430 DEAHiTING OF LAKE OF HAAELEM. 



There is no reason to donbt that these differences are due tc- 

 the draining of the lake. In summer, solar irradiation has acted 

 more powerfully on the now exposed earth and of course on the 

 air in contact with it ; and there is no longer a large expanse of 

 water still retaining, and of course, imparting something of the 

 winter temperature ; in winter, the earth has lost more heat by 

 radiation than when covered by water, and the influence of the 

 lake, as a reservoir of warmth accumulated in summer and grad- 

 ually given out in winter, was of course lost by its drainage. 

 Doubtless the quantity of moisture contained in the atmosphere 

 has been modified by the same cause, but it does not appear that 

 observations have been made upon this point. Facts lately ob- 

 served by Glaisher tend to prove an elevation of not far from 

 two degrees in the mean temperature of England during the 

 course of the last hundred years. For reasons which I have 

 explained elsewhere, the early observations upon which these 

 conclusions are founded do not deserve entire confidence ; but 

 admitting the fact of the alleged elevation, its most probable 

 explanation would be found in the more thorough draining of 

 the soil by superficial and subterranean conduits. 



So far as respects the immediate improvement of soil and cli- 

 mate, and the increased abundance of the harvests, the English 

 system of surface and subsoil drainage has fully justified the 

 eulogiums of its advocates ; but its extensive adoption appears to 

 have been attended with some altogether unforeseen and unde- 

 sirable consequences, very analogous to those which I have de- 

 scribed as resulting from the clearing of the forests. The under- 

 di-ains carry off very rapidly the water imbibed by the soil from 

 precipitation, and through infiltration from neighboring 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 subterranean 

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

 earth, would have reached it only by small instalments after per- 

 colating through hidden paths for weeks or even months, 'and 

 would have furnished perennial and comparatively regular con- 

 tributions, instead of swelhng deluges, to its channel. Thus, when 

 human impatience raslily substitutes swiftly acting artificial con- 

 trivances for the slow methods by which nature drains the surface 



