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.
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