on the Radiation of Heat. 307 



object, he selected his opportunities at different stations : but only 

 once, at Bahia, did he obtain a result which even equalled the 

 mean power of the sun for two years, in this country, in the 

 month of June ; all the cloudy days being included in the average: 

 and which fell short, by one-third, of the maximum effect which 

 often occurred in clear weather, measured by the same means. 

 Mons. Gay-Lussac objects that the thermometers were not always 

 placed at equal distances from the ground and from the vegetation 

 on it, and that they were not equally secured from currents of air, 

 &c. &c. ; but it must be admitted that there is ample room for 

 allowances of this kind, and yet to save the conclusion, which is 

 only general, that the power of solar radiation is less between the 

 tropics than in higher latitudes. He has also forgotten to men- 

 tion, that these results were confirmed by others obtained with 

 instruments of more delicate construction ; in which the thermo- 

 meters were placed in vacuo, one being armed with a case of 

 polished silver to repel the rays, and the other with a blackened 

 surface to absorb them. Bnl I again repeat, that if Captain Sabine 

 but once succeeded at any one station between the tropics in ob- 

 taining the full impression of the sun upon a blackened thermo- 

 meter, or even approached the full impression within one-third, 

 that there is ground for the hypothesis. 



Captain Sabine has not been accused of having sought the 

 maximum power of the sun upon cloudy, windy, or foggy days ; 

 but an argument of equal force is employed to controvert the 

 results of his experiments upon terrestrial radiation at night : for, 

 says M. Gay-Lussac, " On pourra, ce me semble, se borner S, de- 

 duire des observations qui precedent, que, du 24 an 30 Juiliet, 

 pendant le sejour de M. Sabine a Bahia, I'atmosph^re y etait 

 moins calme ou moins pure que dans les jours du meme mois oft 

 M. Daniell h. trouve a Londres un rayonnement nocturne de 8 ou 

 9 centigrades." Those who are practically acquainted with the 

 usual serenity and beauty of an intertropical sky, those more espe- 

 cially who have described the splendour of the stars and the 

 beauty of the constellations within the torrid zone, as differing so 

 much from what we are accustomed to in our turbid atmosphere, 



