“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.

“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.