106 



OPERATIONS OF DRAINING. 



" This accordingly forms, in the greater number of 

 cases, the rule adopted in practice for the laying out of 

 drains upon the surface. The line is drawn nearly at, 

 or a httle above, the line of wetness, or, to use the com- 

 mon expression, betw^een the wet and the dry. 



" Should the line of drain be drawn too much below 

 the line of wetness, as at G, fig. 6, then the trench would 

 fail to intercept the water ; and further, if it w^ere filled 

 with earth, stones, and other substances, in the way to be 

 afterwards described, 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 Fig. 9. 



would be useless. 



" It is for this reason that, in 

 common practice, the rule is, to 

 clear the line of the drain nearly 

 between the wet and the dry, or 

 a little above it, taking care to 

 give it the necessary descent, and 

 to form it of sufficient depth to 

 reach the pervious bed or stra- 

 tum in which the water is con- 

 tained. 



" But as the water may arrive 

 at the surface in different ways, 

 and the wetness be produced by 

 different causes, so variations 

 from this rule of lining out the 

 drain may be required, and the 

 judgement of the drainer is to be 

 shown in adapting the course of 

 his drain to the change of cir- 

 cumstances. 



" Sometimes in a hollow piece 

 of ground feeders may reach the 

 descent, as in fig. 9, and the 

 water may be forced upw^ards by 

 the pressure from each side of 



