LEVELING INSTRUMENTS. 



195 



is two feet long, and is so carefully adjusted by means 

 of the plumb-line as not to vary more than the twen- 

 tieth of an inch from a true level, which is easily ac- 

 complished, then a twentieth of an inch in two feet 

 will be one inch in forty feet, a sufficient degree of ac- 

 curacy for many cases. 



Where greater accuracy is required, as in long and 



nearly level ditches, the "water level" may be used. 



Fig. 163. It may be made of a 



- g fl^ lead tube about three 



A F ^ ' ,, ^ B feet long, bent up an 



inch or two at each end, 

 and stiffened by fasten- 

 ing to a wooden bar, 

 A, B {Fig-. 16S). Into 

 each end is cemented, 

 with sealing-wax, a 

 small and thm phial 

 with the bottom broken 

 off, so that when the tube is filled with water it may 

 rise freely into the pliials. If the tube be now filled 

 with water colored with cochineal or any dye-stuff, and 

 then placed upon the tripod, C, by looking across the 

 two surfaces of liquid in the phials, an accurate level 

 may be obtained. "When not in use, a cork is placed 

 Fig- '64. into each phial. " Sights" of equal height, fast- 

 ened to pieces of cork floating on the water, as 

 shown in Fig-. 164, give a more distinct hne for 

 the eye. The sights are formed of fine threads 

 or hairs stretched across the square openings. 

 To ascertain whether these threads are both of 

 equal heights above the water, let a mark be made 



Leveling Instnimcnt. 



