OPERATIONS OF DRAINING. 



107 



the hollow, and thus form a swamp from A to B. It may 

 not be necessary here to cut a trench on each side along 

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

 in the hollow, and giving egress to the water, may relieve 

 the pressure and remove the swamp. 



'^ Sometimes, upon a sloping surface, one pervious stra- 

 tum, in which the water percolates, may produce more 

 than one hne of springs, as at B and A in fig. 10. Here 

 a single drain, cut at B, will remove the cause of wetness 

 at both swamps, without the necessity of the drain at A. 



" And, in practice, it is well to mark the effects of a 

 drain cut in the higher part of the slope to be drained, for 

 these effects often extend further than might be anticipated, 

 removing springs, oozings, or bursts at a great distance. 



