176 ANNUAL OF SCIENTIFIC DISCOVERY. 



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 develop- 

 ment of the leaves, which present to the sky an immense 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 long nights of 

 autumn. Dew is more abundant in autumn, because the days being 

 then warmer than in spring, and the vapor increasing 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 vapor. The slightest breath of wind 

 disturbs the circulation of the lower stratum, and necessarily dimin- 

 ishes the accumulation of dew. A strong wind impedes the formation, 

 by bringing fresh supplies of heat, and especially by renewing inces- 

 santly the stratum of air comprised between the summit of the plants 

 and the surface of the earth, and thus taking away from it the possi- 

 bility of gradually acquiring that high degree of humidity necessary 

 to the precipitation of the vapor, by reason of the small degree of 

 cold which the plants contract with regard to the surrounding medium. 



The differences of the quantity of dew on different substances all 

 arise, either from their difference of emissive power, or from the 

 diversity of their situation, with regard to the heavenly vault, or from 

 the hygrometric condition of the surrounding space, or from the 

 greater or less obstacles which retard the descent of the air, and thus 

 more or less favor its frigorific reaction ; or lastly, from the proximity 

 of the soil, which permits the return of the air on the radiating sub- 

 stances, and gives rise to that aerial circulation, whence results the 

 gradual cooling and successive augmentation of humidity in the lower 

 stratum of the atmosphere. 



Copiousness of Dew in Tropical Countries. Laying aside every- 

 thing depending on the alternations of the seasons in our temperate 

 climates, and on the differences of vegetation in countries situated in 

 the same latitude, it is easy to convince ourselves, that the greatest 

 difference between the temperature of the day and that of the night, 

 will occur under the torrid zone, and that there also, the dews will, in 

 general, be more abundant than in any other part of the globe. In 

 fact, in cold and temperate countries, the two principal elements of 

 nocturnal radiation proceed (so to speak) in opposite directions ; since 

 the night is long when the earth is destitute of vegetation, and short 



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when the plants are richly clothed with foliage. But, under the equa- 

 tor, vegetation never fails --the night is always long, and almost 

 entirely without twilight; and in the neighboring countries forming 

 the torrid zone, properly so called, when the night time slightly 

 exceeds the period of daylight, the rain falls in torrents, and plants 

 are more richly clothed with leaves, than at any other season of the 

 year. The greatest difference, then, between the temperature of the 

 days and that of calm, clear nights, will occur in the equatorial regions, 

 a short time after the rainy season ; and as there will then prevail in 



