Aware of the abilities and celebrity of many of the writers on this subject--from Whitehurst and Franklin to Reid and Gurney--I must ward off the imputation of self-conceit by expressing my belief that the errors of those who have failed should be chiefly ascribed to excessive cleverness; to unadvised attempts at outwitting nature! I hope to escape that snare. In the execution of my humble task, I shall entirely rely on common sense and common experience. Air is essential to human life, and as respiration destroys its vital qualities, the _ventilation_ of rooms which are intended for habitation should be a primary object in all architectural plans. Architects, however, seldom provide for the ventilation of rooms otherwise than as they provide for the admission of light. Now the properties of light and air, with reference to our domestic requirements, differ in some important particulars--of which it may not be amiss to give a brief enumeration. _Light_ moves with uniform velocity: _air_ is sometimes quiescent, and sometimes moves at the rate of thirty miles an hour. _Light_ diffuses itself with much uniformity: _air_ passes in a current from the point of its entrance to that of its exit. _Light_, whatever be its velocity, has no sensible effect on the human frame: _air_, in the shape of a partial current, is both offensive to the feelings and productive of serious diseases. _Light_, once admitted, supplies our wants till nightfall: _air_ requires to be replaced at very short intervals. _Light_ may be conveniently admitted from above: _air_ requires to be admitted on the level of the sitter. _Light_, by the aid of ground glass, may be modified permanently: _air_ requires to be variously adjusted according to its direction, its velocity, the seasons, the time of the day, the number of persons assembled, &c. 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.