An attentive consideration of the above circumstances leads me to certain conclusions which I shall now state aphoristically, and proceed to describe in more detail.
A room designed for a numerous assemblage of persons—as a reading-room, a lecture-room, or a school-room—should be provided with apertures, adapted to admit spontaneous supplies of fresh air, in such variable quantities as may be required, on at least two of its opposite sides, and within three feet from the floor; also, with apertures in the ceiling, or on a level therewith, to promote the exit of the vitiated air. The apertures of both descriptions may be quite distinct from those which admit light.
Suppose a room to be twenty-four feet square, and sixteen feet in height, with two apertures for light on each side, each aperture being three feet wide by eight feet in height, and rising from the floor. There are not many rooms constructed on a plan so favourable to the admission of fresh air—but it has some serious defects. 1. The air would enter in broad and partial currents. 2. It would not reach the angular portions of the room. 3. The vitiated air might rise above the apertures, and so accumulate without the means of escape.
Now, suppose the same room to have its apertures at eight feet from the floor, and so to reach the ceiling. The escape of the vitiated air might then take place—if not prevented by a counter-current. But whence comes the fresh air for the occupants? There is no direct provision for its admission. The elevated apertures are utterly insufficient for that purpose; and the perpetual requisite is no otherwise afforded than by the occasional opening of a door!
It being thus established that the same apertures can never effectually serve for light and ventilation, I propose with regard to reading-rooms, lecture-rooms, and school-rooms, which require accommodation for books, maps, charts, and drawings, rather than a view of external objects, that the windows should be placed in the upper part of the room—that the admission of fresh air should be provided for by ducts near the floor—and the escape of the vitiated air by openings in, or on a level with, the ceiling.
The number of windows, and their size, must depend on the size of the room. If windows are to admit light only, a smaller number may be sufficient, and they may not be required on more than one side; a circumstance which recommends the plan proposal, as we can seldom have windows on each side of a room, or even on two of its opposite sides, but may devise a method of so admitting air.
Rejecting the use of windows as a means of ventilation, and rejecting artificial currents of every description, I propose the substitution of air-ducts of incorrodible iron, to be inserted horizontally in the walls of at least two opposite sides of the room, within three feet from the floor, and at intervals of about four feet. The ducts to be six or eight inches in diameter, according to the size of the room. The external orifice of each duct to be formed of perforated zinc, and the internal orifice, which may be trumpet-shaped, of
perforated zinc or wire-gauze, with a device which would serve to adjust the quantum of air according to circumstances, and to exclude it at night. By such contrivances, while the offensive and noxious currents which proceed from wide openings would be obviated, the supplies of fresh air would always be equal to the demand. The purest air may not be accessible—but, as Franklin says, "no common air from without is so unwholesome as the air within a close room."
The escape of the vitiated air requires less consideration. If the ceiling of the room be flat, with another room above it, the upper part of each window, in the shape of a narrow slip, might be made to act as a sort of safety-valve; but if the windows are on one side only, corresponding openings should be made on the opposite side, so that there would almost always be, more or less, a leeward opening. A vaulted ceiling, without any other room over it, seems to be the most desirable form, as the vitiated air would rise and collect towards its centre, where there could be no counter-current to impede its egress.
It is the union of those two objects, the admission of fresh air and the riddance of the vitiated air, skilfully and economically effected, which forms the circle of the science of ventilation.