168 Improved Agricultural Drainage Level. 



stations, to determine the height or depth of one place with respect 

 to another. 



To find the difference of level beticeen two places, when the distance 

 does not exceed 200 yards. 



A 



B A 



First place your instrument at A, midway between the two 

 places B and C, and direct your assistant to hold the staff per- 

 pendicular at B ; then point the telescope to it, adjust the instru- 

 ment and read off the staff as directed above. Suppose the 

 reading 67 h inches ; your assistant must now take the staff to 

 station C ; turn the level round on the joint, taking care not to 

 remove the stand, direct the telescope to the staff, adjust and read 

 as before. Suppose this 52 inches. 



The difference of the two readings is 15^ inches, which shows 

 that the ground at B is 15| inches lower than at C. 



If the line of sight or of collimation of the telescope is known 

 to be correct (see pages 171, 172), it is not of so much importance 

 that the Instrument be placed so exactly midway between the 

 two stations, though it is always better to place it so when prac- 

 ticable. 



If there should be buildings or other objects between the 

 two stations, place the Instrument on one side as at A, where 

 you can get a view of both stations, and the difference of the 

 readings will give the difference of level as in the previous ex- 

 ample. 



When the distance of the two places, the difference of level of 

 which is required, is considerable, a series of observations will be 

 necessary. 



In this case divide the distance into portions of 100 or 150 

 yards each, or into unequal portions as circumstances require, and 



