ON THE CONSTRUCTION OF FIRE-PLACES. 195 



The glazed frame then, described above, will prevent, it is evi- 

 dent, the heat when once received into the room, from again 

 escaping from it, as it now docs, with the air ascending in the 

 chimney ; and although the glass is some obstacle to the radiation 

 of the heat from the fire in the first instance, the disadvantage is 

 much more than compensated by its retaining agency afterwards. 



One of our rooms, as now constructed and heated, may be com- 

 pared to a vessel of water of similar shape, with a hole near its 

 bottom, through which the water is constant!} running off, while 

 an attem|)t is making, at the same time, to warm its contents by 

 heat radiating inwards, from the hole and around it. l^he hottest 

 water would always get out first, being nearest the opening from 

 whence the heat came ; and to keep the vessel full, this would be 

 replaced by fresh cold water, entering by one or more openings in 

 the circumf(!renf e. It would require a powerful heat indeed, to 

 raise much the temperature of such a vessel ; and it is evident, 

 that Jio degree of heat so admitted, could warm the contents uni- 

 formly. 



It may be supposed that I have under-rated the proportion of 

 caloric which radiates from a fire into the room, compared with 

 that which ascends the chimney, in calling the former only a fourth 

 part of the whole produced ; but the following considerations, 

 without new experiment, may probably be accounted decisive of 

 the question : — Mr. Leslie, in his experiments on heat, found that 

 a metallic vessel of water, of medium temperature, suspended in 

 the air, lost about half its caloric by radiation, and half by contact 

 with the air. At a higher temperature, however, on account of 

 the increased velocity of the air, caused by its greater expansion, 

 or in an artificial current of air, without higher temperature, it lost 

 much more by contact than by radiation. Now, in a fire are found 

 the two circumstances of extreme heat and great velocity, and to 

 these is added a third, of much greater importance than either, 

 viz: the surface of contact being exceedingly increased bj" the air 

 passing between the pieces of coal, while the surface of radiation, 

 viz: the external surface, remains the same. 



It is a thing not suiliciently adverted to in the management of 

 our fires, that the heat given into the room, is propoitioned rather 

 to the extent of burning surface presented towards the room, than 

 to the depth of the fire, the intensity of the heat backwards, and 

 the quantity of the fuel consumed. I have been trying experi- 

 ments, with a view to ascertain the proportions exactly ; of which, 

 however, I have not as yet had time to prepare an account for 

 publication ; but as the general result, I may state, that a tile, or 

 sheet of iron, laid on the back part of the fire, so as to cover it 

 closely, and to prevent combustion, except in front, rather in- 

 creases than diminishes the radiation of heat towards the apart- 

 ment, and much less fuel is consumed. 



In constructing the glass frame proposed, a part must be made t© 

 open, to allow the putting on of coal, and stirring of the fire. The 

 ^ir, tp feed the fire^ may come from an opening in the external wall, 



