ANGULAR SURVEYING 45 



the sun dazzling. An open and nearly level space affording 

 an unobstructed sight for at least 400 ft. from the transit 

 in opposite directions should be chosen for making the adjust- 

 ments. In setting up the instrument, the feet of the tripod 

 should be planted firmly in solid ground that is not subject 

 to jars from heavy machinery or other causes, so that its posi- 

 tion will not be disturbed. 



First Adjustment. To make the axes of the plate levels 

 perpendicular to the vertical axis of the instrument, so that 

 when the bubbles are centered by the leveling screws the axis 

 of the centers will be truly vertical and the plates will revolve 

 in a horizontal plane. This adjustment is substantially the 

 same as for the compass, and is performed as follows: 



With the upper clamp set and the lower clamp loose, turn 

 the instrument so that the plate levels I and /', Fig. 1, will be, 

 respectively, parallel to the lines determined by the two pairs of 

 leveling screws, and bring each bubble to the middle of its tube 

 by means of the corresponding pair of leveling screws. Next, 

 turn the instrument half way around; that is, revolve it in 

 azimuth through 180, so that each level will be in the reverse 

 position with respect to the same pair of leveling screws. If 

 the levels are in adjustment, the bubbles will remain in the 

 centers of the tubes. If the bubbles do not remain so, but run 

 to either end, bring them half way back to the middle of the 

 tubes by means of the capstan-headed screws attached to the 

 ends of the tubes, and the rest of the way back by the leveling 

 screws. Then revolve the instrument again through 180 

 and observe the positions of the bubbles. Sometimes this 

 adjustment is made by one trial, but it is usually necessary to 

 repeat the operation. 



Second A djustmeni. To make the line of sight perpendicular 

 to the transverse axis of the telescope. 



The manner of performing this adjustment is illustrated 

 in Fig. 5. Set and level the instrument at a point A, and 

 direct the telescope to some well-defined point B a few hundred 

 feet distant. Both clamps being set, plunge the telescope and 

 set another point, as a marking pin or a tack in the top of a 

 stake, a few hundred feet away, on the opposite side of the 

 instrument from B. If the line of sight is tru'y perpendicular 



