128 STEPPES AND DESERTS. 



of leaves also A similar operation 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 equi- 

 noctial zone, the forest air contained in the intervals betv/een 

 the strata of leaves becomes cooled by the process of 

 radiation ; and by reason of the great quantity of its thin 

 appeiidicular 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. p. 195-205). I have sought thus to 

 develope in detail the comp^cated effects which make up the 

 total action of extensive forests upon the atmosphere, because 

 they have been so often touched upon in reference to the 

 important question concerning the climates of ancient 

 Germany and Gaul. 



As in the old continent European civilization has had 

 its principal 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 temperature than Europe, 

 which is, as it were, a projecting western peninsula to Asia, 

 as Brittany is to the rest of France. But in this remark 

 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 



