and the Mode of its Communication. 143 
vessel, and received heat from it, (though it became specifically 
lighter than it was before,) could not make its escape upwards , 
into the atmosphere, being confined and prevented from moving 
upwards by the thin projecting hoop of paper, there is no doubt 
but that the time of cooling was prolonged by this arrangement ; 
for, there being much reason to believe that the propagation of 
heat downwards, in air, from one particle of that fluid to another, 
is either quite impossible, or so extremely slow as to be imper- 
ceptible, as a succession of fresh particles of cold air was pre- 
vented from coming into contact with the bottoms of the vessels, 
but very little heat could have been given off immediately to the 
air in those experiments. 
In order to be able to form some probable conjecture re- 
specting the quantity so given off, in cases where the succession 
of fresh particles of air is free and uninterrupted, I made the 
following experiment. 
Exper. No. 31. The two conical vessels used in the last ex- 
periment, (which I shall now distinguish by calling the one 
No. 5, and the other No. 6,) being left suspended in the air, to 
the two horizontal arms of their wooden stand, at the height of 
44 inches above the floor of the room, (the pewter platters, the 
earthen dishes, and the stands on which they were placed being 
removed,) both the vessels were again filled with boiling hot 
water, and exposed to cool in the air. 
The vessel No. 5 remained in a vertical position, or with its 
fiat bottom in a horizontal position, as before ; but the vessel 
No. 6 was now reclined, so that its axis, and consequently the 
plane of its flat bottom, made an angle with the plane of the 
horizon, of 45 degrees. In this position of the vessel No. 6 , it is 
evident that the air, heated by coming into contact with its 
