M. Mellonl on Dew. 369 



Voyages^ tome i., page 625.) The reason of this strange 



phenomenon 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, 

 .tiij 9il j yd 



Dew becomes more abundant as we approach the Equator. 



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 

 phenomenon is very much modified by the extent, the nature, 

 and the position of the land, according as it is more or less 

 surrounded by the sea, more or less covered by mountains, 

 ravines, lakes, meadows, marshes, or running streams. The 

 borders of Egypt, of the Red Sea, of the Persian Gulf, of 

 Chili, and of Bengal, are celebrated for the richness of their 

 dews (see the Voyages de Volney, t. i., p. 51 ; of Burck- 

 hardt, p. 423 ; of JViebuhr, p. 10 ; of Ker Porter, t. ii., p. 123 ; 

 of Le Gentil, t. i., p. 624 ; of Ruppel, p. 186) ; the deserts 

 of Central Africa, and the interior provinces of Bahia ; of 

 Fernambouc, Urmia, and Mazandecan, in Brazil and Persia, 

 by the almost total absence of this nocturnal phenomenon. 

 (Voyages of Spix and Martins, t. ii., p. 624 ; of Oliver in 

 Persia, t, i., pp. 123, 145 ; of Ker Porter, t. ii., pp. 63, 69.) 





