LEVELING 99 



point whose elevation is known, or has been established ; 

 it is set, with the legs of the tripod spread in such a way 

 that when they are firmly set in the soil, the lower plate 

 of the leveling head E, Fig. 36, is approximately level. 



2. The upper plate and spindle are then adjusted by 

 the use of the thumb-screws of the leveling head, so that 

 the spirit level indicates level in whatever direction the 

 telescope is turned. In practice, the telescope is turned 

 so that it stands in line with two opposite thumb-screws, 

 and adjustment is made to bring the telescope to level. It 

 is then turned so that it stands in line with the other 

 pair of thumb-screws and adjusted as before. The 

 telescope is now turned back to its first position for re- 

 adjustment, then reversed, then turned to its second posi- 

 tion and reversed, and in each case the thumb-screws 

 are used, if necessary, to perfect the adjustment to bring 

 the telescope to level. When thus adjusted, the telescope 

 should stand level in all positions. 



130. Cautions. The following cautions should be 

 observed in setting up the level : 



1. Tighten the thumb-screws only sufficiently to hold 

 the telescope firmly. More than this is likely to do injury 

 to screws and plates. 



2. Remember that once the level is carefully adjusted, 

 continual care must be exercised to keep it in adjustment. 

 It should not be struck; one should not stand too near 

 the feet of the tripods; the bubble of the spirit level 

 should be frequently observed. 



131. Determining the height of the level. The 

 height of the instrument is determined in the following 

 manner : 



1. A leveling rod is held by an assistant, or rodman, 

 on a point whose elevation is known, or has been es- 



