

 original nautical azimuth compass (described 280 

 years ago by Gilbert, Physician in Ordinary to 

 Queen Elizabeth, in his great Latin book, On 

 tJie Magnet and on the Rarth a great Magnet], 

 which is very much the same as that still in use 

 in many of the best merchant steamers. In using 



