ILLUSTRATIONS (18). CLIMATE OF AMERICA. 99 



plane. In the initial condition of radiation of all the leavea 

 which form the summit of a tree, and which partially cover 

 each other, those which are directly presented towards the 

 unclouded sky, will be first cooled. 



This production of cold (or the exhaustion of heat by 

 emission) will be the more considerable in proportion to the 

 thinness of the leaves. A second stratum of leaves has its up- 

 per surface turned to the under surface of the former, and will 

 give out more heat by radiation towards that stratum than 

 it can receive from it. The result of this unequal exchange 

 will then be a diminution of temperature for the second 

 stratum also. A similar action will extend from stratum to 

 stratum, till all the leaves of the tree, by their greater or less 

 radiation, as modified by their difference of position, have 

 passed into a condition of stable equilibrium, of which the 

 law may be deduced by mathematical analysis. In this 

 manner, in the serene and long nights of the equinoctial zone, 

 the forest air, which is contained in the interstices between 

 the strata of leaves, becomes cooled by the process of radia- 

 tion ; for a tree, a horizontal section of whose summit would 

 hardly measure 2000 square feet, would, in consequence of 

 the great number of its appendicular organs (the leaves), 

 produce as great a diminution in the temperature of the air 

 as a space of bare land or turf many thousand times greater 

 than 2000 square feet.* I have thus sought to develope 

 somewhat fully the complicated relations which the action of 

 great forest regions exerts on the atmosphere, because they 

 have so often been touched upon in connection with the im- 

 portant question of the climate of ancient Germany and Gaul. 



As in the old continent, European civilization has had its 

 principal seat on the western coast, it could not fail to be 

 early remarked that under equal degrees of latitude the oppo- 

 site eastern littoral region of the United States of North 

 America was several degrees colder, in mean annual tempe- 

 rature, than Europe, which is, as it were, a western peninsula 

 of Asia, and bears much the same relation to it as Brittany 

 does to the rest of France. The fact, however, escaped notice 

 that these differences decrease from the higher to the lower 

 latitudes, and that they are hardly perceptible below 30. 

 lor tks west coast of the New Continent exact observations 

 * Asie centrale, t. iii. pp. 195 205. 



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