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 {416} 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."