LAYING OUT A DRAIN OR SYSTEM 129 



11, from the elevation of stake 11, gives 11.70 feet as 

 the elevation of the bottom of the ditch at that point. 



(i) Introduce these ditch-bottom elevations into column 

 8 of Table XV on their proper lines, 7 feet on line 1, 

 8.74 feet on line 7, and 11.70 feet on line 11. 



(j) Before we can go further in finding values for columns 

 8 and 9, we must determine the fall or grade of the drain. 



The elevation of bottom of ditch at stake 11 is 11.70 feet. 

 The elevation of bottom of ditch at stake 7 is 8.74 feet. 

 The fall of the drain from stake 11 to stake 7 is 2.96. 

 The distance from stake 11 to stake 7 is (500 feet 300 



feet = ) 200 feet. 



The fall to a 100 feet of this distance is (2.96 feet-i-2 = ) 

 1.48. 



Notice the way in which this fall is introduced in 

 Table XVI. 



The stakes along this drain are 50 feet apart, so that the 

 fall from one stake to another is one-half of 1.48 feet, or 

 .74 feet. In other words the bottom of the ditch at stake 

 10 will be .74 feet lower than at stake 11 and .74 feet 

 lower at stake 9 than at stake 10 and so on. 



11.70 feet = elevation of bottom of ditch at 11. 



.74 

 10.96 feet = elevation of bottom of ditch at 10. 



.74 

 10.22 feet = elevation of bottom of ditch at 9. 



.74 

 9.48 feet = elevation of bottom of ditch at 8. 



.74 



8.74 feet = elevation of bottom of ditch at 7. 



