76 A MANUAL FOR NORTHERN WOODSMEN 
about the vertical axis until B is again sighted. Clamp the 
plate, revolve the telescope, and observe if point C is in 
line. If not, set a third point D in the new line. Then, 
to adjust, the cross-hair ring must be moved until the 
vertical hair appears to have moved to the point E, one- 
fourth the distance from D toward C, since, in this case, 
a double reversal has been made. 
The cross-hair ring is moved by loosening one of the 
screws which hold it in the telescope tube and tightening 
the opposite screw. The process of reversal should be 
repeated until no further adjustment is required. When 
finally adjusted, the screws should hold the ring firmly but 
without straining it. 
c. To make the horizontal axis of the telescope per¬ 
pendicular to the vertical axis of the instrument, so that 
the telescope in its revolution will generate a vertical 
plane. Set up the instrument and level it carefully. Sus¬ 
pend a fine, smooth plumb line twenty or thirty feet long 
some twenty feet away from the instrument with a weight 
on the lower end hanging freely in a pail of water. Set the 
line of sight carefully on the cord at its upper end. Clamp 
both plates and bring the telescope down until it reads on 
the lower end of the cord. If the line of sight does not cut 
the cord, raise or lower the adjustable end of the horizon¬ 
tal axis until the line of sight does revolve in a vertical 
plane. Constant attention must be given to the plate 
bubbles to see that they do not indicate an inclined verti¬ 
cal axis. 
If more convenient two points in a vertical line may be 
used, as points on a buildihg. Set on the top point and turn 
down to the bottom one, marking it carefully. Revolve 
both plate and telescope 180° and set again on the bottom 
point. Raise the telescope again and read on the top point. 
The second pointing at the top point should correspond 
with the first. If it does not, adjust as above for half the 
difference. 
d. To make the telescope bubble parallel to the line of 
sight. This adjustment is performed in the same way as 
for a level, as explained on pages 89 and 90. 
e. To make the vernier of the vertical circle read zero 
