"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... wide open for a moment, and hold two lighted candles in the door-way, one near the top of the door, and the other near the bottom... 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 [convince them] that the change of air which actually takes place must be very considerable... 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."