On the Effects of Nocturnal Radiation within the Tropics. 141 



This apparent anomaly, if correctly stated, as I have every 

 reason to believe, can hardly be attributed to a decrease in the 

 quantity of radiant heat with which the earth parts at the mo- 

 ment of sunset; for the amount of nocturnal radiation being 

 mainly dependent on the quantity of caloric that the earth has 

 acquired during* the day, must obviously, cceteris paribus, be 

 greater in the torrid zone than in the more temperate regions 

 of Europe. It may, however, I think, be explained by taking 

 into account the following circumstances. First, the heat of the 

 sun's rays within the tropics is probably sufficiently intense to 

 warm not only the surface of the earth, but to penetrate to a 

 certain depth below the surface. The consequence is, that after 

 sunset, when the earth begins to cool rapidly by nocturnal radia- 

 tion, the heat that has penetrated into the interior is gradually 

 brought back to the surface, and thus contributes to prevent the 

 cooling of the stratum of air in immediate contact with it. The 

 second circumstance, and probably that which tends most to 

 prevent nocturnal radiation producing the same effects as with 

 us, depends, I think, on the greater quantity of water which 

 must exist in the atmosphere, under the form of elastic vapour, 

 in a country where the mean temperature is so much higher than 

 it is in Europe. Professor TyndalPs recent experiments have 

 shown to what extent aqueous vapour is capable of intercepting 

 the passage of radiant heat, since he has calculated that even in 

 England, in ordinary clear weather, the tenth part of the heat 

 radiated by the earth into space is arrested at a distance of 

 less than ten feet from its surface. In the tropical regions, par- 

 ticularly those which are at no very great distance from the sea*, 

 the quantity of aqueous vapour which the atmosphere can take 

 up and maintain in the elastic form must be far more consider- 

 able, and consequently the decrease in the earth's radiation ren- 

 dered still more apparent. 



The following fact tends to corroborate, to a certain extent, 

 the results my son obtained in Queensland. My friend M. 

 Lucien De la Rive undertook last year, at my request, to make a 

 few observations on the effects of the cold produced by nocturnal 

 radiation in the plains of Egypt bordering on the Nile. Un- 

 luckily circumstances prevented their being made with regularity, 

 the weather not always being favourable, and M. De la Rive's 

 stay in Egypt having been shorter than he had intended. But 

 he made a sufficient number to be able to inform me that the 

 differences of temperature he observed about sunset, between the 

 stratum of air in contact w T ith the earth and that of the atmo- 



* My son's station in the neighbourhood of Peak Downs was not much 

 above a hundred miles in a direct line from the sea, and in the neighbour- 

 hood of Isaac's River. 



