COMMON FIRE. 



opening A i above it, as indicated by the arrows. The former 

 portions, passing through the burning fuel, supply to it the 

 oxygen gas necessary to combine with it, and thus maintain the 

 combustion. These portions after passing through the interstices 

 of the fuel, and after the oxygen or a part of it, has combined 

 with the fuel, issue from the top of the fuel, being then a mixture 

 of azote, such portion of oxygen as may not have combined with 

 the fuel, carbonic acid and aqueous vapour, the latter being the 

 products of the combination of the oxygen with the carbon and the 

 hydrogen of the fuel. 



All these gases issuing from the burning fuel at a high tem- 

 perature, and mixing with the cold air which enters the chimney 

 through the opening A I, render the column of air in the flue so 

 warm as to give it the buoyancy necessary to sustain the draught. 



When the fire is first kindled in the grate, if the air in the 

 chimney have the same temperature as the external air, it will 

 have no buoyancy, and there will be no draught. In this case 

 the chimney will generally be found to smoke. This inconvenience 

 may be sometimes removed by opening the windows, so as to fill 

 the room with air as cold as the external air, and therefore 

 colder than the air in the chimney. If, however, this be found 

 insufficient, the air in the flue may be warmed and the necessary 

 draught produced by holding under the chimney any blazing 

 combustible. 



The draught through the grate may be greatly increased in 

 intensity by stopping up, either partially or completely, the opening 

 A I. By this expedient, all the air necessary to replace that which 

 ascends in the chimney must pass through the fuel in the grate. 

 If the magnitude of the opening be for example three times the 

 magnitude of the front and bottom of the grate, four times as 

 much air will thus pass through the fuel as would pass through it 

 when the opening AF is not closed, supposing the draught in the 

 chimney to be the same in both cases. 



But, in fact, the draught in the chimney will be greatly 

 augmented by this process : for, so long as the opening A I is not 

 closed, the air which fills the chimney will consist of a mixture of 

 that which passes through the burning fuel, which is raised to a 

 high temperature, and the much larger portion which passes into 

 the chimney through the opening A I, and which, being cold, 

 lowers the temperature, and therefore diminishes the buoyancy of 

 the air in the chimney. But when all the air which passes 

 through A I, by closing that opening, is made to pass through the 

 burning fuel, it is raised to a high temperature, which not being 

 lowered by admixture with any air not passing through the fuel, 

 fills the chimney with air raised to a very elevated temperature, 



