not guarantee parallelism in this case with the line of sight un- 

 less the maker has provided for it. The certain test is the peg 

 method. As a matter of fact this is a dumpy level proposition 

 that is being conducted on a wye-and-collar basis to no very 

 serious purpose. If, in the peg method, the collimated sight- 

 line is brought into the predetermined horizon on the rod and 

 the bubble will not remain centered on being reversed, mount 

 the striding level with the release button on the side of the 

 telescope nearest the vertical arc, center the bubble carefully to 

 those conditions and thereafter use the striding level in this 

 position only. This method disregards the collars entirely as 

 a means of adjustment, and a disparity in their diameters is of 

 no concern. 



The Wind Adjustment, or lateral adjustment, tends to bring 

 the axis of the vial vertically over the telescope axis. Tem- 

 porarily remove the clamping post in the hub of the telescope 

 and revolve the vial from side to side on the collars. Correct 

 the error as described for the wye level, p. e 8. This adjustment 

 is not extremely important for, while the clamp allows an un- 

 restrained seating on the collars, there is still too little opportun- 

 ity for side movements to cause serious displacements in the 

 bubble. 



The Vernier of the Vertical Arc must be adjusted to the limb 

 when the telescope is in a horizontal position, as indicated by 

 the striding level. If the indices do not coincide, slightly 

 loosen the screws that secure the vernier plate and gently tap 

 the scale until a coincidence is perfected; then tighten the 

 set screws, keeping the reading edges in contact. The zero of 

 the vernier is set at the 30 mark so that small angles of 

 elevation or depression will not be confused. 



The Control Bubble should be perfectly centered when the 

 zeros of the vernier and the vertical arc are coincident. If not, 

 center the bubble with its own adjusting screws. When in- 

 struments are provided with this attachment the index coinci- 

 dence, provided for in the paragraph above, is accomplished by 

 turning the tangent screw that regulates the movement of the 

 vernier scale and control bubble mount. When the coinci- 

 dence is perfected, bring the control bubble to the center of the 

 srale with its own adjusting nuts. In this position the axis of 

 the bubble is not only normal to the index of the vernier 

 but is parallel to the line of sight. 



In an extremity, the telescope may be considered level, 

 therefore, when the zeros of the arc and vernier scale corres- 

 pond and the control bubble is in the center of its run. In 

 this way the control bubble may be delegated to perform the 

 office of a striding level that has been damaged or lost 



56 



