76 LEVELING 



air. Alcohol is used extensively for the levels of surveying 

 instruments, but is rather slow acting. Ether, though more 

 sensitive and quick acting, is affected too greatly by changes 

 of temperature to be used in surveying instruments. A mix- 

 ture of alcohol and ether gives excellent results. Since the 

 air bubble rises to the highest point of the inner surface of the 

 level tube in which it is confined, and since the upper portion of 

 the inner surface of the tube is ground truly to the arc of a 

 circle in the plane of its longitudinal section, it follows that 

 a tangent to this arc at the center of the bubble is a horizon- 

 tal line. A line tangent to the inner upper surface of the 

 bubble tube at its center is called the axis of the bubble, or axis 

 of the level tube. When the bubble is in the center of the tube, 

 this line will be tangent to the center of the bubble, and con- 

 sequently, will be a horizontal line. Hence, the axis of the 

 level tube is horizontal when the bubble is in the center of the 

 tube. 



Adjustments of the Wye Level. There are three adjust- 

 ments of the wye level, as follows: 



1. To make the line of sight, or line of collimation, par- 

 allel to the axis of the collars, or rings, on the telescope by 

 which it rests in the wyes. 



2. To make the axis of the level tube bubble parallel to 

 the axis of the collars, and, consequently, parallel to the line 

 of collimation. 



3. To make the axis of the level tube perpendicular to 

 the vertical axis of the instrument, so that when the instru- 

 ment is leveled up the bubble will remain centered while the 

 telescope is revolved horizontally. 



First Adjustment. Plant the tripod firmly; choose some 

 distant and clearly denned point, the more distant the better, 

 so long as it is distinctly visible and sharply defined. Remove 

 the pins from the clips, clamp the spindle, and by means of the 

 tangent screw and leveling screws bring the intersection of the 

 cross-hairs to coincide exactly with the point sighted. Revolve 

 or turn the telescope in the wyes through one-half a revolution, 

 that is, until it is bottom side up. If the intersection of the 

 cross-hairs is still on the point of sight, it shows that the line 

 of sight coincides with the axis of the collars. But if, when the 



