48 AGRICULTURAL ENGINEERING 



until the bubble does not move in the tube as the telescope 

 is turned about the vertical axis. 



Second Adjustment. To make the line of sight parallel 

 to the bubble axis: ■ 



Select a level piece of ground for the work, and locate 

 three points in a straight line, 100 feet apart. At one end 

 point (Sta. A) drive a hub, at the mid-point locate the level 

 and take a reading on a rod held on the first hub with the 

 instrument carefully leveled. Turn the instrument in the 

 opposite direction, and, after leveling carefully, drive a hub 

 at the second point (Sta. B) until the same rod reading is 

 obtained as at Station A. These two stations now have the 

 same elevation, because any error of the instrument will be 

 the same in both cases. Now bring the instrument near 

 Station A (two or three feet off) and adjust the line of sight 

 until the same rod readings are obtained on both stations. 

 The rod on Station A may be read by looking through the 

 instrument in the reverse way and locating the line of sight 

 on the rod with the point of a pencil. After adjusting, the 

 operation should be repeated as a check. 



QUESTIONS 



1. Define a level surface. A level line. A leveling instrument. A 

 datum plane. 



2. What is meant by the elevation of a point? 



3. Describe a leveling rod. What is the difference between a 

 speaking and non-speaking rod? 



4. How are leveling rods graduated? 



5. What is the purpose of a bench mark? 



6. Describe the plumb line. How may it be used to determine a 

 level line? 



7. Describe the construction of the water level. 



8. Describe the bubble tube and its use in leveling. 



9. Describe the construction of the engineer's level. 



10. What is meant by the "line of sight"? 



11. Describe the fundamentals of the adjustment of the leveL 



