EXPERIMENT STATION BULLETINS. 



477 



spirit level. When the tripod is set the spindle can be adjusted so 

 that the telescope swinging upon the spindle is always ''level." 



As one looks through the telescope he sees apparently near the far end, 

 two lines — one horizontal and the other perpendicular — crossing each 

 other at the center of the opening. When the telescope stands level, i. e., 

 when the level attached to the telescope indicates level : 



(a) A line passing from the eye of the observer through the small 

 opening through which he looks, and through the horizontal cross-hair 

 is also level, and is parallel to the datum plane. 



Fig. 10. Cheaper forms of drainage or grade levels. Reading from 

 left to right: — Gurley, Jackson, Queen. 



(b) The distance of the horizontal cross-hair above the datum plane 

 is called also the height of the instrument. 



(c) Every point that falls directly back of, or behind the horizontal 

 cross-hair as the observer looks through the telescope, is the same dis- 

 tance above the datum plane as is the instrument. 



CHEAPER LEVELS. 



The instrument shown in Fig. 9 is a rather expensive instrument for 

 one, to buy who has only a limited amount of draining to do. 



There are to be had a number of cheaper levels, also called drainage 

 levels. Sometimes they are called grade levels. They are not so ac- 



