102 



OPERATIONS OF DRAINING. 



off by an impervious stratum below, and often upon the 

 borders. The first object here is to make an outlet, of 

 sufficient size and depth to carry off the water ; the sec- 

 ond to carry a main drain through the marsh or swamp ; 

 and the third to lay lateral and other under-drains, accord- 

 ing to the extent of the ground, to collect and conduct 

 the waters into the main drain. The under-drains should 

 not enter the main drain at right angles, but diagonally, 

 inclining down the stream. If waters come in from the 

 margins of the low ground, they must be arrested then 

 by under-drains, and conducted off, as represented in fig. 

 5. Care should be taken to sink the main drain, and the 



Fig. 5. 



Cross Section. 



others, particularly those around the margin of the swamp, 

 into the subsoil, or impervious stratum, so that the water 

 shall not pass under the drain into the lower ground. If 

 the surface-water that flows into the main drain be con- 



