THE SURVEYOR S COMPASS 5 



a. The plate is exactly vertical to the spindle in a new 

 compass, but the soft metal of most instruments is liable 

 in use to become bent. If that occurs to any considerable 

 degree, it will be shown by the needle and the bubbles. 

 The instrument should then be sent to the maker for repairs. 



b. To make the plane of the level bubbles perpendicular 

 to the axis of the socket, level the instrument, turn it 180, 

 and, if the bubbles are out, correct one half the movement 

 of each by means of the adjusting-screw at the end of the 

 bubble-case. Now level up again and revolve 180, when 

 the bubbles should remain in the center. If they do not, 

 adjust for half the movement again and so continue until 

 the bubbles remain in the center of their tubes for all posi- 

 tions of the plate. 



c. d. When the pivot is in the center of the circle and 

 the needle is straight, the two ends of the needle will cut 

 the circle exactly 180 apart in whatever position the in- 

 strument may be set. If the needle does not so cut, one 

 or both of these conditions is not fulfilled. If the differ- 

 ence between the two end readings is constant for all posi- 

 tions of the needle, then the pivot is in the center of the 

 circle but the needle is bent. If the difference in readings 

 is variable for different parts of the circle, then the pivot is 

 off center and the needle may or may not be straight. 



To adjust the pivot, first find the position of the needle 

 which gives the maximum difference of end readings; 

 then, using the small brass wrench commonly supplied 

 with the compass, bend the pivot a little below the point at 

 right angles to the direction of the needle until one half 

 the difference in end readings is corrected. Repeat the 

 test and adjust again if necessary. When the needle cuts 

 opposite degrees, or when it fails to do that by a constant 

 quantity in all parts of the circle, the pivot point is in the 

 correct position. 



With the above adjustment attended to, straighten the 

 needle. To do this, set the north end of the needle on some 

 graduation mark and bend the needle until the south end 

 cuts the circle exactly 180 from it. 



e. To make the sights perpendicular to the plane of the 

 bubbles, level the instrument carefully, hang a plumb 



