568 Of Chimney Fireplaces. 



ings, communicating with the air of the atmosphere) 

 immediately above the roof of the house. 



If this air-tube be situated in the middle of a build- 

 ing, it is evident that a horizontal canal or tube of 

 communication must be carried from its lower orifice to 

 some open place without the building, in order to estab- 

 lish a free circulation of fresh air, both upwards and 

 downwards, in the air-tube. I say both upwards and 

 downwards^ for sometimes the current of air in the tube 

 will be found to set upwards, and sometimes down- 

 wards. Its direction will depend on the winds that hap- 

 pen to prevail, or rather on the eddies they occasion in 

 the air out of doors in the neighbourhood of the build- 

 ings ; and it is no small advantage that will arise from 

 leaving both ends of the air-tube open, that the tube 

 will always be supplied with a sufficiency of air, what- 

 ever eddies the winds may occasion. It is easy to per- 

 ceive how powerfully this must operate to prevent 

 those puffs of smoke which, in high winds, are fre- 

 quently thrown into some rooms by the eddies, and the 

 partial rarefactions of the air that they occasion ; but 

 this is far from being the only or the most important 

 of the advantages that will be derived from this air-tube. 

 Those who consider what an immense quantity of air is 

 required to supply the current that sets up the chimney 

 of an open fireplace, where there is a fire burning, must 

 perceive what an enormous loss of heat there must be, 

 when all this expense of air is supplied by the warmed 

 air of the room, and that all this warmed air is necessa- 

 rily and constantly replaced by the cold air from with- 

 out, which finds its way into the room by the crevices 

 of the doors and windows. But all this waste of heat, 

 or any part of it, at pleasure, may be prevented by the 



