270 Observations on Surveying Instruments. 



as standing upright on its staff, with its forward end, or end 

 farthest from the observer, elevated at an angle of about for- 

 ty-five degrees. 



A, a hollow plate, covering the whole bottom of the com- 

 pass box, and about four tenths of an inch in thickness on the 

 outer edge, a part of which plate projects and forms axles at 

 D and d. 



B and C, two bars about four tenths of an inch thick, encir- 

 cling the compass box and attached by screws to the joints 

 D, d, and turning on the aforesaid axles. 



E, a plate, the upper part of which forms the card, or face 

 of the compass, to which is attached the graduated ring. This 

 plate, provided with a nonius represented at O, and with two 

 spirit levels in the face of the compass, is turned at pleasure 

 by the trundle P, and made fast by the screw T. The noniua 

 should always be made to turn off, at least fifteen degrees, 

 Compasses are usually faulty in this respect, 



G, a semicircle, graduated to degrees, and attached to a, 

 projection of the circular bars. 



H, an index with a nonius, by which the divisions on the 

 semicircle may be read off for every five minutes, attached 

 by a screw, to the end of the axle at D, and made fast to 

 the semicircular arch, when necessary, by the thumb screw 

 at e. 



K, a trigonometer attached by screws to the circular bar, but 

 which may be conveniently carried in the pocket. It has an 

 index or hand with a fiducial edge, so divided as to correspond 

 with the size of the divisions on the plate. One end of this 

 index turns around a centre at g, and the other comes over 

 the edge of the plate, (which forms an arc of a circle, and is 

 graduated up to 45°, thence backward to 90°,) the index be- 

 ing the hypothenuse, and the divisions on the plate, the legs, 

 of a right angled triangle. 



L, ball and stem. Around the stem of the ball is a small 

 circular plate in contact with the screws a, 6, c, whieh pass 

 through the top of the socket, and by which the compass may 

 be nicely adjusted to a level, when great accuracy is re- 

 quired. 



M, a screw to regulate the ball, and N, another to fasten 

 the socket to the staff. 



P, screw, which by means of a spiral spring, raises the 

 needle off the centre-pin. 



