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."

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.

I have restricted myself to the means of _ventilation_, which is requisite
at all seasons of the year, but am quite aware that _warmth_, or a
temperature above that of the external air, is sometimes indispensable to
health and comfort, and therefore to the free exercise of the faculties. I
believe, however, that the means proposed for the admission of fresh air
might also be made available for the admission of heated air, and that
either description of air might be admitted independently of the other, or
both descriptions simultaneously.

A vast increase of reading-rooms, lecture-rooms, and school-rooms, may be
safely predicted, and as the due ventilation of such rooms is a project of
undeni