TOTAL INFLUENCE OF THE FOREST ON TEMPERATURE. 175 



least so mucli modified by local conditions which are infinitely 

 varied, that no general formula is applicable to the question. 



In the report to which I referred on page 161, Gay-Lussac 

 says : " In my opinion we have not yet any positive proof that 

 the forest has, in itself, any real influence on the climate of a 

 great country, or of a particular locality. By closely examining 

 the effects of clearing off the woods, we should perhaps find that, 

 far from being an evil, it is an advantage ; but these questions 

 are so complicated when they are examined in a climatological 

 point of view, that the solution of them is very difficult, not to 

 say impossible." 



Becquerel, on the other hand, considers it certain that in trop- 

 ical climates the destruction of the forests is accompanied with an 

 elevation of the mean temperature, and he thinks it highly prob- 

 able that it has the same effect in the temperate zones. The fol- 

 lowing is the substance of liis remarks on this subject : 



" Forests act as frigorific causes in tliree ways : 



" 1. They shelter the ground against solar irradiation and main- 

 tain a greater humidity. 



" 2. They produce a cutaneous transpiration by the leaves. 



" 3. They multiply, by the expansion of their branches, the 

 surfaces which are cooled by radiation. 



" These tliree causes acting with greater or less force, we must, 

 in the study of the climatology of a country, take into account 

 the proportion between the area of the forests and the surface 

 which is bared of trees and covered with herbs and grasses. 



" We should be inclined to beheve, d priori, according to the 

 foregoing considerations, that the clearing of the woods, by rais- 

 ing the temperature and increasing the dryness of the air, ought 

 to react on climate. There is no doubt that, if the vast desert of 

 the Sahara were to become wooded in the course of ages, the 

 sands would cease to be heated as much as at the present epoch, 

 when the mean temperature is twenty-nine degrees [Centigrade, 

 = 85° Fahr,]. In that case, the ascending currents of warm air 

 would cease, or be less warm, and would not contribute, by de- 

 scending in our latitudes, to soften the climate of Western Eu- 

 rope. Thus the clearing of a great country may react on the 

 climates of regions more or less remote from it. 



" The observations by Boussingault leave no doubt on this 



