128 LAND DRAINAGE 



is little less than 3 feet deep at any point. But it is 5 

 feet deep at stake 5 and nearly 5 feet deep at stake 9. 

 A 5-foot cut makes rather expensive digging. A compro- 

 mise would be better in this case. 



(e) Let us establish a new point, d, 2j feet below the 

 top of stake 7 and a new point, e, 1\ feet below top of 

 stake 11, and draw a new line d to e, to represent the 

 bottom of a drain from stake 7 to stake 11. This mate- 

 rially lessens the amount of digging at stakes 9 and 10. 



(/) Let us adopt the line ade as the bottom of the 

 drain. 



Observe that the drain will be 3 feet deep at stake 1, 

 2| feet deep at stake 7, and 2J feet deep at stake 11. 

 These depths we have established for convenience and 

 economy in the work of digging. If this were a main 

 drain it might be necessary, because of the laterals, to 

 make the line (acb) the bottom of the drain. If, how- 

 ever, this drain were to be a lateral instead of a main, 

 the line (ade) would be better for the bottom of a drain. 



Observe, also, that the diagram we are using for this 

 purpose brings out, more clearly, the relative depths of 

 the drain at the several points. 



(g) Introduce these depths in column 9 of the table 

 3 feet on line 1, 2.5 feet on line 7, and 2.5 feet on line 11. 



(h) If the depth is 3 feet at stake 1, the bottom of the 

 ditch is 3 feet below the top of stake 1, or it is 3 feet lower 

 than stake 1. If then we subtract the depth of the ditch, 

 3 feet, from the elevation of the stake, we have (10 feet 

 3 feet = ) 7 feet as the elevation of the bottom of the 

 ditch, at stake 1, above datum. Subtracting the depth 

 of the ditch, 2j feet, at stake 7, from the elevation at 7, 

 gives 8.74 feet as the elevation of the bottom of the ditch 

 at stake 7. Subtracting the depth of the ditch at stake 



