142 
KIKTLEY F. MATHEB 
of the micrometer screw pushes must be kept securely tightened. 
Should it become loose very erratic readings will result. The 
surface of this plate will gradually wear at the point where the 
micrometer screw bears against it until a distinct socket is made. 
Ultimately this becomes so pronounced that not only does it 
throw out the relation of the straight line push of the screw to 
the circular movement of the arc, but the point of the screw 
will not hit exactly the same spot on successive readings, and as 
a result three or four readings from the same station to the same 
object will fail to check. When that happens, the bearing plate 
should be stirfaced with a file and a new gradienter table 
constructed. 
The compass needle should always be raised from its pivot 
and clamped immediately after it has been used. Protect the 
pivot in every way possible, for unless the pivot is sharp and 
perfect the needle may be sluggish and unreliable.’’ Place the 
alidade as nearly as possible in the magnetic meridian before 
releasing the needle, and thus avoid the blow to the needle 
resulting from sudden contact with the compass box. The 
danger of destroying the polarity of the needle is another reason 
for guarding against reckless treatment of the alidade as a whole. 
When working in the rain, the compass box is the most vulner- 
able part of the instrument. Unless the glass cover is securely 
sealed all around, moisture will penetrate the box and put the 
needle out of commission by causing it to adhere to the inside of 
the glass. If this occurs, the box must be opened, the needle 
removed, and all parts thoroughly dried before proceeding with 
the work. 
