Ventilation has for its object the preservation, within buildings, of an atmosphere as free as possible from accumulation of carbonic acid, or any other impurity, by affording ingress to fresh air and egress to the vitiated. Practically, the great problem is how to attain this end with as little admission of cold as possible, and without draughts. Draughts are not only highly dangerous, on account of the well known, but ill understood, sympathy between the secretion of different parts of the integument and various internal organs, but are deservedly regarded with much dislike, a dislike which may be so great that impure air will be endured in preference. To avoid draughts, the communication of a heated room with the external air should be constant, free, and directed away from the position of the inmates, or the air should be heated by some special contrivance before it gains admission. The dense air from without rushes into an apartment with the greater force the narrower the aperture of entrance; and no arrangement can well be imagined more likely to produce exposure to draughts than a room with a warm fire on one side, and insufficient ventilation taking place through the key-holes and chinks of windows and doors on the other. The heated air passes up the chimney, and cold air rushes in streams with great velocity through the narrow apertures opposite. The density of cold air gives it such force, in rushing into a heated enclosure, that it travels inwards or falls down through an opening in the ceiling very compactly; and, therefore, in good ventilation, means should be taken to diffuse the entering streams of cold air, and direct them away.