smoke from the engines, subway cars with dust-laden air from the tunnel, is naturally inefficient and of questionable benefit. Efficiency in ventilation must come through wider streets and courts, cleaner thoroughfares, the abolition of smoke and dust nuisances, and last, but not least, through the design of buildings, engineering work, public conveyances and their equipment on sanitary lines.
Laws have been in effect in several states which prescribe a fixed amount of fresh air to be supplied per capita in schools and theaters. These laws do not cover the standard of purity, except perhaps as expressed by the carbonic acid test, which does not measure the worst forms of contamination. They do not always define temperature and other qualities essential to secure its benefit to people. Moreover, it is almost hopeless to enforce them in the proper spirit. Discretion might often be in order where natural conditions will help, but can not be conceded while the exact volume of artificial supply is prescribed. The chief benefit of such legislation lies in its educational effect on people. The urgent need to-day is to bring before the public again and again the most objectionable causes of impure air, especially those of preventable nature, and to promote sound judgment as to the logical and practical means of relief.
Ventilation can be effected by natural, artificial or mechanical means. Each of these three methods has its field for application. Natural ventilation is incidental to the design and construction of a building. Frame houses are subject to considerable leakage through the shrinking wood-work of walls, windows and doors and through their greater exposure to the air generally. Such ventilation may also be called spontaneous. It is generally sufficient in exposed wooden dwellings, at times even greater than necessary. Brick and stone buildings are also subject to more or less spontaneous ventilaton, which, however, does not always meet the need. In such cases, the general design of the building should be arranged deliberately to encourage a natural ingress and egress of air. For residences and offices not unduly crowded, this may suffice with a fair exposure, but often it should be supplemented by artificial means. This implies that the building must have certain features which induce a decided movement of air, such as shafts leading from kitchen and inside rooms, also fire-place flues and vents from special sources of odor. With such provisions an active removal of foul air may be effected by differences of temperature, increased possibly by waste heat available. The leading idea should be to give the most advantageous direction to the natural currents of air. Systematic supply of fresh air, combined in some form with the heating apparatus, is appropriate in many cases, particularly as it permits some control over the purity, temperature and humidity of the air entering the building.