on the Radiation of Heat. 309 



comparable with a thermometer covered with black wool, with 

 which the utmost effect I ever obtained in the month of February, 

 in this country, was 36°. The difference, therefore, of the power 

 of the sun in the two situations was 31°; from which, let any 

 reasonable deduction be made for reflection, and the remainder 

 will be' amply sufficient to support my conclusion. 



M. Gay-Lussac has entirely omitted to mention a confirmation 

 of my argument, upon which I place very great reliance, and 

 which, possibly, it was more agreeable to him to overlook, than to 

 refute by the same species of reasoning which has been deemed 

 sufficient for Captain Sabine and me. M. de Humboldt " often 

 endeavoured to measure the power of the sun between the tropics, 

 by two thermometers of mercury perfectly equal, one of which re- 

 mained exposed to the sun, while the other was placed in the shade. 

 The difference resulting from the absorption of the rays in the ball 

 of the instrument never exceeded 3°. 7. (6°.6. Fahr.) ; sometimes 

 it did not even rise higher than one or two degrees." 



What ! Did M. de Humboldt also select days for his experiments, 

 " quand I'atmosphere etait moins calme ou moins pure que dans 

 les jours du meme mois a Londres ?" or was he so little skilled in 

 experiment as not to know how to place his instrument so as to 

 receive the full impression which he wished to measure ? 



Captain Sabine tried the very same experiment with a naked 

 thermometer at Jamaica, and obtained the same result, namely, 

 3.1 centigrade degrees between the sun and the shade. 



Will Monsieur Gay-Lussac say, that we have no analogous 

 experiments in these northern latitudes to compare with those of 

 the uncoated thermometers? I refer him to the Ephemerides of the 

 Meteorological Society of the Palatinate, published in 1783 and 

 following years ; in which he will find a register of the power of 

 the sun at Manheim, measured by equal and carefully-adjusted 

 thermometers, with naked bulbs, nearly every day in the year for 

 several years. He will there see a difference of from 5 to 7 octo- 

 gesimal degrees (6.3 to 8.7 centig.) often recorded. 



I will now take the opportunity of introducing another argument 

 in favour of my hypothesis, tlerived from a very different source, 



