"In consequence of the _lightness_ of heated air, it always rises to the upper parts of rooms and buildings, when it either escapes, or, becoming cooled and _heavier_, again descends. If, in cold weather, we sit under a skylight in a warm room, a current of cold air is felt descending upon the head, whilst warmer currents, rising from our bodies and coming into contact with the cold glass, impart to it their excess of heat. Being thus contracted in bulk, and rendered specifically heavier, they in their turn descend, and thus a perpetual motion is kept up in the mass of air. This effect is attended with much inconvenience to those who inhabit the room, and is in great measure prevented by the use of double windows, which prevent the rapid cooling and production of troublesome currents in the air of the apartment. "We generally observe, when the door of a room is opened, that there are two distinct currents in the aperture; which may be rendered evident by holding in it the flame of a candle. At the upper part it is blown outwards, but inwards at the lower part; in the middle, scarcely any draught of air, one way or other, is perceptible. "The art of ventilating rooms and buildings is in a great measure dependent upon the currents which we are enabled to produce in air by changes of temperature, and is a subject of considerable importance. As the heated air and effluvia of crowded rooms pass upwards, it is common to leave apertures in or near the ceiling for their escape. Were it not, indeed, for such contrivances, the upper parts of theatres and some other buildings would scarcely be endurable; but a mere aperture, though it allows the foul air to escape, in consequence of its specific lightness, is also apt to admit a counter-current of denser and cold air, which pours down into the room, and produces great inconvenience. This effect is prevented by heating, in any convenient way, the tube or flue through which the foul air escapes. A constantly ascending current is then established; and whenever cold air attempts to descend, the heat of the flue rarefies and drives it upwards. Thus the different ventilators may terminate in tubes connected with a chimney; or they may unite into a common trunk, which may pass over a furnace purposely for heating it.