EFFECTS OP FORESTS OJT LOCAL RAINFALL. 159 



being abandoned to the action of gravity, begin to fall. By the 

 theory of the resistance of fluids, the velocity of descent in air of a 

 given density ia as the square root of the diameter of the globule. 

 The larger globules therefore fall fastest, and if (as must happen) 

 they overtake the slower ones, they incorporate, and the diameter 

 being thereby increased, the descent grows more rapid and the 

 encounters more frequent, till at length the globule emerges from the 

 lower surface of the cloud, at the vapour plane, as a drop of rain ; 

 the size of the drop depending on the thickness of the cloud-stratum 

 and its density." 



From what has been advanced it appears that rain is simply the 

 precipitation from the air of moisture which happened to be held by 

 it in excess of the quantity which it could, at its reduced tempera- 

 ture, hold in solution. It differs from the cloud or fog only in being 

 formed of drops produced by the mutual attraction of smaller drops 

 which are rapidly drawn by gravitation to the ground, instead of 

 floating long suspended, as the constituent lesser drops had been, in 

 the air. And this it may be well to bear in mind in proceeding to 

 the consideration of the local effects of forests on the rainfall. 



Section II. — The EffecU of Forests on the Qnantity of the Local 



Rainfall. 



I have had occasion in a previous chapter to refer to the existence 

 of an opinion held by many, and widely-diffused, that forests and 

 mountains attract rain. I have otherwise explained the phenomena 

 which have been attributed to some attraction exercised by them 

 upon clouds, alleging and maintaining that the clouds seen in such 

 situations must have been produced there ; and I am disposed to 

 allege and maintain something similar in regard to rain. 



The effects of trees on the humidity of the earth and atmosphere 

 has received the attention of several men of science and observation 

 in Britain, some of whom — Sir John Herschel and others — have 

 spoken out very decidedly on the subject. But in Britain it has 

 been less of a practical question, demanding attention as a means of 

 avoiding threatening calamities, than it has been in other lands ; and 

 while many of those who have given their attention more especially to 

 this subject speak decidedly, they speak with the caution which science 

 demands. Boussingault says: "My opinion is that the felling of 

 trees over a large extent of country has always the effect of lessening 



