492 



STATE BOARD OF AGRICULTURE. 



the elevation of the stake at which it was taken, and snbtracting' from this sum the 

 fore-sight reading gives the elevation of the stake at which the fore-sight reading 

 was taken. 



Proceed in this manner nntil the elevations of all the stakes have been found, in 

 each case recording the height of instrument and elevation of stake in the proper 

 places. 



At this point compare your results with those recorded in table V, below. 

 If they do not agree in all cases go over your work to discover and correct your 

 difficulty. 



PALL AND DEPTH OP DITCH. 



To help in determining the fall and depth of ditch, a diagram or pro- 

 file such as is shown in Fig. 17 is very helpful. 



Table V is complete up to and including the elevations of the stakes, 

 and is identical with table IV as you have completed it if you have 

 cared to make the computations as directed in boldface type above. 



In making the computations which remain we shall use Table V, and 

 into it introduce the results which we obtain from these computations. 



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 expense in digging and filling. 



(2) To have the ditch suflSciently deep. InsufiQcient depth would 

 endanger the tile from frost or even from plow points and it might fail 

 to lower the ground w^ater sufficiently for best results. 



A good way to proceed is something as follows : 



1. Eeferring to Fig. 17, we find that conditions will permit a depth 

 of three feet at stake 1, which is practically the outlet. Three feet is 

 a satisfactory depth. Let us establish on our diagram, Fig. 17, point a, 

 three feet below the top stake 1. 



2. For trial let us establish a point, b, three feet below the top of 

 stake 11. 



3. 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 

 a b that this plan brings the drain very close to the surface at stakes 

 7 and 8. 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 



