M. Melloul on Dew. 305 



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 ac- 

 quired cold; and since the difference of temperature be- 

 tween the radiating body and the surrounding medium is in- 

 dependent of the absolute value of the prevailing temperature, 

 the grass surrounded by a colder air still further lowers its 

 temperature, and communicates a newdegreeof cold to the air, 

 which reacts, in its turn, on the grass, and compels it to ac- 

 quire a temperature still lower, and so on in succession. 

 Meanwhile the medium loses its state of equilibrium, 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 gra- 

 dual 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 produced 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 indefinitely. 



The greatest part of the nocturnal cooling is due to the 

 development of the leaves, which presents to the sky an im- 

 mense number of thin bodies having large surfaces, and 

 almost completely isolated ; this is the reason why the dews 

 are so feeble in winter, and less copious in the nights of the 

 early parts of spring, than in the equally long nights of au- 

 tumn. Dew is also more abundant in autumn, because the 

 days being then warmer than in spring, and the vapour in- 

 creasing more rapidly than the temperature, the same degree 

 of cold (such as the invariable depression of the temperature 

 of plants below that of the atmosphere) condenses a greater 

 quantity of vapour. The slightest breath of wind disturbs 

 the circulation of the lower atmospheric stratum, and neces- 



VOL. LIII. NO. CVI. — OCTOBER 1852. 2 B 



