M. BECQUEREL, ON THE CLIMATIC EFFECTS OF FORESTS. 325 



we may feel an oppressive heat in the woods. We may attribute it 

 simply to the absence of currents of air, and this may be true to a 

 certain point, but this appears to be the cause of this warming : when 

 the branches and leaves, becoming warm, become so many radiators of 

 heat. 



We will now explain the mode of influence by which trees affect the 

 temperature of the air around their trunks and branches, and may be 

 able from this to draw the conclusion that the mean temperature of a 

 place may be ameliorated by this cause. To aid in the solution of this 

 question, it will be necessary to consult observations upon temperature 

 made in places wooded and cleared, under the same latitudes, with the 

 same geological conditions, and at the same height above sea-level. 



[The author here cites from Jefferson's Notes on Virginia to show the 

 immediate effect of clearing upon temperatures, as elsewhere more fully 

 mentioned.] 



We will pass to observations that inspire more confidence, such as 

 those discussed by M. Boussiugault, and made by him and Humboldt, 

 and by Roulin, Eivero, and others, in localities between 11° N. and 5^ 

 S. latitude, where the celestial radiations act with full effect. 



The mean temperature, on account of its small variations in the course 

 of the year, is shown directly by that of the earth in the shade at 3 

 decimeters (11.8 inches) below the surface. 



Observations show that the temperature of the torrid zone varies 

 from 26*^.5 to 280.4 (centigrade), and that the abundance of forests and 

 the humidity have a tendency to cool the clin^ate, while dryness and 

 aridity would have the opposite effect. These effects are olDserved at 

 different heights on the Cordilleras, where the temperatures of middle 

 latitudes occur. 



Shall we inquire whether it is the same in localities wooded and not 

 wooded, without the tropics, where, the mean temperatures being the 

 same, the means of summer and of winter are different? No observa- 

 tions have yet been made upon this point. Former and subsequent ob- 

 servations tend to show, on the contrary, that clearing over a great area 

 does not sensibly change the mean temperature. 



Humboldt collected a large number of thermometric observations 

 made at different points in North America, for the purpose of learning 

 whether the mean temperatures had changed in a course of years. He 

 remarks that he had records for sixty- three years (1771 to 1834), and 

 returns from thirty-five military posts at which the thermometer has 

 been observed, from the southern point of Florida at 24<^ 35' of latitude, 

 to Council Bluffs on the Missouri, and from a study of these records he 

 arrived at the following conclusions : 



These observations tend to show, contrary to the opinion very gen- 

 erally received, that since the first settlement of Europeans in Pennsyl- 

 vania and Virginia, the climate has not become more uniform, more 

 mild in winter or cooler in summer, either this side of or beyond the 

 Alleghenies, in consequence of the great number of forests. 'We can- 

 not, however, concur in this; and Humboldt himself admits that clear- 

 ing tends to ameliorate the temperature, by removing three tendencies 

 to cooling, viz : first, shelter to the soil from the sun's rays, and mainte- 

 nance of greater humidity; second, evaporation from the leaves; and, 

 third, the multiplication of surfaces, which have a cooling effect by 

 reason of nocturnal radiatious. 



M. Boussiugault, as we have already seen, came to opposite conclu- 

 sions, since he found that where the forests are abundant the humidity 



