M. Melloni on Dew. 367 



shall now add, that in these countries where the foliage is 

 generally narrow and vertical, like that of New Holland, the 

 nocturnal temperature ought to be less diminished, relative 

 to the diurnal temperature, than in places of the same lati- 

 tude covered with plants analogous to those which grow in 

 ether countries. 



Copiousness of Dew in Tropical Countries. 



But, laying aside everything depending on the alternations 

 of the seasons in our temperate climates, and on the differ- 

 ences of vegetation in countries situated under the same la- 

 titude, 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 coun- 

 tries, the two principal elements of nocturnal radiation pro- 

 ceed (so to speak) in opposite directions ; since the night is 

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

 the plants are richly clothed with foliage. But under the 

 equator, vegetation never fails, the night is always long, and 

 almost entirely without twilight ; and in the neighbouring 

 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 

 and clear nights will occur in the equatorial regions, a short 

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

 in the atmosphere a high degree of humidity, the dew itself 

 also 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 between 

 the progression of the vapour and that of the temperature. 

 In fact, the dews are so copious in the equatorial regions, 

 that M. de Humboldt does not hesitate to compare their 

 effect with those of rain itself. 



2b2 



