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 CHAPTER V.—DESIGN OF RADIATION. Heat loss in buildings.—There have been considered in previous chapters the various kinds of radiators in use and the amount of heat given off by them under different conditions; and the subject now approaches a study of the particular way in which the heat is utilized. The object of any heating system is, of course, to maintain a uniform temperature in the building in question, and to do this it is necessary: First, to replace the heat lost by convection and radiation from the windows and walls of the building to the colder surroundings outside; second, to heat to the required temperature any air that may be intentionally admitted for ventilation; and third, to heat also the air that may be admitted unintentionally through cracks of window frames and porous walls and opening doors. The amount of heat required for this last cause is much greater than is generally supposed.

The amount of air that will pass through the walls of an apparently tight room is incredible to those who are not familiar with it by actual experiment. The author knows of no experimental data on the subject, but Dr. John S. Billings, in his valuable work on "Ventilation and Heating," describes a simple and interesting experiment that may be undertaken by any one. Take a room of average proportions, heated by a hot-air furnace or indirect radiation, and the air will, on a fairly cold day, be coming through the register with a very considerable velocity. If now this velocity be measured by an anemometer, or other means, and all doors and windows be closed and the measure be again taken, it will generally be found that there is scarcely an appreciable reduction in this velocity. If now all the cracks of the doors and windows be carefully stopped up with cloth or paper, the reduction in the velocity of the incoming air will still be but very little reduced. Some of the air in such cases escapes directly through the plastered and papered walls, but more through floors and into the outside air through brick walls, between the floors, and at such points as are not plastered and papered. In cold weather it will generally