112 STEPPES AND DESERTS. 



condition of strong radiation and of the formation of dew, the cooling 

 of the grassy surface is also promoted by the particles of air which 

 are already cooled sinking to the ground as being the heaviest. In 

 the vicinity of the Equator, under the clouded sky of the Upper 

 Orinoco, the Rio Negro, and the Amazons River, the plains are 

 clothed with dense primeval forests ; but to the north and south of 

 this wooded region there extend from the zone of palms and lofty 

 dicotyledonous trees, in the northern hemisphere, the Llanos of the 

 Lower Orinoco, the Meta and the Guaviare, and in the southern 

 hemisphere the Pampas of the Rio de la Plata and of Patagonia. 

 The space thus occupied by Savannahs or grassy plains in South 

 America is at least nine times as great as the area of France. 



The wooded region acts in a threefold manner in diminishing the 

 temperature ; by cooling shade, by evaporation, and by radiation. 

 Forests, -which in our temperate zone consist of trees living together 

 in "society," i. e., many individuals' of one, or of a few kinds, of the 

 families of Coniferse or Amentacese, oaks, beeches, and birches, but 

 in the tropics, of an immense variety of trees living separately or 

 "unsocially," protect the ground from the direct rays of the sun, 

 evaporate fluids elaborated by the trees themselves, and cool the 

 strata of air in immediate contact with them by the radiation of heat 

 from their appendicular organs or leaves. The latter are far from 

 being all parallel with each other; they are, on the contrary, variously 

 inclined to the horizon, and, according to the law developed by Leslie 

 and Fourier, the influence of this inclination upon the quantity of 

 heat emitted by radiation is such, that the power of radiation (pou- 

 voir rayonnant) of a measured surface a, having a given oblique 

 direction, is equal to the " pouvoir rayonnant" which would belong 

 to a surface of the size of a, projected on a horizontal plane. Now 

 in the initial condition of radiation, of all the leaves which form the 

 summit of a tree and partly cover each other, those are first cooled 

 which are directed without any intervening screen towards the un- 

 clouded sky. The cooling result (or the exhaustion of heat by 

 emission) will be the more considerable the greater the thinness of 

 the leaves. A second stratum of leaves has its upper surface turned 

 to the under surface of the first stratum, and will give out more heat 

 by radiation towards that stratum than it can receive by radiation 



