and Subterranean Water Storage. 81 



■waters of the lake had ag;ain risen, and had covered large tracts 

 of land formerly under cultivation. Many other examples arc 

 on record of results of the same nature following immediately on 

 the change from forest land to cultivation, or conversely from 

 cultivation to a fresh growth of forest. 



And it is clear that some such result must follow. When the 

 earth is bare it parts more rapidly with its heat, and evapora- 

 tion from the surface is more complete than when it is clothed 

 with forest. A larger quantity of the rain that falls is also then 

 retained near the surface for any limited time and the evapora- 

 tion that afterwards takes place is slower and lasts longer. But 

 even the precipitation of moisture as rain is less regular. 

 Where the ground is bare a soil is longer in being formed, more 

 liable to be removed by torrents, and the streams that flow 

 through it or convey its rainfall to the sea necessarily become 

 more rapid. Beneath a forest a swamp is often formed by the 

 natural accumulation of trunks of trees and other vegetation, 

 Avhile on cultivated land such a result is almost impossible. 

 On the other hand, the rapid flow of rivers is more apt to pro- 

 duce a bar or other impediment at the contact with the sea. 

 England has suffered less than other parts of Europe from un- 

 reasonable disforesting because from its geographical position 

 there is almost always a supply of moisture both in the atmo- 

 sphere and from rain, but it seems certain that a difference has 

 there also taken place. There has been a change of climate in 

 every essential sense of the Avord — a change in the mean tempe- 

 rature, annual, monthly, and daily — a reduction in the range of 

 temperature — a change in the amount and distribution of rainfall 

 — and a change also in the habits of the rivers and streams. 

 Many of the smaller of these have been entirely lost and sAval- 

 lowed up, and some are canalised or otherwise altered in volume. 

 They have also undergone alteration in the condition of the 

 freshets, and in the usual period and amount of the maximum 

 and minimum discharge of their waters. 



The actual quantity of woodland that should be left in a 

 given area to secure a fair climate must evidently depend on 

 many circumstances. In France the extent of the forests in 

 1750 was estimated at about 40 millions of English acres, while 

 in 1860 the area had been reduced to one-half, or 20 millions. 

 It is now believed that the former was not too much, and that at 

 present a great increase is necessary if the climate and rainfall 

 are to remain unaltered. There is certainly far too little forest 

 in most parts of the continent of Europe.* 



* See Marsh's ' Man and Nature/ p. 28 ; Ansted's 'Physical Geography,' ISfie, 

 p. 421. 



VOL. III.— S. S. G 



