216 SCIENCE OF COMMON THINGS. 



How to construct a chimney. Use of a cowl upon a chimney. 



the lines of the current, descending the chimney and 

 circulating round the apartment. 



143 7* Why does an apartment often smell disagreeably of soot in sum- 

 mer-time ? 



Because the air in the chimney (being colder than 

 the air in the apartment) descends into the room, and 

 leaves a disagreeable smell of soot behind. 



1438 How ought chimneys to be constructed? 



A chimney should be constructed in such a way 

 that the flue or passage will gradually contract from the 

 bottom to the top, being widest at the bottom, and the 

 smallest at the top. 



1439 Why is it expedient to construct a chimney in iliis manner f 



At the base of the chimney, the hot column of air fills 

 the entire passage ; but as the hot air ascends it 

 gradually cools and contracts, occupying less space. If, 

 therefore, the chimney were of the same size all the 

 way up, the tendency would be, that the cold external 

 air would rush down to fill up the space left by the con- 

 traction of the hot column of air. This action would 

 still farther cool the hot air of the chimney and 

 diminish the draught. 



14:40 Why will a long chimney smoke, unless the fire be pretty fierce f 



Because the heat of the fire will not be sufncient to 

 rarefy all the air in the chimney. 



1441 Why will the chimney smoke, unless the fire be fierce enough to 

 heat all the air in the chimney-flue f 



Because the cold air (condensed in the upper part of 

 the flue) will sink from its own weight ; and sweep 

 the ascending smoke back into the room. 



144S What is the use of a cowl upon a chimney-pot f 



It acts as a screen, to prevent the wind from blowing 

 into the chimney. 



1443 What harm would the wind do if it were to blow into a chim- 

 ney? 



1. It would prevent the smoke from getting out ; 

 and 



2. The cold air (introduced into the chimney by the 



