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.
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