78 Rainfall, Natural Drainage, 



of streams once the outlets of mountain valleys. In Greece the 

 whole country was wooded in the time of Homer, and probably 

 for centuries afterwards. There is abundant proof that the period 

 of destruction, even on the Mediterranean shores, commenced less 

 than 2000 years ago, and had hardly affected a very large area 

 till within the last thousand years. These shores are now, to a 

 very great extent, absolutely bare of vegetation of all kinds. 



In discussing the influence of man, not only on the destruc- 

 tion and growth of trees and crops, but as exerted on climate, 

 it is quite necessary to appeal to figures and definite state- 

 ments, as we should otherwise be accused of exaggeration. 

 In America, the most recently cleared of all countries, it is 

 certain that in the year 1860 there was an area of some 250,000 

 square miles of country (160 millions of acres) under profitable 

 cultivation, replacing the same or nearly the same area of 

 primaeval forest land that existed there 300 years ago. The 

 climate and rainfall of North America before the disforesting 

 are very imperfectly known, but all the evidence that exists 

 favours the conclusion that the rainfall has diminished, that the 

 streams have become more rapid, and that the climate is warmer 

 in summer and cooler in winter. 



Northern Europe was in a similar way a country of forests 

 in the time of Caesar, although now there are no large forest tracts 

 remaining. There also the climate has changed, the rainfall has 

 diminished, and the air is drier than before. Wherever civilized 

 man appears the forest disappears, and cultivation takes its place. 

 Let us consider the result of this change in the various elements 

 of climate as far as we have facts to guide us. 



There is good proof that hail and other storms depending on 

 electrical causes have been far less frequent and severe where 

 forests have been cut down. This has been noticed especially in 

 the Alps, where much wood has been removed within a short 

 time. Nor is this remarkable, for very important chemical 

 changes are caused by vegetation, and when these, after proceeding 

 without interruption over thousands of square miles for many 

 centuries, are suddenly and abruptly terminated, the result 

 cannot but be felt. A surface covered by forest is generally 

 believed to absorb more carbonic acid and exhale more oxygen 

 than meadows or fields. 



But forests act, not only indirectly, but directly and very 

 manifestly, on temperature. During a large part of the year 

 cultivated lands are bare, or nearly so ; forest lands never. And 

 this of itself is a very important matter. If the power of cal- 

 careous sand to retain heat be taken at 100, arable calcareous 

 soil may be represented by 74'3, argillaceous soil at 68"4, and 

 common garden earth somewhat less. Humus, such as is 



