Fire and Heat 65 



The law applied to draft. The draft, as in the fireplace, 

 is easily understood by the aid of the facts you have studied 

 experimentally. The air, which is directly heated by the fire, 

 expands and, as it expands, occupies more and more space, 

 so that a cubic foot of the expanded air weighs much less than 

 a cubic foot of the colder, unexpanded air. Since the pressure 

 of the air is the result of its weight, the downward pressure 

 of the air in the chimney cannot be so great as that of the 

 outside air. The outside, colder air falls toward the place where 

 the pressure is diminished through expansion due to rise in 

 temperature. The heavier air forces its way into the fireplace 

 where it is heated and then in turn is forced upward through 

 the chimney by incoming volumes of colder, heavier air. 



The process is the same as that observed in the course of 

 the smoke-laden air passing through the box and its chimneys. 

 It is due to the unequal heating of the air by the candle in the 

 one instance, by the fireplace in the other instance. The 

 principle is identical in both cases, in fact in all examples of 

 draft, whether in range, furnace, or fire in the open air. 



Control of draft. Heating appliances have been equipped 

 with certain more or less effective means of control of draft 

 and of escape of heated air. It is through such means that all 

 the heat produced by a fire does not follow the path above 

 described and waste itself in the great outdoors. Dampers 

 of different kinds are familiar. What devices of this kind 

 are on your range at home? When a fire has burned long 

 enough to establish a good draft and a bed of hot coals, the 

 draft is partly cut off by closing dampers in the range or chim- 

 ney. The escape of the heated air is checked so that the range 

 may become more uniformly heated. Dampers are provided 

 to direct the course of the heated air through the range to its 

 several parts, as the oven, for example. In many pipes there 

 is what is known as the " air damper," which when open admits 

 cold air directly from the room. This cold air reduces the 

 temperature of the air in the pipe and chimney, and checks 



