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 be noticed that with all windows and doors closed, there is a decided current of air flowing through any building, in the same direction as the wind out of doors, and this is always most noticeable at the floors.

It may be stated in general that brick buildings are much tighter than wooden; and fireproof buildings which have wooden flooring laid on some kind of a concrete filling are much tighter than the ordinary brick buildings. It is perhaps a valuable thing that the walls of buildings are not less porous than they are, for in far too many cases there is no ventilation in winter, except what is obtained in this way. It is, however, much better to make the walls tight and provide some proper inlet for ventilation, especially as from a sanitary standpoint the floor is the worst place to let cold air into an otherwise warm room. An ordinary brick building can be much improved in this respect, if, during construction, the walls around the joists and for some inches above and below be painted with a heavy coat of asphalt paint. It is for these reasons mainly that all rules for proportioning radiation surface are very largely empirical.

The heat required for a definite amount of ventilation can be very accurately calculated. A cubic foot of ordinary air, at 60 or 70 degrees Fahr., weighs about 0.0745 pound, or there are 13.4 cubic feet per pound; and the specific heat is about 0.24, so that one British thermal unit will heat 55.8 cubic feet of air 1 degree Fahr. This factor is, however, subject to considerable variation according to the final temperature considered and the degree of moisture, and is usually taken at 55.

The heat lost by radiation and convection from the walls of buildings has been variously calculated by different authorities, from Péclet down, and a great variety of results have been given. It is unquestionably very difficult to determine it experimentally, because of the fact that the loss of heat from walls, etc., depends first upon the construction of the walls, but more especially upon the condition of the air outside, the loss of heat being very greatly increased by a slight wind blowing against the exposed surface. The loss of heat is greatest from the glass surface of windows. Mr. W. J. Baldwin, in his book on "Steam Heating for Buildings," which has for some years been a standard, publishes a table of the relative "heat-transmitting power of various building substances," which is given here.