LEVELING 95 



opening. These lines are called the cross-hairs. When 

 the telescope stands level, i.e., when the level attached 

 to the telescope indicates level : 



1. A line passing from the eye of the observer through 

 the small opening through which he looks, and through 



FIG. 36. Level commonly used in drainage work. A, telescope; B, 

 spindle; C, spirit level; D, eye-piece; E, leveling head; F, ratchet 

 to adjust objective ; O, objective ; S, leveling screws. 



the horizontal cross-hair, is also level and is parallel to 

 the datum plane. 



2. The distance of the horizontal cross-hair above the 

 datum plane is called also the height of the instrument. 



3. Every point that falls directly back of, or behind the 

 horizontal cross-hair, as the observer looks through the 



