4 Temperature of the Terrestrial Globe. 



atmospheric strata which are condensed by their own weight, heat 

 them very unequally : those which are rarest are likewise coldest, 

 because they extinguish and absorb a smaller part of the rays. The 

 heat of the sun, coming in the form of light, possesses the property 

 of penetrating transparent soHds or liquids, and loses this property 

 entirely, when by communication with terrestrial bodies, it is turned 

 into heat radiating without light. 



This distinction of luminous and non-luminous heat, explains the 

 elevation of temperature caused by transparent bodies. The mass 

 of waters which cover a great part of the globe, and the ice of the 

 polar regions, oppose a less obstacle to the admission of luminous 

 heat, than to the heat without light, which returns in a contrary di- 

 rection to open space. The pressure of the atmosphere produces 

 an effect of the same kind : but an effect, which, in the present state 

 of the theory, and from want of observations compared with each 

 other, cannot be exactly defined. Whatever it may be, we cannot 

 doubt that the effect which should be attributed to the impression 

 of the solar rays upon a solid body of very large dimensions, by far 

 surpasses that which would be observed in exposing a common ther- 

 mometer to the same rays. 



The radiation of the most elevated strata of the atmosphere, the 

 cold of which is very intense and almost constant, has an influence 

 upon all the meteorogicaj facts of our observation ; it can be ren- 

 dered more sensible by reflexion from the surface with concave mir- 

 rors. The presence of the clouds which intercept these rays, miti- 

 gates the cold of the nights. 



We see that the surface of the terrestrial globe is placed between 

 a solid mass, the central part of which may surpass that of matter 

 heated to v/hiteness, and an immense girdle, whose temperature is 

 below that requisite for the congelation of mercury. 



All the preceding results can be applied to other planetary bo- 

 dies. They can be considered as placed in a medium whose con- 

 stant and common temperature is little below that of the terrestrial 

 poles. This same temperature of the heavens is that of the surface 

 of the most distant planets ; for the impression of the rays of the 

 sun, even when augmented by the disposition of the superficies, 

 would be too feeble to occasion sensible effects ; and we know by 

 the condition of the terrestrial globe, that, in the planets whose 

 formation cannot be less ancient, there exists upon the surface no 

 longer any elevation of temperature to be attributed to internal heat. 



