468 MELLONI ON THE NOCTURNAL COOLING OP BODIES. 



here, as in the experiments of LesHe and Rumford, a portion of 

 the radiation which produces the cooHng of the thermometers 

 arises from points situated at a certain depth below the surface. 



This property of heat, which reveals itself during the trans- 

 formation of ordinary heat into radiating heat, perfectly accounts 

 for a phsenomenon which some observers consider sufficient to 

 completely overthrow the theory of Wells as to the formation of 

 dew. It is generally seen at the present day, that, according to 

 the English philosopher, dew is a necessary consequence of noc- 

 turnal radiations, which give rise, in plants and other bodies 

 exposed to the free air, to the cooling necessary to precipitate 

 the transparent and invisible aqueous vapour pervading the 

 atmosphere. Now if we admit as true the tendency of bodies to 

 become colder under a serene sky, say the adversaries of Wells' 

 theory, this tendency will be compensated by the heat of the 

 surrounding air, especially when the body is very thin, and con- 

 sequently whose mass is very small compared with the extent of 

 its surface. 



Thus the spiders' webs so profusely scattered in the country 

 during certain seasons of the year can scarcely descend below 

 the surrounding temperature, and ought to remain sensibly dry 

 during the whole night ; and yet precisely the opposite is ob- 

 served, since, other circumstances being the same, these little 

 corpuscles are more abundantly bathed in dew than any other 

 substance. But the objection implies an absolute ignorance of 

 the fact which we have just mentioned, and of the elements of 

 physics. 



In fact, as the contact of the air takes place only at the sur- 

 face, and radiation occurs not merely at the surface but also at 

 points situated at a certain depth, bodies which radiate towards 

 a clear sky during the night may be compared to a vessel full of 

 water the bottom of which is pierced vA'ith a number of holes, 

 whilst, to compensate the loss sustained, we caused to arrive, by 

 means of a second recipient of the same form and dimensions, 

 water flowing from a single opening of equal diameter to one of the 

 preceding, suppc sing it even the largest of all. The water would 

 enter without ceasing into the vessel, and yet nevertheless the level 

 of the liquid would necessarily fall. It is in this manner that 

 the temperature diminishes in a body radiating towards the sky 

 in spite of the heat communicated to it by the contact of the air. 



Suppose now that a given quantity of matter is successively 

 formed into discs of larger and larger size and diminishing thickr 



