"The small tin ventilators, consisting of a rotating wheel, which we
sometimes see in window-panes, are perfectly useless, though it is often
imagined, in consequence of their apparent activity, that they must be
very effectual; but the fact is, that a very trifling current of air
suffices to put them in motion, and the apertures for its escape are so
small as to produce no effectual change in the air of the apartment:
they are also as often in motion by the ingress as by the egress of air.
"From what has been said, it will be obvious that our common fires
and chimneys are most powerful ventilators, though their good services
in this respect are often overlooked. As soon as the fire is lighted,
a rapid ascending current of air is established in the chimney, and
consequently there must be a constant ingress of fresh air to supply
this demand, which generally enters the room through the crevices of the
doors and windows. When these are too tight, the chimney smokes or the
fire will not draw; and in such cases it is sometimes necessary to make
a concealed aperture in some convenient part of the room for the
requisite admission of air, or to submit to sitting with a window or
door partly open.
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