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. \Yhen 

 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 building. 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 



