430 ^f ^^^^ Propagation of Heat 



body is supposed to lose, when exposed to the cold of 

 winter, above what it communicates to the surrounding 

 atmosphere in warm summer weather ; but is it not 

 more than probable that the difference of the quantities 

 of Heat, actually lost or communicated, is infinitely 

 less than what they have imagined ? These inquiries 

 are certainly very interesting ; and they are undoubtedly 

 within the reach of well-contrived and well-conducted 

 experiments. But taking my leave for the present of 

 this curious subject of investigation, I hasten to the 

 sequel of my experiments. 



Finding so great a difference in the conducting powers 

 of common air and of the Torricellian vacuum, I was 

 led to examine the conducting powers of common air of 

 different degrees of density. For this experiment I 

 prepared the thermometer No. 4, by stopping up one 

 of the small glass tubes passing through the stopple, 

 and opening a passage into the cylinder, and by fitting a 

 valve to the external overture of the other. The in- 

 strument, thus prepared, being put under the receiver 

 of an air-pump, the air passed freely out of the globe 

 and cylinder upon working the machine, but the valve 

 above described prevented its return upon letting air in- 

 to the receiver. The gage of the air-pump shewed the 

 degree of rarity of the air under the receiver, and con- 

 sequently of that filling the globe and cylinder, and im- 

 mediately surrounding the thermometer. 



With this instrument, the weather being clear and 

 fine, the mercury in the barometer standing at 27 inches 

 9 lines, the thermometer at 15°, and the hygrometer at 

 47°, I made the following experiments. 



