"Cold air, being heavier than warm, will fall below the latter, and press it upwards to make way for itself. Thus the air in the neighborhood of the fireplace will press the hot smoke up into the chimney-throat. If this throat is only large enough to take the smoke, hot air only will enter the flue, and the draught will be rapid. But if the throat is larger than necessary, that part of the cool air of the room which enters the fireplace and becomes most heated by the fire, and next in buoyancy to the smoke, will, in its turn, be pressed up by the cooler air behind it, and enter the flue alongside of the smoke. Indeed, the entire volume of the air of the room, being warmer than the outside air, will tend to enter the flue with the smoke, so long as there be room provided for its entrance. The heat of the column, and consequently the rapidity of its rise, will therefore be proportionately diminished."