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. 
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 200 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 
