272 



Practical Jgricidtnre — Draining. 



Vol. II- 



the surface, extending over a considerable space, x.rxxx, Fig. 46, the effects appearing in the 

 wetness of the ground farther down the slope, as yi/y. The line where liie wetness 

 begins, and which is generally rendered perceptible by the change of color of the soil, the 

 tendency to produce subaquatic plants, and other indications of wetness, marks for the most part 

 nearly the course which the line of the drain should follow. By cutting a drain nearly in this line 

 as from G to A, and from L to A, sufficiently deep to reach the ])orous stratum in which the water 

 percolates, we shall intercept it before it reaches the surface, and by carrying it away in some conve- 

 tiient outlet, AB, remove the cause of wetness. 





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Figr. 46. 



This, accordingly forms, in the greater number 

 of cases, the rule adopted in practice for the lay- 

 ing out of drains upon the surface. The line is 

 drawn nearly at, or a little above, the line of wet- 

 ness, or, to use the common expression, between 

 the wet and the dry. 



Should the line of drain be drawn too much be- 

 low the line of wetness, as at G, Fig. 44, then 

 the trench would fail to intercept the ^vater ; and 

 further, if it were filled with earth, stones, and 

 other substances, in the way to be afterwards de- 

 scribed, the whole, or a part, of the water would 

 pass over it, and the injury be unremoved. 



Again, should the line be too much above the 

 line of wetness, as at H, the drain would fail to 

 reach the channel of the water, and so would be 

 useless. 



It is for this reason that, in common practice, 

 the rule is, to draw the line of the drain nearlyj, 



between the wet and the dry, or a litile above it, 

 taking care to give it the necessary descent, and 

 to form it of sufficient depth to reach the pervious 

 bed or stratum in which the water is contained. 



But as water may arrive at the surface in diffe- 

 rent ways, and the wetness be jiroduced by diffe- 

 rent causes, so variations from this rule of lining 

 out the drain may be required, and the judgment 

 of the drainer is to be shown in adapting the 

 course of his drain to the change of circumstances. 



Sometimes in a hollow piece of ground, feeders 

 may reach the descent, as in F"ig. 47; and the 

 water may be forced upwards by the pressure 

 from each side of the hollow, and thus form the 

 swamp from A to B. It may not be necessary 

 ht;re to cut a trench on each side along the lineoC 

 wetness at A and B ; a single trench C, cut in the 

 hollow, and giving egress to the water, may re- 

 lieve the pressure and remove the swami>. 





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Fig. 47. 



Sometimes upon a sloping surface, one pervious stratum, in which water percolates, may produce 

 more than one line of springs, as at B and A, in the following figure. Here a single drain cut at B 

 will remove the cause of wetness at both swamps, without the necessity of the drain at A. 



