MELLOXI ON THE NOCTURNAL COOLING OF BODIES. 547 



of them easily permeable even to its centre, by the air of the 

 surrounding sea. This invasion is, moreover, favoured by the 

 trade-winds which prevail constantly in those latitudes. Now 

 we know that the air in the midst of vast seas preserves a 

 nearly uniform temperature. The stratum of air cooled by the 

 contact of the soil will, then, be warmed by mixing with the 

 air which is constantly reaching it from the sea, and the differ- 

 ence betweeen the temperatures of the day and night being ex- 

 tremely small, dew can scarcely be formed at all, or at any rate, 

 in very slight quantity. 



Perfectly analogous causes jDrevent the formation of dew on 

 ships which traverse the vast solitudes of the ocean. But w^hat 

 is truly singular, is the appearance of the phsenomenon on board 

 these same ships on arriving afterwards in the neighbourhood 

 of terra firma. Thus, the navigators who proceed from the 

 straits of Sunda to the Coromandel coast, know that they are 

 near the end of their voyage when they perceive the ropes, sails, 

 and other objects placed on the deck, become moistened with 

 dew during the night. (Le Gentil, Voyages, tome i. page 625.) 

 The reason of this strange phaenomenon will readily be seen, if 

 we start from the fact (well established by experience), that, in 

 the equatorial regions, the sea-air preserves, not only a nearly 

 constant temperature by day and night, but also an hygrometric 

 state considerably removed from the point of saturation ; and 

 that the reverse is the case with regard to the air on land, which 

 in the day-time is drier than the air of the sea, but which in the 

 night may readily acquire, in countries sufficiently abounding 

 in water, or near enough to the coast, a much greater humidity 

 in consequence of the frigorific actions and reactions of which 

 we have before spoken. Now the land-wind, which always 

 blows by night on the borders of tropical countries when the 

 sky is clear, transports this humid air to a certain distance out 

 at sea. Then, the feeble degree of cold acquired by substances 

 freely exposed on the deck, totally unable, as it is, to condense 

 the vapour of the sea-atmosphere, is nevertheless sufficient to 

 precipitate that of the air which has been in nocturnal contact 

 ■with the soil. 



We conclude that dew, feeble or non-existent towards the 

 poles by reason of the extreme brevity of the summer nights, 

 becomes more and more abundant as we approach the equator; 

 that, notwithstanding, the general course of the phaenomenon 



