EXPERIMENT STATION BULLETINS. 479 



guides and is fitted with a clamping screw. It is open in the center 

 to expose a portion of the face of the rod. 



USING THE LEVEL. — OBJECT. 



In using the level 



1. There is always a starting point whose elevation above datum is 

 known or whose elevation above datum is arbitrarily set or, to put it 

 more correctly, below which a datum plane is arbitrarily established. 

 Ordinarily in simple drainage Avork this arbitrary height, or elevation 

 is 10 ft. 



2. There are one or more other points whose elevations are not 

 known, but which it is desired to determine. 



STEPS IN THE OPERATION. 



To determine the elevations sought the procedure is as follows: 



1. The level is set within range of the point whose elevation is known 

 or assumed. 



(a) The legs of the tripod are spread and firmly set in the earth 

 so that the lower plate of the leveling head E (Fig. 9) is approximately 

 level. 



(b) The upper plate and spindle are then adjusted by the use of 

 the thumb-screws of the leveling head so that the spirit level indi- 

 cates level in whatever direction the telescope is turned. In practice 

 the telescope is turned so that it stands in line with one pair of thumb- 

 screws and adjustment is made with these screws to bring the telescope 

 level. The telescope is then turned so that it stands in line with the 

 other pair of thumb-screws and adjustment is made as before. The tele- 

 scope is now turned back to its first position for readjustment, then re- 

 versed, and in each case the thumb-screws are used, if necessary, to per- 

 fect the adjustment to bring the telescope to level. When thus adjusted 

 the telescope will stand level in all positions. 



Caution. — Tighten the thumb-screws only sufficiently to hold the tele- 

 scope firmly. More than this is likely to do injury to screws and plates. 



2. The height of the instrument is determined. 



(a) A leveling rod is held, by an assistant, or rodman, upon the 

 point whose elevation is known or assumed. The rod should be held 

 perpendicular, with face toward the level. 



(b) The person in charge of the adjusted level turns the telescope 

 toward the rod, places the eye to the eyepiece D and moves the objective 

 O out or in until the figures upon the face of the leveling rod are clearly 

 seen or, if the rod is too far away for that, till the view of the target 

 is clear out. The eye piece may need adjusting to bring out the cross- 

 hairs. 



Look now to see that the spirit level indicates level, and if necessary 

 adjust. 



(c) If the figures on the leveling rod appear sufficiently clear to the 

 one in charge of the level, as he looks through the telescope, he should 

 read and record the height on the rod at which the horizontal cross hair 

 crosses the face of the rod; or 



