254 Polmaise Method of Heating 



houses VA^here such flues exist. The waste of heat by un- 

 der-ground channels of conduction is immense, whether 

 these channels be hot-air drains, smoke-flues, or cast iron 

 pipes, in consequence of the great absorption of heat by 

 the cold mass of materials around them ; and if hot air be 

 conducted through channels beneath the floor, and com- 

 municating with the atmosphere at a considerable distance 

 from the fire, a very small portion of the heat generated 

 will enter the atmosphere by the aperture of ingress. This 

 fact may be easily demonstrated by experiment. For ex- 

 ample, let a stream of air, heated to 150°, be forced by its 

 own specific gravity through a tube 100 feet in length ; by the 

 time it has travelled to the end of the tube it will be reduced 

 nearly to the temperature of the external atmosphere. In an 

 under-ground drain the reduction of temperature must be 

 much greater, as the solid materials of which the latter is 

 composed, will abstract the heat more rapidly than the atmos- 

 phere. It is impossible to calculate the amount of heat 

 absorbed by an under-ground channel of conduction, but it 

 must be very great. This advantage is gained by the common 

 flue, which not only conducts the heat, but radiates it along 

 its course, and the more a flue is exposed on all sides to the 

 atmosphere, the more heat is radiated from its surface. Al- 

 though the formation and materials of Polmaise drains and 

 smoke are the same, their characters are clearly different. 

 A smoke-flue is a medium both of conduction and radiation, 

 — a hot-air drain a medium of conduction only. 



From what has been said, I think it will require no great 

 depth of penetration to perceive the superiority of common 

 flues over Polmaise or hot-air drains, apart from the consider- 

 ation of the heat lost by escaping with the smoke, a circum- 

 stance which is inevitable in a Polmaise stove, as smoke will 

 not ascend from a hot-air furnace without carrying along 

 with it a large amount of caloric, and this caloric cannot be 

 economized without conducting the smoke through the house, 

 and carrying off the caloric by radiation into the atmosphere. 

 I have here taken a practical view of hot-air drains, and if 

 any advantage arise from their use in the experience of 



