and the Economy of Fuel. 1 5 



communication double, one within the other ; the inter- 

 nal canal by which the heat passed, and which was 5 

 inches wide internally, and 6 inches high, being itself 

 placed, and, as it were insulated, in a canal still larger, 

 in such a manner that the canal by which the heat 

 passed (which was constructed of very thin bricks, or 

 rather tiles) was surrounded on every side with a wall, 

 2 inches thick, of confined air. The surrounding canal 

 being formed in the solid body of the mass of brick- 

 work, this arrangement of the double canals was en- 

 tirely concealed. The double canals and the double 

 vault over the fire-place were intended to serve the same 

 purpose ; namely, to confine more effectually the heat, 

 and prevent its escape into the mass of brick-work, and 

 its consequent loss. 



Having found, in the course of my experiments, that 

 confined air is the best barrier * that can be opposed to 

 heat, to confine it, I endeavoured to avail myself of that 

 discovery in these economical arrangements, and my 

 attempts were not unsuccessful. 



Not only the fire-place itself, and the canals of com- 

 munication between the fire-place and the boilers, were 

 surrounded by confined air, but it was also made use of 

 for confining the heat in the boilers, and preventing 

 its escaping into the atmosphere. This was done by 

 making the covers of the boilers double. These covers 

 (see the Figures i and 2, Plate I.) which were made of 

 tin, or rather of thin iron plates tinned, were in the 

 form of a hollow cone. The height of the cone was 

 equal to about one third of its diameter, and the air 

 which it contained was entirely shut up, the bottom of 



* See Philosophical Transactions, 1792, Part I. See also Vol. I., pp. 401 

 and following. 



