40 The Theory of Irrigation. [January, 



in proportion. Accordingly it is a fact of experience (as 

 stated by Marsh in his " Man and Nature " previously 

 referred to) that, other things being equal, dry soils, and the 

 air in contact with them, are perceptibly warmer during the 

 season of vegetation, when evaporation is most rapid, than 

 moist lands with the atmospheric stratum resting upon 

 them. Under-drains, also, like surface-drains, withdraw 

 from local solar action much moisture which would other- 

 wise be vapourised by it, and, at the same time, by drying 

 the soil above them, they increase its effective hygroscopicity, 

 and it consequently absorbs from the atmosphere a greater 

 quantity of water than it did when, for want of under- 

 drainage, the soil was always humid, if not saturated. 

 Under-drains, then, contribute to the dryness as well as to 

 the warmth of the atmosphere, and as dry ground is more 

 readily heated by the rays of the sun than wet, they tend 

 also to raise the mean, and especially the summer, tempera- 

 ture of the soil. 



Although the immediate improvement of soil and climate, 

 and the increased abundance of the harvests have fully 

 testified to the advantages of surface and subsoil drainage as 

 adopted in England ; its extensive application appears to 

 have been attended with some altogether unforeseen and 

 undesirable consequences, very analagous to those resulting 

 from the clearing of the forests. The under-drains carry off 

 very rapidly the water imbibed by the soil from precipitation, 

 and through infiltration from neighbouring springs or other 

 sources of supply. Consequently, in wet seasons, or after 

 heavy rains, a river bordered by artificially drained lands 

 receives in a few hours, from superficial and from subter- 

 ranean conduits, an accession of water which, in the natural 

 state of the earth, would have reached it only in small 

 instalments, after percolating through hidden paths for 

 weeks or even months, and would have furnished perennial 

 and comparatively regular contributions, instead of swelling 

 floods to its channel. By thus substituting swiftly acting 

 artificial contrivances for the slow methods by which nature 

 drains the surface and superficial strata of a river basin, 

 the original equilibrium is disturbed ; the waters of the 

 heavens are no longer stored up in the earth to be gradually 

 given out again, but are hurried out of man's domain with 

 wasteful haste ; and while the inundations of the river are 

 sudden and disastrous, its current, when the drains have 

 run dry, is reduced to a rivulet. 



It has thus been shown that a great similarity exists in 

 the consequences arising from the destruction of forests and 



