NATURAL PHILOSOPHY. 177 



the atmosphere a high degree of humidity, the dew itself will be very 

 abundant at this season. On the other hand, since the torrid zone 

 possesses the highest known atmospheric temperature, the nocturnal 

 cooling ought to precipitate there a larger quantity of water than in 

 any other country, by reason of the divergence above mentioned 



J f W ^J 



between the progression of the vapor and that of the temperature. 

 In fact, the dews are so copious in the equatorial regions, that Hum- 

 boldt does not hesitate to compare their effects with those of rain itself. 

 Want of Dew in Polynesia. A curious fact, and one not much 

 known, which seems at first sight to contradict what we have been 

 saying, is the extreme feebleness, or absolute non-existence of dew, in 

 that extensive assemblage of small islands in the torrid zone, generally 

 fertile, and more or less rich in plants, which geographers have denom- 

 inated Polynesia. But with little attention it will be seen that this 

 apparent anomaly affords one of the most striking confirmations of the 

 truth of the theoretical views unfolded in this memoir. In fact, what- 

 ever may be the humidity of these small islands, scattered here and 



*' * 



there in the vast ocean, and their tendency to the cooling produced 

 by the long nights and the luxurious vegetation, the small extent ot 

 their territories renders the atmospheric column superincumbent on 

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

 surrounding sea. This invasion is, moveover, favored 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 tem- 

 perature. 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 difference between the temperatures of the 

 day and night being extremely small, dew can scarcely be formed at 

 all, or at any rate, in very small quantity. 



Perfectly analogous causes prevent the formation of dew on ships 

 which traverse the vast solitudes of the ocean. But what is truly 

 singular, is the appearance of the phenomena, on board these same 

 ships, on arriving afterwards in the neighborhood 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. The reason of 

 this strange phenomena will be readily 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-tune, 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 the humid air to a certain distance out at sea. Then, 



