546 MELLONI ON THE NOCTURNAL COOLING OF BODIES. 



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 ele- 

 ments of nocturnal radiation proceed (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 

 D-reatest difference, then, between the temperature of the days 

 and that of calm and clear nights, will occm- 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 noctiu'nal cooling ought to preci- 

 pitate there a larger quantity of water than in any other country, 

 by reason of the divergence above mentioned between the pro- 

 gression of the vapour and that of the temperature. In fact, the 

 dews are so copious in the equatorial regions, that M. de Hum- 

 boldt does not hesitate to compare their effect with those of rain 

 itself. 



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 the 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 denominate 

 Polynesia. 



But, with a little attention, it will soon be seen that this ap- 

 parent anomaly affords one of the most striking confirmations of 

 the truth of the theoretical views unfolded in the course of this 

 memoir. In fact, whatever may be the humidity of these small 

 islands, scattered here and there in the vast ocean like oases in 

 a desert, and their tendency to the cooling produced by the long 

 nights and luxuriant vegetation, the small extent of their terri- 

 tories renders the atmospheric column superincumbent on each 



