ANNOTATIONS A.ND ADDITIONS. 127 



ing 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 Amentaceae, 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 

 Courier, 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 from it. The result of this unequal exchange 

 will thus be a loss of temperature for the second stratum 



