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.

"In some of our theatres, the gas chandelier is made a very effectual
ventilator. It is suspended under a large funnel, which terminates
in a cowl outside the roof; and the number of burners heat the air
considerably, and cause its very rapid and constant ascent through the
funnel, connected with which there may be other apertures in the ceiling
of the building. But in these and most other cases, we may observe that
the vents are not sufficiently capacious; and the foul air from the
house, and from the gas-burners themselves, not being able readily to
escape, diffuses itself over the upper part of the building, and renders
the galleries hot and suffocating--all which is very easily prevented by
the judicious adjustment of the size of the ventilating channels to the
quantity of air which it is requisite should freely pass through them.

"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 wha