538 Of Chimney Fireplaces. 



said upon that subject in various parts of this essay, 

 and as they may, in all cases (a very few only excepted), 

 be completely remedied by making the alterations in 

 fireplaces here pointed out, I do not think it necessary 

 to enumerate them all in this place, or to enter into 

 those long details and investigations which would be 

 required to show the precise manner in which each of 

 them operates, either alone or in conjunction with others. 



There is, however, one cause of smoking chimneys 

 which I think it is necessary to mention more particu- 

 larly. In modern-built houses, where the doors and 

 windows are generally made to close with such accuracy 

 that no crevice is left for the passage of the air from 

 without, the chimneys in rooms adjoining to each other, 

 or connected by close passages, are frequently found to 

 affect each other; and this is easy to be accounted for. 

 When there is a fire burning in one of the chimneys, as 

 the air necessary to supply the current up the chimney 

 where the fire burns cannot be had in sufficient quanti- 

 ties from without, through the very small crevices of 

 the doors and windows, the air in the room becomes 

 rarefied, not by heat, but by subtraction of that por- 

 tion of air which is employed in keeping up the fire, or 

 supporting the combustion of the fuel, and, in conse- 

 quence of this rarefaction, its elasticity is diminished, 

 and being at last overcome by the pressure of the ex- 

 ternal air of the atmosphere, this external air rushes in- 

 to the room by the only passage left for it, namely, by 

 the open chimney of the neighbouring room ; and the 

 flow of air into the fireplace, and up the chimney where 

 the fire is burning, being constant, this expense of air is 

 supplied by a continued current down the other chimney. 



If an attempt be made to light fires in both. chimneys 



