98 VIEWri OF NATURE 



the Llanos of Venezuela and the Pampas of Buenos Ayres, 

 are covered with small monocotyledons, belonging to the 

 family of the Cyperacege, and with grasses, whose dry pointed 

 stalks, and whose delicate, lanceolate leaves radiate* towards 

 the unclouded sky, and possess an extraordinary power of 

 emission. Wells and Daniell* have even seen in our latitude, 

 where the atmosphere has a much less considerable degree of 

 transparency, the thermometer fall to 14. 5, or 18 Fahr. on 

 being placed on the grass. Melloni has most ably shownf 

 that in a calm, which is a necessary condition of a powerful 

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

 stratum of grass is promoted by the falling to the ground of 

 the cooler particles of air, as being the heavier. 



In the vicinity of the equator, under the cloudy sky of the 

 Upper Orinoco, the Rio Negro and the Amazon, the plains 

 are covered with dense primeval forests ; but to the north and 

 south of this woody district, there extend, from the zone of 

 palms and of tall dicotyledonous trees in the northern hemi- 

 sphere, the Llanos of the Lower Orinoco, the Meta, and 

 Guaviare ; and in the south, the Pampas of the Rio de la 

 Plata and of Patagonia. The area thus covered by grassy 

 plains, or Savannahs, in South America, is at least nine times 

 greater than that of France. 



The forest region acts in a threefold manner, by the coolness 

 induced by its shade, by evaporation, and by the cooling pro- 

 cess of radiation. Forests uniformly composed in our tem- 

 perate zone of" social" plants, belonging to the families of the 

 Coniferae or Amentaceae (the oak, beech, and birch), and under 

 the tropics composed of plants not living socially, protect the 

 ground from direct insolation, evaporate the fluids they have 

 themselves produced, and cool the contiguous strata of air by 

 the radiation of heat from their leafy appendicular organs. 

 The leaves are by no means all parallel to one another, and pre- 

 sent different inclinations towards the horizon ; and according 

 to the laws established by Leslie and Fourier, the influence of 

 this inclination on the quantity of heat emitted by radiation 

 is such, that the radiating power of a given measured surface a, 

 having a given oblique direction, is equal to the radiating 

 power of a leaf of the size of a projected on a horizontal 



* Meteor. Essays, 1827, pp. 230, 278. 



f Sull' Abbassamento di Temperatura durante le Notti placide t 

 terene, 1847, pp. 47, 53. 



