"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 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. Any imperfect action of the chimney, or descending current, is announced by the escape of smoke into the room, and is frequently caused by the flue being too large, or not sufficiently perpendicular and regular in its construction. When there is no fire, the chimneys also generally act as ventilators; and in summer there is often a very powerful current up them, in consequence of the roof and chimney-pots being heated by the sun, and thus accelerating the ascent of the air. In a well-constructed house there should be sufficient apertures for the admission of the requisite quantity of air into the respective rooms, without having occasion to trust to its accidental ingress through every crack and crevice that will allow it to pass. These openings may either be concealed, or made ornamental, and by proper management may be subservient to the admission of warm air in winter." * * * * *