46 AGRICULTURAL ENGINEERING 



for determining the difference of elevation between two 

 points is known as the level, and involves primarily the ele- 

 ments just described, — the bubble axis, a line of sight paral- 

 lel to the bubble axis, and a vertical axis perpendicular to the 

 bubble axis about which it may be revolved. 



To assist in extending the line of sight, leveling instru- 

 ments are provided with telescopes. The sights in this case 

 are provided by cross wires or cross hairs set in the tele- 

 scope. 



Fig. 35. A level known as a Wye Fig-. 36. A "dumpy" level. 



level with horizontal circle and com- 

 pass. 



THE ADJUSTMENTS OF THE LEVEL 

 The Need of Adjustment. Accurate and rapid work 

 cannot be done with a level unless it be in proper adjustment. 

 Even the best instruments will not remain in adjustment 

 indefinitely, and tests of their condition should be made often. 

 In practice some of the best engineers make it a rule to test 

 their instruments every day. Everyone who uses a level 

 should know how to test and adjust it. Its adjustment is not 

 a difficult matter, yet it requires some study to master the 

 methods used. Every instrument maker of repute will fur- 

 nish full and complete directions for adjusting each instru- 

 ment of his manufacture, and these directions should be 

 given preference over general directions applicable to all 



