DEVELOPMENT OF HOUSE HEATING 119 



CONCLUSION. There is nothing mysterious about the draft 

 of a chimney. A chimney will "draw" if the laws of physics 

 have been regarded in the construction of the chimney and in 

 the operating of the stove. A column of heated air is lighter 

 than a column of cold air and will, therefore, be pushed up 

 through and out of the top of a chimney, unless there is some 

 other force opposing its motion. 



PROBLEMS 



1. How much will a column of air 1 ft. square and 30 ft. high 

 weigh if 12 cu. ft. weigh 1 lb.? 



2. If a chimney is 1 ft. square and 30 ft. in height and the air within 

 it is heated to 273C., how much will the air within it weigh if 1 cu. 

 ft. of air at 0C. weighs H2 lb.? 



3. The ordinary residence chimney for a stove is 9 in. square and 

 about 30 ft. in height. If the air out of doors is at 0C. and the air 

 within the chimney heated to an average temperature of 250C., a 

 similar column of air outside is how many times as heavy as that 

 within the chimney? What will be the result? 



137. Construction of the Chimney. Certain rules should be 

 followed in constructing a chimney, otherwise some interfering 

 forces may prevent the chimney from " drawing" properly. 



1. The chimney must be large enough to carry the volume 

 of air and smoke which is to be turned into it. 



2. The chimney must not be so large that the gases within 

 it do not become heated to a temperature considerably higher 

 than the surrounding air. 



3. The chimney should be as straight as possible and as 

 smooth as possible on its inside. 



4. It is best not to place the chimney in or against the out- 

 side wall. Why? 



5. The higher the chimney, the longer the column of heated 

 air and hence the greater the difference in weight between this 

 column and a similar column of outside air. 



6. It is well to line the chimney inside, making it double- 

 walled to further protect it from the cold, just as our houses 

 are double-walled. 



