64 ON THE LOSS OF HEAT 



off' the smoke, a descending current is produced, and the smoke 

 is driven into the room as a necessary consequence. The pas- 

 sage of the external air through the small crevices of the room, 

 is not only diminished by the increased friction which it sustains 

 in passing through a large number of crevices, instead of only 

 one of equal capacity, but the pressure is absolutely prevented 

 from exerting its full influence in raising the column of air 

 within the chimney, by which the smoke is made to ascend. 

 If we open a window, the air within the chimney, which be- 

 fore was the heavier column, will become the lighter, and con- 

 sequently the current will be inverted, and the evil thereby 

 instantly corrected. 



It is not my intention to notice the various causes which 

 operate to produce what are termed "bad draughts" to chim- 

 neys; there is one cause however of considerable importance, 

 which will be noticed. Chimneys which are new, are found very 

 frequently, if not invariably, to smoke, when an attempt is made 

 to use them before they become perfectly dry. This being attri- 

 buted to their bad construction in many cases, alterations are 

 consequently made, without knowing the true cause, which will 

 generally be found to be entirely owing to their not being dry. 

 The materials of which they are composed being damp, they 

 are consequently good conductors of heat, and unless very 

 large fires are made, it is difficult to elevate the temperature 

 of the air, throughout the chimney, sufficiently, to produce an 

 ascending current ; but when the chimney beeomes dry, and 

 covered with carbonaceous matter, it presents a bad conducting 

 surface, and, if then found to smoke, this may be attributed to 

 its bad construction, for whieh, however, no necessity exists 

 in any case, save that the highly important class of artisans, 

 who wield the trowel, have, too generally, discarded science 

 from their craft. 



A sufficient quantity of air must be admitted into every 

 room to supply the demands of respiration and combustion, but 

 any excess is injurious. The usual manner of admitting air 

 for these purposes, through the joints or crevices of the doors, 

 windows, and other parts of the room, appears very objectionable. 



