182 



LAND DRAINAGE 



ff-e-o, 



of its water column 

 stands even with the 

 mark just placed upon 

 it. Then the man at 

 stake 7 carefully marks 

 on the inner edge of 

 stake 7 the height of 

 the top of the water 

 column. In like man- 

 ner the height of the 

 mark on stake 7 is in- 

 dicated on stake 6, and 

 that on stake 6 is in- 

 dicated on stake 5, and 

 so on till a similar mark 

 is placed upon stake 1. 

 It is not necessary to 

 explain that all of the 

 marks so placed on the 

 inner edges of the stakes 

 are in the same hori- 

 zontal plane and on 

 the same level. This 

 plane is a datum plane 

 and from it we make 

 our actual computa- 

 tions. 



238. The height of 

 grade bars. The 

 height at which the 

 grade bar should stand 

 at stake 1 is now de- 

 termined and marked 



