428 



Of the Propagation of Heat 



Though the difference of the whole times of cooling 

 from 80 to 10 in these two experiments appears to 

 have been very small, yet the difference of the times 

 taken up by the first twenty or thirty degrees from the 

 boiling point is very remarkable, and shows with how 

 much greater facility Heat passes in moist air than in 

 dry air. Even the slowness with which the mercury in 

 the thermometer No. 4 descended in this experiment 

 from the 3Oth to the 2oth, and from the 2oth to the 

 loth degree, I attribute in some measure to the great 

 conducting power of the moist air with which it was sur- 

 rounded; for the cylinder containing the thermometer 

 and the moist air being not wholly submerged in the 

 freezing water, that part of it which remained out of the 

 water was necessarily surrounded by the air of the at- 

 mosphere ; which, being much warmer than the water, 

 communicated of its Heat to the glass ; which, passing 

 from thence into the contained moist air as soon as that 

 air became colder than the external air, was, through 

 that medium, communicated to the bulb of the inclosed 



