104 LAND DRAINAGE 



within range of the level. The height of the instrument 

 at this new position is now determined, the back-sight 

 reading being taken at a point within range, and whose 

 elevation is already found, or whose elevation can be found 

 from data already obtained. If the new points whose 

 elevations are sought are in the same line of stakes as those 

 already found, it is desirable to take the back-sight read- 

 ing at the stake whose elevation was last found. The 

 work from this point proceeds as above described. 



141. Using cheaper kinds of levels. The cheaper 

 kinds of drainage levels are of necessity more crudely 

 made and cannot, therefore, be so delicately adjusted as 

 the better made and more expensive instruments. 



In leveling with these cheaper instruments, usually only 

 one fore-sight reading is taken with each setting up. 

 One back-sight reading must also be taken, because this 

 is necessary to determine the height of the instrument. 

 In using the cheaper level, the precaution should always 

 be observed of setting the instrument nearly equidistant 

 from the point whose elevation is known and the point 

 whose elevation is to be determined. In practice, in 

 leveling for drains where the fall is large, it is possible, 

 with care, to take two, three, or even four fore-sight read- 

 ings with each setting up of the instrument. But here, 

 as above, the level should be set very nearly midway be- 

 tween the point whose elevation is known and the farthest 

 point whose elevation is to be determined with this setting 

 of the instrument. 



If but one fore-sight reading were taken with each set- 

 ting up of the instrument in determining the elevations 

 of the points recorded in the tables above, the readings 

 would appear as seen in the following table : 



