474 MELLONI ON THE NOCTURNAL COOLING OF BODIES. 



suffice to condense on the leaves a portion of the elastic and in- 

 visible vapour which pervades the surrounding air. 



It must be added, that the hypothesis of cooling from 1° to 10° 

 according to the position of the body, leads directly to the con- 

 sequence admitted by M. Pouillet himself, namely, that in many 

 cases where the atmospheric moisture is but little, certain plants 

 ought to be covered with dew, and others preserve their habitual 

 state of dryness ; and those who have had occasion to cross the 

 fields when the sun is below the horizon will no doubt have re- 

 marked that there is absolutely no dew, or else it is found distri- 

 buted in nearly equal proportions on all low plants, whatever be 

 the position of their leaves with regard to the sky. Now the 

 fact of an absolute absence of dew, or of its general diffusion in 

 a series of surfaces situated at all sorts of inclinations, indicates 

 clearly that the hypothesis of an extreme humidity of the air is 

 the only one which has place in nature when there is a formation 

 of dew. And the cause of this fact, which results so evidently 

 from the presence of dew on all low plants, as well as from hy- 

 grometric observations, is connected, if I am not mistaken, with 

 another fact, which has long been known to philosophers, but 

 which has hitherto remained isolated in science, in spite of its 

 great importance in phaenomena of nocturnal radiation. 



Wilson was the first to show that the effect of the radiation 

 of bodies towards the sky is sensibly the same at all temperatures ; 

 so that in nights equally calm and serene, the same substance is 

 always cooled to the same extent whatever may be the tempera- 

 ture of the atmosphere. 



Snow, for instance, would, according to the experiments of 

 Parry and of Scoresby, be cooled by about 9^, when the tem- 

 perature of the air descends to —1°, or to —2% or to —21°, or 

 to —22°; so that the bulb of a thermometer introduced into the 

 first layer of the snow-mantle which covers the soil of the north- 

 ern regions during the greatest part of the year would mark 

 — 10^ or —11° in the first case, and —30^ or —31° in the second. 

 I have never as yet found myself in circumstances favourable to the 

 verification of *^he observations of these two celebrated navigators; 

 but I have been able to convince myself of the truth of the prin- 

 ciple of Wilson, by experimenting with thermometers surrounded 

 with radiating substances, in the same manner as M. Pouillet 

 has done. Nevertheless as the mean values obtained by me do 

 not comprise so extended a range of temperature as that em- 



