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XVII. On the Effects of Nocturnal Radiation within the Tropics, 

 By Professor Marcet, of the Academy of Geneva*. 



IT is now a recognized fact, that about the period of sunset 

 and during a great part of the night, provided the sky be 

 clear and the atmosphere calm, the temperature of the stratum 

 of air in immediate contact with the earth is lower by 2° or 3° 

 Cent, than the temperature of the atmosphere at an altitude of 

 a few feet. The observations relative to the cold produced by 

 nocturnal radiation, on which the above data are founded, have 

 hitherto been confined to the temperate climates of Europe; 

 they have never, to my knowledge, been repeated in the warmer 

 atmosphere of the torrid zone. I therefore thought it expedient 

 to take advantage of a residence of my son during several months 

 in Australia, district of Peak Downs, Queensland (latitude 22° 

 south), to obtain from him a series of observations on the effects 

 of nocturnal radiation similar to those which I. had myself made 

 at different periods in the neighbourhood of Geneva f, and which 

 have more recently been repeated by Professor Martens at Mont- 

 pellierj. With this view 1 sent him, with the necessary instruc- 

 tions, a couple of carefully graduated Centigrade thermometers, 

 in which the tenth part of a degree could be easily distinguished. 

 My impression, I own, was that the phenomenon of a nocturnal 

 increase of temperature at a few feet above the ground would, in 

 all probability, be more apparent in a very hot country situated 

 within the tropics than in the more temperate climates of 

 Europe, both on account of the greater transparency of the 

 atmosphere and because, as the surface of the earth receives 

 during the day a far greater quantity of caloric, it appeared 

 natural to infer that the nocturnal radiation would be propor- 

 tionably greater. It was therefore with considerable surprise 

 that, on examining the observations made by my son at his sta- 

 tion on Peak Downs in March and April 1862, I found my con- 

 jecture to be completely erroneous. According to these obser- 

 vations, the increase of temperature of the air between 5 and 6 

 feet above the ground, compared to that of the stratum at about 

 an inch and a half above the surface of the earth, appears to be 

 so slight as to be frequently barely appreciable, generally not ex- 

 ceeding from 0°*1 to 0°*2 Cent., and only in one instance reach- 

 ing 0°*4. And this was the case under circumstances reckoned 

 most favourable to nocturnal radiation, the nights being gene- 

 rally splendid, and the sky almost always perfectly clear. 



* Communicated by the Author. 



f See Memoir es of the Societe de Physique et d'Histoire Naturelle of 

 Geneva, vol. viii. 1839; and Bibliotheque Universelle for November 1861. 

 J See Memoires of the Academie des Sciences of Montpellier, vol. v. 



