MELLONI ON THE NOCTURNAL COOLING OF BODIES. 535 



being heated by contact with the terrestrial surface, and may 

 easil)-^, on its again ascending, evaporate the first drops of water 

 deposited on the lower portions of grass, and again allow them 

 to be afresh precipitated on the higher leaves. This successive 

 transportation of dew will never occur in wet or very moist soils ; 

 and the lower portions of grass will there preserve the water con- 

 densed on their surface. But both in one case and in the other, 

 the first appearance of the phaenomenon will take place at a 

 short distance from the soil, and will afterwards extend itself to 

 portions of the plants more and more elevated, just as if the dew 

 rose out of the ground and gradually rose in the atmosphere. 

 Such, in fact, was the opinion of the ancient philosophers, gene- 

 rally adopted by those of the last century ; and such is still the 

 fundamental idea of the hypothesis maintained at this day by 

 certain experimenters, who consider the phasnomena which we 

 have been describing as altogether contrary to the explanation 

 of dew derived from the coohng produced by radiation. 



Another fact, which, according to the same experimenters, also 

 supports this alleged contradiction, is, the abundance of dew in 

 perfectly calm weather. It is very true that great tranquillity 

 in the atmosphere is remarkably favourable to the deposition of 

 dew; that the least wind diminishes it, instead of increasing it, 

 as has been wrongly maintained of late ; and that frequently the 

 water deposited amounts to a much greater quantity than could 

 arise from the elastic vapour contained in the small quantity of 

 air placed in contact with the leaves and other radiating sub- 

 stances. 



But we have seen that the lower stratum of the atmosphere 

 (as we have termed it) loses its state of equilibrium in conse- 

 quence of the nocturnal radiation of vegetables, and takes a rota- 

 tory movement, which commences in the first place by cooling 

 the whole fluid mass of which it consists, and afterwards conti- 

 nues when the air deposits the vapour it contains; so that the 

 fluid in contact with the leaves changes at every instant, be- 

 comes cooler and cooler, and, by fresh precipitations, increases 

 the liquid drops scattered over the surfaces of bodies. 



Let us add, that the quantity of water deposited does not de- 

 pend solely on the vapour disseminated through the atmosphere, 

 but also, and principally, on the humidity of the soil ; and that 

 it is most copious when the ground is saturated with water, as 

 any one may easily convince himself in countries where artificial 



