44 



AGRICULTURAL ENGINEERING 



Corks to be 



Used When 



L e vel is Carried 



a tube. Water is then poured in until it appears at a con- 

 venient height in both glass tubes at the same time. The 

 surface of the water in each of the two tubes gives two points 



in a level line, which may 

 be extended to a distant 

 leveling rod by sighting 

 over the surface of the 

 liquid. 



A water level may be 

 made as shown in Fig. 31; 

 A and B are short lengths 

 of glass tubing attached 

 to a board, about three 

 feet apart, and connected 

 on the lower sides with a 

 length of rubber tubing. 

 For field use, the board 

 is bolted to a staff which 

 may be pushed into the 



ground to hold the instrument erect, and corks are provid- 

 ed for the upper ends of the tubes to prevent loss of the 

 liquid while the instrument is being carried. When leveling, 

 these corks should be removed. 



The bubble tube is the basis of nearly all leveling 

 instruments. It consists of a round glass tube bent so that 

 the upper inside sur- 

 face is an arc of a 

 circle lengthwise, or 

 on a longitudinal sec- 

 tion. This tube is 



sealed at each end and nearly filled with ether, the 

 remaining space being filled with the vapor of the liquid. 

 The upper surface of the tube is usually graduated 



Fig. 31. A home made water level. 



Fig. 32. A bubble tube. 



