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 
