176 TOTAL INFLUENCE OF THE EOEEST ON TEMPEEATUEE. 



point. This writer determined the mean temperature of wooded 

 and of cleared points, under the same latitude, and at the same ele- 

 vation above the sea, in locahties comprised between the eleventh 

 degree of north and the fifth degree of south latitude, that is to 

 say, in the portion of the tropics nearest to the equator, and where 

 radiation tends powerfully during the night to lower the temper- 

 ature under a sky without clouds." * 



The result of these observations, wliich has been pretty gener- 

 ally adopted by physicists, is that the mean temperature of cleared 

 land in the tropics appears to be about one degree Centigrade, or 

 a little less than two degrees of Fahrenlieit, above that of the 

 forest. On page 14Y of the volume just cited, Becquerel argues 

 that, inasmuch as the same and sometimes a greater difference is 

 found in favor of the open ground, at points within the tropics 

 so elevated as to have a temperate or even a polar chmate, we 

 must conclude that the forests in Northern America exert a re- 

 frigerating influence equally powerful. But the conditions of 

 the soil are so different in the two regions compared, that I think 

 we can not, with entire confidence, reason from the one to the 

 other, and it is much to be desired that observations be made on 

 the summer and winter temperature of both the air and the 

 ground in the depths of the North American forests, before it is 

 too late. 



Secent inquiries have introduced a new element into the prob- 

 lem of the influence of the forest on temperature, or rather into 

 the question of the thermometrical effects of its destruction. I 

 refer to the composition of the soil in respect to its hygroscopicity 

 or aptitude to absorb humidity, whether in a hquid or a gaseous 

 form, and to the conducting power of the particles of which it is 

 composed.*!* 



* Becquerel, Des CKmats, etc., pp. 139-141. 



f Composition, texture and color of soU are important elements to be con- 

 sidered in estimating the effects of the removal of the forest upon its thermo- 

 scopic action. "Experience has proved," says Becquerel, "that when the 

 soil is bared, it becomes more or less heated [by the rays of the sun] accord- 

 ing to the nature and the color of the particles which compose it, and accord- 

 ing to its humidity, and that, in the refrigeration resulting from radiation, we 

 must take into the account the conducting power of those particles also. 

 Other things being equal, silicious and calcareous sands, compared in equal 

 volumss with different argillaceous earths, with calcareous powder or dust. 



