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Show 112 STEPPES AND DESERTS. condition of strong radiation and of the formation of dew1 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.1 many individuals of one, or of a few kinds, of the families of Coniferre or Amentacere1 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 (pouvoir 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 unclouded 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 |
