LEVELING 



75 



a spring bearing against its opposite side. The position of the 

 socket V is controlled by the four leveling screws S, which, 

 together with the lower leveling plate M, and the tripod P, 

 are substantially the same as in a transit, except that a level 

 does not commonly have a shifting center. 



The telescope is in every respect similar to that of the transit 

 except that it is longer, and having no horizontal axis, it cannot 

 be revolved in a vertical plane. 



FIG. 1 



The spirit level EF is also similar to that attached to the 

 telescope of a transit, but in a leveling instrument, it is usually 

 more accurate and sensitive. It consists of a hermetically 

 sealed glass tube, curved slightly in a manner corresponding to 

 the short upper arc of a large vertical circle, having the upper 

 portion of its inner surface on a longitudinal section ground 

 truly to the arc, and so nearly filled with alcohol, or a mix- 

 ture of alcohol and ether, as to leave only a small bubble of 



