and the Mode of its Communication. 91 



jecting 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 propa- 

 gation of heat downwards, in air, from one particle of 

 that fluid to another, is either quite impossible or so ex- 

 tremely slow as to be imperceptible, as a succession of 

 fresh particles of cold air was prevented 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 

 respecting the quantity so given off in cases where the 

 succession of fresh particles of air is free and uninter- 

 rupted, I made the following experiment. 



Experiment No. 31. The two conical vessels used 

 in the last experiment (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 flat 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 bottom, had 

 full liberty to escape upwards, and to make way for 

 other particles of colder air to come into contact with 

 the hot surface and be heated, rarefied, and forced up- 



