126 LAND DRAINAGE 



In this work we use the elevations as they now appear 

 in Table XV. Two precautions are to be observed in 

 this part of the work : 



1. Not to have the ditch unnecessarily deep at any one 

 or more points. Unnecessary depth means added ex- 

 pense in digging and filling. 



2. To have the ditch sufficiently deep. Insufficient 

 depth would endanger the tile from frost or even from 

 plow points, and it would very likely fail to lower the 

 ground water sufficiently for best results. 



169. The grade or fall. A good method of procedure 

 is something as follows : 



(a) Referring to Fig. 49, we find that conditions will 

 permit a depth of 3 feet at stake 1, which is practically 

 the outlet. Three feet is a satisfactory depth. Let us 

 establish on our diagram, Fig. 49, point a, 3 feet below 

 stake 1. 



(b) For trial let us establish a point, b, 3 feet below the 

 top of stake 11. 



(c) If the fall in our ditch is to be constant, from point 

 b to point a, a straight line connecting the two points 

 will indicate the bottom of the ditch. We draw such a 

 line. 



It is very evident, as one looks at the diagram, after 

 drawing the line ab, that this plan brings the drain very 

 close to the surface at stakes 7 and 8. At either stake, if 

 one applies the scale, the depth is found to be not over 

 18 inches, and, while drains are sometimes laid as shallow 

 as this, a greater depth is desirable. It is further found 

 that this drain would be only 27 inches deep at stake 3. 



(d)' Let us establish a point at c, 3 feet below the top 

 of stake 7, and draw a dotted line from a to c and From c 

 to 6. We have now indicated the bottom of a drain that 



