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 effect of wind. He states, however, that it is well known that in extremely cold weather there is invariably but little wind, so that he considers this a safe coefficient to use.

Mr. Wolff has constructed a diagram of heat transmission from buildings, which embodies the German coefficients with some slight modifications based on his own very extended practice. He states that he has used this diagram in the calculations of heating surface for buildings, during the past six years, with the most satisfactory results. This is a valuable recommendation, and the author takes much satisfaction in presenting the diagram herewith.

In using his diagram for proportioning radiation surface, Mr. Wolff calculates by it the number of heat units lost from the exposed wall and glass surfaces and further makes allowance for the direction of winds on the outside exposure, as shown on the small diagram on page 65 facing the main one. As indicated 5 to 25 per cent. is added to the calculated amount of heat dissipated in transmission through the actual wall surface and 10 per cent. for reheating the air constantly leaking in. An allowance is also advised, to the amount of 10 per cent., for the transmission of heat through floors, ceilings, etc. Where the rooms are not large, one calculation is made for all these factors by adding to the heat transmission as obtained by means of the main diagram, the percentage given in the small circle.

For "wooden floors" in cheaply constructed buildings, the author would recommend even more allowance than Mr. Wolff gives, since where such floors are used a great loss of heat comes from a large amount of cold air, which, even with a light wind blowing, will work through the brick walls where the joists are set in and where the walls are unsealed by lathing or plaster, and find its way into the rooms. This is a common source of heat loss in cheaply constructed brick houses and apartment buildings, and is the usual cause of the cold floors which are noticeable in many buildings in cold weather. In fireproof structures, on the contrary, the construction of the walls and floors is much more substantial and offers but little opportunity for air to blow in between the floor and ceiling.

Baldwin's rule for direct radiation.—The wide variation in these coefficients of heat transmission have led to a corresponding variation in the rules laid down for proportioning radiating surface.