72 Dr. Heberden’s account of the heat of July , 1825 ; 
which it is placed, the sensations of the body depend altoge- 
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 8o° 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 
ioo°, 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. 
