LAYING OUT A DRAIN OR SYSTEM 123 



enter them in columns 4 and 6 of the table ; or he may 

 rule a table for the purpose. 



166. Computations in detail. Observe that the back- 

 sight reading taken at stake 1 is introduced on the line 

 belonging to stake 1, and that in like manner each back- 

 sight reading is introduced on the line of the stake at 

 which it is taken. Observe, also, that each fore-sight 

 reading is introduced upon the line of the stake at which 

 it was taken. 



1. We assume the elevation of stake 1 to be 10 feet 

 above datum. This we record on line 1 in column 6. 

 Figure 47 shows the location of the datum plane. 



2. Add the first back-sight reading, 4.75 feet, to the 

 elevation of stake 1. This gives 14.75 feet as the height 

 of the instrument above datum. The height should be 

 recorded on line 2 in column 4. Subtract the fore-sight 

 reading, 4.25 feet, from this height of instrument. This 

 gives 10.50 feet as the elevation of stake 2. This eleva- 

 tion we record on line 2 in column 6. 



3. Add the back-sight reading, 5 feet, to the eleva- 

 tion of stake 2. This gives the height of the instrument, 

 15.50 feet, in its second position. Record properly. 

 Subtract from 15.50 feet the fore-sight reading, 5.17, 

 and we have 10.33 feet as the elevation of stake 3. This 

 elevation we record on line 3 in column 6. 



Observe (1) That with each setting of the instru- 

 ment one back-sight and one fore-sight reading were 

 taken. (2) That adding the back-sight reading to the 

 elevation of the stake at which it was taken, and subtract- 

 ing from this sum the fore-sight reading, gives the eleva- 

 tion of the stake at which the fore-sight reading was taken. 



Proceed in this manner until the elevations of all 

 the stakes have been found, in each case recording the 



