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 more economical ways of heating than by laboriously carrying expensive fuel to separate fires in each individual room. Stoves were built to get more uniform combustion and save some of the heat lost up the flue, and gradually various forms of distributing heat through many rooms from one central fire were developed to economize labor. Heated air, heated water and steam were all in turn experimented with as a means of distributing heat, and systems employing them have been rapidly and scientifically evolved to meet various requirements, and are to this day very widely used. But since the time of Tredgold, heating by steam has increased in extent and popularity year after year, especially since the increase in size of buildings began to be very rapid, and its economy of operation and incidental advantages of convenience and simplicity have become more and more apparent, until at the present day, in some form or another, it is used almost universally in all installations requiring distribution of heat over any considerable area. In this country it is well within the memory of most men in active life when even our largest factories and office buildings were heated by means of open fires and stoves, but the development from a primitive life to a congested and complex civilization has been phenomenally rapid, especially in the last quarter century, and the greatest advances in steam heating, as well as in most practical sciences, have been made in that period. These have chiefly been due to the almost universal application of steam power and the tremendous economy effected by the use of exhaust steam for heating.

The problem of mechanical ventilation, therefore, though growing out of much the same economic conditions, was solved, to a large extent, independently of the question of heating; and with the development of heat distribution by steam much was lost in the way of ventilation. The old-time fireplace and stove insured a certain amount of ventilation, to say nothing of the mental exhilaration of the former, but heating by steam was accomplished with no ventilation whatsoever. Hygienically, therefore, it was a step in the wrong direction, but economically the lack of ventilation made it more advantageous, as ventilation requires the heating of all incoming air. Heat in cold weather was the prime essential, and it was always possible to obtain some amount of ventilation by what might be called the "natural circulation" of air through doors and windows. The fallacy of resorting to such