Deviation of the Compass. 39 



place and declination of the sun, that a moveable arm, bearing 

 a lens for the concentration of the sun's rays to a point at the 

 axis of the instrument, can be made to traverse in an oblique 

 plane exactly coincident, from the eastern to the western horizon, 

 with the plane of the sun's motion. The speck from the rays of 

 the sun, concentrated by passing through the lens, is received 

 upon a small disk of ivory, and made to coincide, by a vertigi- 

 nous motion of the instrument, with a dot at the centre of the 

 disk. As, however, the speck from the lens, when the instru- 

 ment is adjusted for latitude and dechnation, will always fall 

 either above or below the centre of the disk, except when the 

 azimuth of the arm, in reference to the instrument, is the same 

 as that of the sun in reference to the earth, the simple act of 

 placing the instrument so, that the speck may fall upon the dot, 

 will of necessity put it in the exact meridional position. By com- 

 paring, then, the course steered by the binnacle compass with 

 the true meridian pointed out by the celestial compass, the va- 

 riation and deviation conjointly will, under existing circum- 

 stances, be correctly exhibited, as it were by direct observation ; 

 or, which may be of equal importance, the true direction of 

 the ship's head will be at once determined. The apparent time 

 is likewise given by the horary circle, without calculation or ad- 

 ditional trouble. And besides these properties, so useful and 

 important in practical navigation, the celestial compass has other 

 capabilities, such as the determination of the latitude of' the place ^ 

 within remarkably small limits, by a single observation \ yet, in 

 this case, as in all others in which the instrument is used, every 

 result is independent of the visible horizon. 



This instrument, so scientific in principle, and so strikingly 

 satisfactory in its results, especially as regards the determination 

 of the local attraction of ships, will, it is to be hoped, when bet- 

 ter known, find general acceptance with nautical men. 



