408 FORESTS AND THEIR CLIMATIC INFLUENCE. 



(luce in a high degree the effects indicated above ; effects wliicli tlie 

 neighboring strata of air cannot fail to manifest. On the other hand, 

 the nocturnal radiation, which is very great under a sky almost without 

 clouds, must act powerfully in hastening the refrigeration of the leaves. 



The following fact is, to a certain point, referable to the heat which 

 woods emit when they have been warmed by solar radiation. Every 

 one Ivuows that at noon of a hot summer day the air in dense woods is 

 almost of a stifling heat. It is usual to attribute this effect to the 

 absence of currents of air, and that, to a certain extent, may bo true; 

 but a concurrent cause of no little efficiency is the fact that, when the 

 leaves and branches of trees have for some time been exposed to the 

 calorific action of the sun's rays, they themselves become foci of heat. 



T\"e have thus explained the sort of influence exerted by trees on the 

 temperature of the air which surrounds the trunk and branches; yet we 

 cannot confidently deduce the conclusion that the mean temperature of 

 the place is further ameliorated by this state of things. To aid us in 

 solving this question, it is necessary to consult the observations of tem- 

 perature made in wooded and unwooded places, situated under the same 

 latitude, having the same geological conditions, and at the same height 

 above the level of the sea. 



Jefferson, in a work translated (1780) by the Abbe Morellet, drew, 

 from observations made at Williamsburg and Monticello, (Virginia,) the 

 conclusion that, since the clearing away of the forest, a very sensible 

 change had taken i^lace in the climate; the heat, as well as the cold, 

 had become less vehement than before, as was testified by persons of 

 no very advanced age. The snows, he says, are less frequent and less 

 abundant, often not remaining in the valleys more than two or three 

 days, and very rarelj' a week, while, within the memory of the living, 

 they are known to have been frequent, deep and durable. Hy old per- 

 sons it is stated that the ground was covered with snow three months 

 in the year, and that rivers, which now freeze very rarely, were usually 

 congealed every winter. These assertions, it will be seen, are based on 

 testimony against which we must be on our guard, for it may well be 

 that years of extraordinary inclemency were taken for those of an 

 average temperature. Let us turn to observations which inspire more 

 confidence, such as those discussed by M. Eoussingault, and made by 

 MM. Boussingault himself, Humboldt, Eoulin, Eivero, «S:c., in localities 

 comprised between the 11th degree of north latitude and the 5th degree 

 of south latitude, where the celestial radiation prevails, during the 

 night, in all its force. 



The mean temperature, by reason of the slight variations in the 

 course of the year, is immediately given by that which is presented by 

 the earth, in the shade, at 3 decimetres (1 foot) below the surface. Ob- 

 servations show that the temperature of the torrid zone varies from 

 260.5 (SOo F.) to 280.1, (83° F.,) and that the abundance of forests and 

 humidity tend to the refrigeration of the climate, while dryness and 

 aridity produce contrary effects. These effects have been observed at 

 different heights on the Cordilleras, where w^e find the mean tempera- 

 tures of tem])erate regions. It has l3een asked whether this is the case 

 in localities wooded and denuded of wood, situated beyond the tropics, 

 where the mean temperatures being the same, the means of summer and 

 winter are different 1 No observation has yet been made as to this point. 



Observations subsequent to the i^receding tend to show, on the con- 

 trary, that disboscation on a great scale does not sensibly change the 

 mean temperature. Humboldt has collated a great number of thermo- 

 metric observations made at different points of Xorth America, in order 



