ON DRAINING. 



405 



ing to some other outlet, although perhaps not visible. 

 The mode of di'aining now recommended is founded on 

 this principle, that an outlet in an inferior elevation takes 

 off the pressure which causes the springs in the higher 

 elevation. In boring, it will sometimes be necessary to 

 pierce quite through the first open stratum, and again 

 through another bed of clay, until the porous bottom 

 containing the water is reached. 



It is sometimes necessary to carry a di'ain across a mill- 

 lead or some other aqueduct, the level of which is higher 

 than the ground to be drained. This is done by means 

 of a square box, of the size required to hold the water 

 which the drain emits, placed in a groove carried across 

 or below the lead, towards the river or stream flowing in 

 a lower level. On the bank of the river it will be ne- 

 cessary to make a back ditch, and form an embankment 

 between it and the river, to keep off all flood- water. 

 Another back ditch may be made if necessary along the 

 side of the lead. 



When land is water-locked, owing to an impervious 

 soil or subsoil, and on examination it is found impossible, 

 or too expensive, to procure a fall, the water will some- 

 times be got rid of by boring through the impervious 

 soils into the open strata below. The water can be pre« 

 viously collected in drains, made in the manner repre- 

 sented in Figure I. 



The earth taken from the main drains, conductors and 

 tabling, may be used in filling up any small hollows or 



