ANGULAR SURVEYING 47 



repeat the operation several times in order to obtain an exact 

 adjustment. 



Third Adjustment. To make the transverse axis of the 

 telescope perpendicular to the vertical axis of the instru- 

 ment, so that when the instrument is leveled the transverse 

 axis of the telescope will be horizontal. 



Suspend a fine, smooth plumb-line from a rigid support 

 at as high an elevation as convenient and at a distance from 

 the instrument not exceeding the length of the line. The 

 weight should be suspended in a pail of water, care being 

 exercised that it does not touch the bottom of the pail and that 

 the line is not exposed to wind. With both plate bubbles 

 in the middle of their tubes, direct the line of sight to the upper 

 end of the plumb-line; then, turning the telescope slowly 

 downwards, notice whether the intersection of the cross-hairs 

 exactly follows the line throughout its length. If it does follow 

 it, the line of collimation revolves in a vertical plane. The 

 plumb-line will usually vibrate slightly, but its mean position 

 can be estimated by the eye. If the intersection of the cross- 

 hairs does not coincide with the plumb-line throughout its length, 

 but diverges to one side as it approaches the bottom of the line, 

 the error must be corrected by raising or lowering one ^ 

 end of the transverse axis of the telescope, which is 

 adjustable by means of screws placed in one of the 

 standards. If the intersection of the cross-hairs 

 diverges on the side of the plumb-line toward the 

 adjustable end of the transverse axis, this end is to 

 be lowered; if on the opposite side, it is to be raised. 



This adjustment can also be tested and made 

 in the following manner: Level the instrument, 

 and direct the telescope to some well-defined point 

 on a church spire or other high object, as the 

 point A, Fig. 6. Having set both the upper and 

 the lower clamp, depress the object end of the 

 telescope and set a point in the line of sight on the j| 1 

 ground at the base of the object; loosen the upper v 

 clamp, reverse the instrument in azimuth, plunge 

 the telescope, sight again on the high point, again turn 

 the telescope downwards, and notice whether or not the line 

 5 



