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 to the fact that the amount of ventilation secured is practically proportional to the amount of heat required, and in warm weather but little, if any, ventilation is obtained. Furthermore, experience shows that in order to ensure reliability it is necessary to have a separate flue for almost every room and to locate the radiators, directly beneath the vertical flue, so that in buildings of any size, especially those more than one or two stories in height, the arrangement of radiators in the basement becomes difficult, and the system of air flues, which are necessarily large, is complex and expensive in space and also in construction.

In order to avoid the difficulties of the system of "indirect radiation" and yet secure some ventilation, a combination has been developed which goes under the significant title of "direct-indirect radiation." In this the radiators are located in each separate room, but they are of special construction, and provided with air connections through the walls of the building so arranged that a certain amount of air can be admitted through this connection so as to pass around the radiator, becoming heated by contact with it. The room is therefore heated both by direct radiation and by the incoming current of fresh hot air, and considerable ventilation is secured.

In this system, as in the "indirect," ventilation in warm weather is dependent on open windows and doors, and it has been as yet but little used. It has, however, in a few cases been adapted to office buildings and hotels, and in the opinion of the writer we may look for a very decided development of the "direct-indirect" during the immediate future in buildings of the more sparsely-peopled character, where the amount of ventilation required per square foot of floor area is comparatively small. But for such buildings this system only achieves its best results when combined with a mechanical system for exhausting the air.

As already mentioned, we have, perhaps, arrived to-day at a point in the advancement of hygienic science where some system of artificial or mechanical ventilation is looked upon as necessary for all buildings of what the author has called the densely-peopled class. It is difficult to define the limits of such buildings, as the height and nature of the room and length of time occupied affect the question, but in a general way any room or apartment in which each individual occupies less than 40 square feet of floor area should be included in such a classification, especially if occupied