196 ESSAY 



Radiation of heat by the earth to the heavens 

 must exist at all times ; but, if the sun be at 

 some height above the horizon, the degree of 

 which is hitherto undetermined, and probably 

 varies according to season, and several other 

 circumstances, the heat emitted by it to the 

 earth will overbalance, even in places shaded 

 from its direct beams, that which the earth 

 radiates upwards. I suspended at midday, on 

 the 24th of July, 1813, in the open air over a 

 grassplat, while the sky was wholly covered 

 with very dense clouds, and the weather calm, 

 two delicate thermometers, one of which was 

 naked, but the other cased in gold paper. At 

 two observations, having an interval of 10 mi- 

 nutes between them, the thermometer in the 

 gilt case was 2 lower than that which was naked. 

 A white paper case was then drawn over the 

 gilt one, upon which, after 5 minutes, the 

 covered instrument was observed to be at the 

 same height with the naked. The outer white 

 case having, in the next place, been taken from 

 the covered thermometer, but that which was 

 gilt suffered to remain, the two instruments 

 were in a few minutes found again to differ 2. 

 A thermometer on the grassplat was, during 

 these experiments, higher than the naked in- 

 strument in the air by 2, and than that in the 

 gilt case by 4. It is evident, therefore, that 



