72 Dr. Heberden's account of the heat of July, 1825 ; 



which it is placed, the sensations of the body depend aUoge- 

 ther upon the rapidity with which its own heat is carried off. 

 And this is by no means confined to the actual temperature 

 of the air ; but whatever alteration of quality increases its 

 power of conducting heat ; and, above all, whatever currents 

 increase the succession of its particles in contact with the 

 body, the same will increase the sensation of cold. Hence 

 it is, that in very hot weather, the same stream of air which 

 would heat a chamber, will nevertheless be cool to the feel- 

 ing ; on the other hand, when the thermometer was more 

 than 80° below the freezing point, Captain Parry observed, 

 that while the air was still, the cold was borne without in- 

 convenience. 



It therefore occurred to me, that the proper way to esti- 

 mate the sensible cold, would be, first to raise a thermometer 

 to a height something exceeding the natural heat of the 

 human body, and then to observe at what rate the quicksilver 

 contracted upon exposure to the air. For this purpose I used 

 a thermometer with a very small bulb, which might show 

 the alteration of heat in a short time. This I held to the fire 

 till it rose to about 120**, and then carried it in a warm glove 

 into the open air. I had with me an assistant with a watch 

 in his hand : and as soon as the mercury had descended to 

 100°, he began to count the seconds, while I continued to 

 observe the thermometer, marking the degree of heat at the 

 end of every ten seconds during half a minute. The result 

 rather exceeded my own expectations ; and (being, as far as 

 I know, the only experiments of the kind, ) I have thought 

 the Society might not dislike to be made acquainted with 

 them. 



