FORESTS AND THEIR CLIMATIC INFLUENCE. 409 



to ascertain if tbe meau temperature liad undergoue chauges after a con- 

 siderabie lapse of years. For about sixty-three years, from 1771 to 1S31, 

 lie tells us, tbermometric observations bad been maintained at tbirty- 

 five military posts, so that we bave far more exact ideas on tbe climate 

 of Nortb America tban existed in tbe times of Jefferson, Barton and 

 Volney. 



Tbese stations were distributed over a space of 40° of longitude and 

 extended from tbe point of Florida and Thompson's Island, under 24° 

 35' of latitude, to Council Bluft's, on tbe Missouri. On discussing tbe 

 observations communicated to bim, Humboldt arrives at tbe following 

 conclusions: 



Tbese observations, be says, tend to demonstrate, contrary to an 

 opinion quite generally adopted, that, since tbe first estaljlisliuient of 

 Europeans in Pennsylvania and Virginia, tbe climate, on either side of 

 the Alleghanies, has not, in consequence of tbe destruction of numbers 

 of forests, become more uniform, more mild in winter and cool in sum- 

 mer, tban it was before; nevertheless, as Humboldt himself acknowl- 

 edges, denudation ought to ameliorate the mean temperature by effect- 

 ing the disappearance of three frigoritic causes ; first, the protection of 

 the ground from tbe solar radiation and the maintenance of a greater 

 humidity; secondly, the production of aqueous transpiration by tbe 

 leaves; thirdly, the multiplication by tbe expanded branches of the 

 surfaces which are cooled through tbe eifect of nocturnal radiation. M. 

 Boussingault, as we bave previously seen, has arrived at contrary con- 

 clusions, indicating that tbe abundance of forests and the humidity 

 thence resulting tend to render tbe climate cooler, and that dryness or 

 aridity produces an opposite effect. 



It inigbt be, however, that, the mean temperature remaining the same, 

 the disfribution of beat in the course of tbe year may be changed, and 

 that thus the climate may have been modified ; but it will not suffice to 

 invoke the authority of documents relating to cultivation at tbe present 

 time, for these documents will not bear a serious examination, as we 

 have shown in our treatise on climates. 



Still, it is possible that a step in advance may be made by taking into 

 consideration observations which have thus far not received due atten- 

 tion. The observations regarding temperature which we bave made in 

 the interior of single trees and at tbe periphery of their branches show, 

 as has been already said, that trees are affected like all bodies exposed 

 or unexposed to solar radiation; that is to say, that they are heated or 

 chilled according to their absorbent, reflecting, and conducting powers. 

 Tbese observations evince, moreover, that their calorific state depends 

 in great part on tbe solar action. What can we thence infer in relation 

 to the influence of trees on tbe temperature of tbe air and the changes 

 which that temperature undergoes as tbe effect of denudation ? These 

 changes result not only from the cause of which we have been speaking, 

 but further, we repeat, from the nature of the soil, according as it is dry 

 or humid, calcareous, sandy, or argillaceous. Let us analyze the effects 

 which may thus be produced. 



We will'first consider a wooded soil: Tbe trees, as has been just stated, 

 become heated or cool ; but what results from this when tbe soil is dry 

 and when it is humid ? If the former, it will be without influence ; if it 

 is humid, the evaporation of water will maintain a constant humidity, 

 the degree of which will de])end on tbe temperature v^'hich the trees have 

 acquired and which will be independent of that resulting from the exu- 

 dation by tbe leaves. Tbe humidity caused by trees, when other things 

 are equal, will be greater in wooded countries with an argillaceous 



