Marcet on Nocturnal Radiation. 143 



attributed to an absence of nocturnal radiation from the soil, 

 for this radiation, which depends on the heat acquired by the 

 earth during the day, should evidently be more intense, other 

 things being equal, in torrid than in temperate regions. We 

 must, therefore, as it appears to me, seek for the explanation in 

 the two following circumstances : — First. In tropical regions the 

 heat of the sun is so intense that his rays warm not only the 

 surface of the earth, but penetrate the soil to a certain depth. 

 It follows from this that at sunset, when the surface of the 

 earth begins to cool by radiation, the diurnal heat concentrated 

 in the interior arrives at this surface, warms it again, and pre- 

 vents the refrigeration of the stratum of air in immediate con- 

 tact with it. The second circumstance which, according to my 

 opinion, may explain the absence in certain tropical regions of 

 the effects produced in temperate climates by nocturnal radia- 

 tion, depends on the great quantity of water which the atmos- 

 phere holds in the form of elastic vapour, in a country whose 

 mean temperature is elevated, as Queensland. The recent 

 researches of Professor Tyndale have shown to what extent 

 the elastic vapour of water operates to intercept the obscure 

 heat emitted by the soil. This physicist has calculated that 

 even in England, where the air must contain infinitely less 

 vapour than in the torrid zone, it nevertheless suffices to inter- 

 cept, at a distance of less than ten feet from the surface, the 

 tenth part of the heat emitted by the soil. In tropical countries, 

 and especially in those which are not far removed from the sea, 

 the quantity of aqueous vapour contained in the atmosphere 

 is no doubt more considerable, and the quantity of intercepted 

 heat should be proportionately greater, and in consequence the 

 effects produced by the nocturnal radiation of the soil less 

 noticeable. 



" I have the more confidence in my son's observations as 

 M. Lucien de la Rive has arrived at analogous results on the 

 plains of Egypt in the neighbourhood of the Nile. M. dela 

 Rive did not, it is true, attach to his observations the im- 

 portance they probably deserve, partly because they were not 

 very numerous, and partly because during the season at which 

 they were made a decided wind frequently arose at sunset, and 

 mingled the strata of the air so as to modify to a certain 

 extent the phenomenon of temperature resulting from external 

 radiation. I confess, however, that the analogy between his 

 results and those obtained by my son in a country still nearer 

 the equator, leads me to think, in the absence of more ample 

 information, that the phenomenon of nocturnal radiation, and 

 the effects which it produces, exhibit themselves in climates 

 of the torrid region in a very different manner from that in 

 which they are displayed in the temperate zone. 



