MEI.I.ONI ON THE NOCTURNAL COOLING OF BODIES. 537 



radiating body, such as a piece of wood or stone, placed on a 

 moist soil, towards sunset, is abundantly covered with dew on 

 its lower side before a single drop of liquid appears on the upper 

 surface. The body submitted to the frigorific action of the sky 

 is in contact with two masses of air, — the one, at rest and humid, 

 because it is sheltered and situated close to the earth's surface ; 

 the other, less humid, and exposed to the changes of the atmo- 

 sphere. The former then will be more disposed than the latter 

 for the precipitation of vapour, and the dew ought to show itself 

 first on the side turned towards the soil ; it may even exist only 

 on this surface, if the air has but little moisture or is agitated 

 by wind. Hence the experiment of a plate covered with waxed 

 cloth, which, being placed on the grass, was found sometimes 

 to be moistened only on its lower surface, by no means proves 

 that the dew is exhaled from the ground, like those clouds of va- 

 pour ivhich are seen to arise from a vessel full of hot water. 



Neither does the humidity which sometimes appears, towards 

 the end of the night, on the surface of a dry soil, constitute an 

 argument favourable to this hypothesis and contrary to the prin- 

 ciple of nocturnal radiation, as some have maintained. In fact, 

 two causes may contribute, either conjointly or separately, to 

 the production of the pha^nomenon. Every one knows, that, 

 when a moist soil becomes dry by means of wind or solar radi- 

 ation, the water which has penetrated to a certain depth ascends 

 by capillary action, and again moistens the surface when the 

 permanent cause of the drying has ceased. Moreover, the un- 

 covered soil is itself endowed, like the grass, with a proper radi- 

 ation of its own, capable of cooling and of bringing upon it the 

 deposition of the atmospheric vapour, especially in the long and 

 humid nights of autumn, during which the cold engendered by 

 the radiation of the surface penetrates more deeply, and can no 

 longer be compensated by the heat of the internal strata. 



The details into which we have entered are more than suffi- 

 cient to prove that the reproach which has been cast several 

 times on the partisans of Wells's theory, that they have neglected 

 to take into account the moisture of the soil, is altogether un- 

 founded ; these philosophers, on the contrary, in accordance with 

 the vulgar notions in this respect, refer the whole atmospheric 

 humidity by which dew is caused, to the water spread over the 

 surface of the earth. In fact, vapour, in its elastic and invisible 

 state, penetrates the atmosphere, not only by the means of rain, 



VOL. V. PART XX. 1 O 



