THE TRANSIT 



75 



perpendicular to the vertical axis of the instrument. Set 

 up the transit and bring the bubbles to the center of their 

 respective tubes. Turn the plate 180 about its vertical 

 axis, and see if the bubbles remain in the center. If they 

 move from the center, turn the capstan-headed screws on 

 the bubble tube until the bubble moves half-way back to 

 the center, or as nearly so as this can be estimated. Each 

 bubble must be adjusted independently. The adjust- 

 ment should be tested again by relevelling and reversing 

 as before, and the process continued until the bubbles re- 

 main in the center when reversed. When both levels are 

 adjusted, the bubbles should remain in the center during 

 the entire revolution about the vertical axis. 



CROSS-SECTION OP THE TRANSIT HEAD 



b. To make the line of sight perpendicular to the hori- 

 zontal axis so that the telescope when revolved will 

 generate a plane. To do this choose open and nearly level 

 ground. Set up the transit carefully over a point A, sight 

 accurately at a point B at about the same level and 20.0 or 

 300 feet away, and clamp both plates. Revolve the tele- 

 scope and set C in line with the vertical cross-hair at about 

 the same distance and elevation. B, A, and C should then 

 be in a straight line. To test this, turn the instrument 



