ANNOTATIONS AND ADDITIONS. 113 



from it. The result of this unequal exchange will thus be a loss of 

 temperature for the second stratum of leaves also. A similar opera- 

 tion will continue from stratum to stratum until all the leaves of the 

 tree, by greater or less radiation, as modified by their diversity of 

 position, have passed into a state of stable equilibrium, of which the 

 law can be deduced by mathematical analysis. In this manner, in 

 the long and clear nights of the equinoctial zone, the forest air con- 

 tained in the intervals between the strata of leaves becomes cooled 

 by the process of radiation; and by reason of the great quantity of 

 its thin appendicular organs or leaves, a tree, the horizontal section 

 of whose summit would measure for example 2000 square feet, would 

 act in diminishing the temperature of the air equivalently to a space 

 of bare or turf-covered ground several thousand times greater than 

 2000 square feet (Asie Centrale, t. iii. pp. 195-205). I have sought 

 thus to develope in detail the complicated effects which make up the 

 total action of extensive forests upon the atmosphere, because they 

 have been BO often touched upon in reference to the important ques- 

 tion concerning the climates of ancient Germany and Gaul. 



As in the Old Continent European civilization has had its princi- 

 pal seats on a western coast, it could not but be early remarked that, 

 under equal degrees of latitude, the opposite eastern coast of the 

 United States was several degrees colder in mean annual tempera- 

 ture than Europe, which is, as it were, a projecting western penin- 

 sula to Asia, as Brittany is to the rest of France. But in this re- 

 mark it was forgotten that these differences decrease from the higher 

 to the lower latitudes, in such manner that they almost entirely 

 disappear from 30 downwards. For the west coast of the New 

 Continent, exact thermometric observations are still almost entirely 

 wanting ; but the mildness of the winters in New California shows 

 that the west coasts of America and Europe, under the same paral- 

 lels of latitude, probably differ little from each other in mean annual 

 temperature. The subjoined table shows what are the correspond- 

 ing mean annual temperatures, in the same geographical latitudes, 

 of the west coast of Europe and the east coast of the New Conti- 

 nent. 



10* 



