544 MELLONI ON THE NOCTURNAL COOLING OF BODIES. 



vapour of the atmosphere, as most treatises on physics and me- 

 teorology assume, but the result of a series of actions and reac- 

 tions between the cold due to the radiation of plants and the cold 

 transmitted to the surrounding air. The grass is cooled but 

 little below the temperature of the air, but it very quickly 

 communicates to it a portion of the acquired cold ; and since 

 the difference of temperature between the radiating body and 

 the surrounding medium is independent of the absolute value of 

 the prevailing temperature, the grass surrounded by colder air 

 still further lowers its temperature and communicates a new 

 degree of cold to the air, which reacts in its turn on the grass, 

 and compels it to acquire a temperature still lower, and so on in 

 succession. Meanwhile the medium loses its state of equili- 

 brium and acquires a sort of vertical circulation, in consequence 

 of the descending motion of the portions condensed by the cold 

 of the upper foliage, and the ascending motion of the portions 

 which have touched the surface of the earth. Now, the gradual 

 cooling and the contact of the soil evidently tend to augment 

 the humidity of the stratum of air, and thus bring it by degrees 

 towards the point of saturation. Then the feeble degree of cold 

 pi-oduced directly by the radiation of bodies, suffices to condense 

 the vapour contained in the air which surrounds them ; and since 

 the causes which give rise to the circulating movement and to 

 the humidity of the air continue through the whole of the night, 

 the quantity of water deposited on the leaves increases inde- 

 finitely. 



The greatest part of the nocturnal cooling is due to the deve- 

 lopment of the leaves, which presents to the sky an immense 

 number of thin bodies having large surfaces, and almost com- 

 pletely isolated : this is the reason why the dews are so feeble in 

 winter and less copious in the nights of the early part of spring 

 than in the equally long nights of autumn. Dew is also more 

 abundant in autumn, because the days being then warmer than 

 in spring, and the vapour increasing more rapidly than the tem- 

 perature, the same degree of cold (such as the invariable de- 

 pression of the temperature of plants below that of the atmo- 

 sphere) condenses a greater quantity of vapour. The slightest 

 breath of wind disturbs the circulation of the lower atmospheric 

 stratum, and necessarily diminishes the accumulation of dew. 

 A strong wind impedes its formation by bringing fresh supplies 

 of heat, and especially by renewing incessantly the stratum of 



