860 Capt. Graydon's Celestial Compasa, 



with small screw caps, which may be screwed on oi" off, for the 

 purpose of giving the hemisphere a slight motion endways or 

 in the direction of its axis, in order to adjust it into a proper 

 position to balance correctly or horizontally in its gimbals. 



The hemisphere C contains a weight or counterpoise within 

 it, so situated that the centre of gravity should fall as nearly 

 as possible in the centre of the hemisphere ; or the centre of 

 gravity should coincide as nearly as possible with the axis of 

 its motion upon its pivot c, so that the horizontal position of 

 the plate K L would not be disturbed by any turning or change 

 of position of the hemisphere C upon its axis. The adjust- 

 ment of its equilibrium may be performed by screwing up or 

 down the small spherical weight Z, which is tapped upon a 

 wire projecting from the top of the fi'ame or tablet P. 



In making use of the instrument for the purpose of finding 

 the deviation of the magnetic needle, the direct light of the 

 sun is made to fall upon a pair of cross wires, or the sun's 

 rays are concentrated upon the tablet P by means of a lens at 

 O instead of the cross wires, so as to direct their shadow upon 

 a piece of ivory, marked with cross lines upon its surface; and 

 the coincidence of the shadow of the cross wires with the inter- 

 section of the lines upon the tablet will determine the required 

 position. 



The dial plate A B is divided into 24 hours, or 360 degrees, 

 and is provided with an hour or index hand E, which is formed 

 to a vernier at one of its extremities, to read off the degrees 

 upon the dial plate. 



P represents a small frame, or square, which is mounted 

 upon a pillar on the hand E, and the axis of which is in the 

 centre of, and perpendicular to, the dial plate. The frame P 

 is adapted to receive a piece of ivory, q, having cross lines or 

 wires, intersecting each other upon its surface. 



The hour hand E has, near one of its extremities, a small 

 pillar or tube o projecting up from it, and into this tube a 

 small round rod s is adapted to slide, having a square frame O 

 at its upper extremity, furnished with cross wires, as repre- 

 sented in the drawing, Plate I. 



The round rod s is graduated, and numbered with tangents 

 to the angles of elevation or depression, above and below the 

 level of the intersection of the cross lines upon the surface q, 

 and it is provided with a vernier scale, formed on the side of the 

 pillar o, to read off the divisions on the round rod. It is also 

 furnished with a clamping and adjusting screw, as shown in the 

 figure, to move it up or down with a slow motion. 



Method 



