

116 LAND DRAINAGE 



Each of these fore-sight readings, subtracted from the 

 height of the instrument, gives the elevation of the stake 

 at which the reading was taken. 



Observe the cautions suggested in paragraphs 130 and 

 137. 



b. If the level to be used is not a high class instrument, 

 it should be set a little to one side of the proposed drain, 

 and about equidistant from stakes 1 and 2. As above, 

 the elevation of stake 1 is assumed to be 10 feet above 

 datum, and is recorded in the proper column after stake 



1 in the notes. 



A back-sight reading should be taken with the rod on 

 stake 1, and recorded in the proper column after stake 1 

 in notes. This reading, added to the height of stake 1, 

 will give height of instrument. 



A fore-sight reading should be taken with the rod on 

 stake 2, and recorded in the proper column, after stake 



2 in notes. This reading, subtracted from the height of 

 instrument, will give elevation of stake 2. 



In like manner the instrument should be set in a 

 similar position between stakes 2 and 3, a back-sight 

 reading should be taken at stake 2, and a fore-sight 

 reading at stake 3. The back-sight reading, added to 

 the elevation of stake 2, will give the height of instrument 

 in the new position, and subtracting the new fore-sight 

 reading from this new height of instrument will give the 

 elevation of stake 3. 



Proceed in this way, taking a back-sight and a fore- 

 sight reading between each two stakes, till the fore-sight 

 reading is taken on the last stake. 



As stated in paragraph 141, where there is a fair fall, 

 these cheaper levels may be set up to take 3, and even 

 5 or 7 fore-sight readings for each back-sight reading. In 



