AZIMUTH COMPASS. 45 



internal reflection in the prism. (Small instruments 

 of this kind, called "prismatic compass," are to be 

 obtained at little expence, and are very convenient.) 



The azimuth of an object being obtained as refer- 

 red to the Local Magnetic Meridian, the next point is 

 to find its azimuth as referred to the Astronomical 

 Meridian. The power of doing this depends on the 

 choice of the object. Navigators almost invariably 

 choose the rising or setting Sun. The latitude of the 

 ship being known with sufficient accuracy for this 

 purpose, and the Sun's declination and consequent 

 distance from the celestial pole being taken from the 

 Nautical Almanac, the solution of a quadrantal equa- 

 tion gives the Sun's azimuth at rising or setting as 

 referred to the Astronomical Meridian. The difference 

 between this and the azimuth as referred to the Local 

 Magnetic Meridian gives the Magnetic Declination. 

 (The poetic reader will find much of this operation 

 correctly described in Falconer's " Shipwreck.") 



For determinations on shore, a " Variation-theodo- 

 lite" was sometimes used, consisting of a common 

 theodolite adapted to the measure of horizontal angles, 

 the axis of its telescope being so much raised that the 

 telescope could be pointed to the pole-star, by which 

 the reading of the horizontal circle for astronomical 

 meridian could therefore be found; and that the tele- 

 scope could also be pointed down to a compass-needle 

 immediately below the theodolite-frame, to the ends of 

 which the telescope-object-glass could be directed, and 

 which ends were made distinctly visible by affixing 



