30 



TILE DKAINAGE 



suppose the line was 800 feet long and the total fall was 

 three feet; the 800 feet would give 16 fifty-foot spaces, 

 and the three feet divided by 16 would give 2i/4 inches, 

 which would be the fall between the stakes. If the sur- 

 face of the ground was perfectly level and uniform, the 

 matter of finding the depth to dig the ditch at each stake 

 would be simple enough. In practice, however, we never 

 find land in this condition. To find these various depths 

 we must first secure readings at the different stakes. To 

 do this place the instrument, wiiether the carpenter's, 

 home-made or surveyor 's level, at some point slightly higher 

 than the highest stake and where all the stakes can be 

 seen. Have a man stand a rod marked off into feet and 

 inches or preferably into feet and tenths of feet on top 

 of the first stake. Sight along the instrument and find 

 what height on the rod is on a level with it. Without 

 changing the height of the instrument, point it at all the 

 stakes in any one line and take the readings on the rod. 

 From these readings and the fall from one stake to another, 

 previously determined, the depth of the ditch is calcu- 

 lated. 



Fig. 4. Showing method of finding depth of ditch. 



From the above diagram it will be noticed that the 

 readings are as follows: outlet, 4.8 feet; first stake, 4 A 

 feet; second stake, 3.7 feet; third stake, 4.0 feet. The out- 

 let reading, plus the height of the outlet stake, equals 8.3 

 feet, which is the difference in level between the instru- 

 ment and the outlet. The fall in this case, we will sav. has 



