34 ANGULAR SURVEYING 



1. The plane tangent to the level bubbles when at the cen- 

 ters of their respective tubes must be perpendicular to the 

 vertical axis of the socket. 



2. The two ends of the needle and the pivot must be in the 

 same vertical plane. 



3. The needle pivot must be in the center of the graduated 

 circle. 



A new compass made by a good manufacturer always satis- 

 fies these conditions, as the instrument is sold by the maker 

 in perfect adjustment. Rough usage, however, a fall, or a 

 hard blow may throw the compass out of order, and it is 

 necessary that the surveyor should know how to test and read- 

 just it. 



To Adjust the Plate Levels. Bring the bubbles to the cen- 

 ters of the level tubes by moving or rotating the plate care- 

 fully by means of the ball-and-socket joint; then revolve the 

 compass horizontally through 180; that is, turn it end for end. 

 If the bubbles remain in the centers of the tubes, the levels are 

 in adjustment. But if in turning the compass end for end, 

 either bubble runs toward one end of the tube, lower that end 

 and raise the opposite end sufficiently to bring the bubble half 

 way back, by means of small screws that attach the ends of 

 the tube to the plate; then bring the bubble to the center by 

 moving the plate as before. Repeat the operation until both 

 bubbles remain in the centers of the tubes in every position 

 of the compass. 



To Straighten the Needle. Level the compass and turn it 

 so that the north end of the needle points exactly toward or 

 cuts some prominent graduation mark of the needle circle, 

 and note the exact reading of the south end of the needle. 

 In order to read either end of the needle accurately, the eye 

 should be directly above a line in the prolongation of the oppo- 

 site end of the needle. Then reverse the compass end for end 

 and turn it so that the south end of the needle cuts the same 

 graduation mark, and observe whether the north end reads 

 the same as the south end did before reversing. If it does not 

 read the same, correct one-half the error by bending the needle 

 carefully, and repeat the operation, using different graduation 

 marks, until exact reversals are obtained. 



