the human body averaging 37·5° Cent., or 99° Fahr.—the lighter air is nearly always within. Therefore, if on the sheltered side a window is lowered at the top, or on any side if the air be calm, the foul air will escape above it; if raised from below, fresh air will enter beneath. But ordinarily it is sufficient to fully provide for the escape of the fouled air—the fresh, as a rule, wall not need so much attention; yet it is better to make ample provision also for this. The best method is by ventilators in the walls—say of a foot square in section, or thereabout—raised but a few inches above the floor below, and lowered but a few inches below the ceiling above; or otherwise at the highest points of the ceiling itself. In this way the currents that are likely to blow on the children's shoulders when the windows are raised are avoided, a matter of importance; for a draught of cold air, blowing upon the shoulders from behind, arrests the action of the skin—probably through the spinal sensory nerves—and causes what is commonly known as a "cold." Even when windows are lowered at the top, draughts will occasionally blow upon the pupils; and, the lower the windows are set in the wall, the stronger and more uncomfortable and injurious is the draught. In order to prevent these draughts, the windows should be set high in the wall and lowered on the sheltered side whenever possible. An ingenious contrivance for the prevention of draughts through open windows has been suggested by Dr. Swinburne, in a paper read before the last annual meeting of the New York State Medical Society. It consists in the attachment of one end of a strip of unstarched muslin to a spring roller fastened to the casement above, and the other end to the upper edge of the window itself. On lowering the window, the muslin is unrolled, and thus stretches across the vacant space. Being held tense by the spring of the roller, it effectually shuts off all draught, while it allows the escape of the foul gases within, and the slow but steady entrance of fresh air.
Even should there be no currents through ventilators or open windows, yet the foul gases will make their escape by diffusion; for, according to the law of diffusion, there is a rapid interchange between gases in free communication. Of course, the outflow of the inside air very materially hastens the rapidity of the interchange; but the outflow will not, can not, be very rapid if there is not sufficient provision for the entrance of fresh air other than through the same apertures through which the outflow itself takes place. Again, the warmer the day, the less the difference between the temperature of the inside and the outside air; hence the buoyancy of the inside air is less, and consequently the ventilation not so effective; so that more attention and greater facilities must be afforded it. Ventilators should never be placed in the hall; here they do but little good. The doors leading from the hall to the rooms are usually closed, and, even if open, the buoyancy of the air as a factor in ventilation is nearly eliminated; for there is a partition between the hall and the room, so that the light
