538 THE POPULAR SCIENCE MONTHLY. 



the human body averaging 3T'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 there- 

 about 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 prob- 

 ably 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 w^all, the stronger and more uncomfortable and 

 injurious is the draught. In order to prevent these draughts, the w^n- 

 dow^s 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. Swin- 

 burne, 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 win- 

 dows, yet the foul gases will make their escape by diffusion ; for, ac- 

 cording to the law of diffusion, there is a rapid interchange betw^een 

 gases in free communication. Of course, the outflow of the inside air 

 very materially hastens the rapidity of the interchange ; but the out- 

 flow 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 w^hich 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 con- 

 sequently 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- 



