Of Chimney Fireplaces. 491 



always enters the room, and an equal quantity of the 

 warm air of the room is driven out of it every time the 

 door is opened, there is much less danger of the air of 

 a room becoming unwholesome for the want of ventila- 

 tion than has been generally imagined ; particularly in 

 cold weather, when all the different causes which con- 

 spire to change the air of warmed rooms act with in- 

 creased power and effect. 



Those who have any doubts respecting the very great 

 change of air or ventilation which takes place each time 

 the door of a warm room is opened in cold weather, 

 need only set the door of such a room wide open for a 

 moment, and hold two lighted candles in the doorway, 

 one near the top of the door and the other near the bot- 

 tom of it : the violence with which the flame of that above 

 will be driven outwards, and that below inwards, by the 

 two strong currents of air which, passing in opposite 

 directions, rush in and out of the room at the same time, 

 will be convinced that the change of air which actually 

 takes place must be very considerable indeed ; and these 

 currents will be stronger, and consequently the change 

 of air greater, in proportion as the difference is greater 

 between the temperatures of the air within the room and 

 of that without. I have been more particular upon this 

 subject, the ventilation of warmed rooms which are 

 constantly inhabited, as I know that people in gen- 

 eral in this country have great apprehensions of the bad 

 consequences to health of living in rooms in which there 

 is not a continual influx of cold air from without. I 

 am as much an advocate for a free circulation of air as 

 anybody, and always sleep in a bed without curtains on 

 that account; but I am much inclined to think, that the 

 currents of cold air which never fail to be produced in 



