COMMON THINGS FI11E. 



Fig. 4. 



activity unless expedients be provided for the supply of the 

 quantity of oxygen which must enter into combination with it. 

 30. The construction of grates, stoves, and chimneys is therefore 



designed to attain this end by 

 causing such a volume of common 

 air to pass through the fuel as is 

 necessary and sufficient to combine 

 with it. The more air which thus 

 passes through the fuel, the more 

 rapid and abundant will be the 

 combination, and the more active 

 and vivid the combustion. 



31. The current of air which 

 passes through a common grate is 

 produced by the draught of the 

 chimney. The column of air in- 

 cluded in the chimney, being 

 raised to a higher temperature 

 than that of the external air, is 

 rarefied and lighter, bulk for 

 bulk, than the external air, and 

 is proportionately more buoyant. 

 It has therefore a tendency to 

 ascend like that which oil would 

 have in water. As it ascends the 

 air from the room must rush in 

 to fill its place. A part of this 

 air will pass through the bottom 

 and front of the grate, and a part 

 will enter at the opening of the 

 fire-place over the grate. This 

 will be more easily understood by 

 fig. 4. The front of the grate is 

 A B, and the bottom B c, having 

 the ash-pit below it. The opening 

 over the grate is A i, and E P G 11 

 is the flue of the chimney. The 

 ascensional force of the column of 

 air in the flue is measured by the 

 difference between its weight and 

 that of an equal volume of the 

 external air. The air which re- 

 places that which ascends in the 

 flue enters the bottom B c, the 

 front B A of the grate and the 

 202 



