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 is a coefficient of heat transmission from a radiator which varies from 1.7 for low-pressure steam heating to 1.9 for steam pressure of 40 pounds and 2.4 for steam pressure of 100 pounds. Taking the usual conditions of $$t = 70\,$$ degrees and $$t_1 = -10\,$$ degrees and with low-pressure steam heating, the factor, $$[ (t - t_1) \div (T - t) a ]\,$$ becomes 0.324, so that with $$n = 1\,$$ the equation for radiation surface becomes:

$R = 0.324 G + 0.08 W + 0.006 C\,$.

This formula differs very considerably, in the factors used, from those already cited, and it will be seen that it is based upon the coefficient of heat transmission for glass of 1 British thermal unit per square foot per degree difference in temperature, and the radiation surface due to the glass area is consequently much less than in the other formulas. The difference is more than made up, however, by the larger allowance for the cubic contents. In the opinion of the author, Prof. Carpenter's coefficient for glass is considerably too small, and his equation gives results which are too large for rooms having large cubic contents with comparatively small window surface, and results which are too small when the proportions are reversed.

Monroe's rule for direct radiation.—The author has recently deduced a formula which is a combination of Willett's and Carpenter's, and is as follows:

$R = (1.3 G + 0.25 W + 0.008 C) J (t - t_1 ) \div (T - t) a\,$. In this the letters stand for the quantities previously assigned, $$J\,$$ being a coefficient depending upon the exposure (being unity in ordinary cases) and for the usual conditions as assumed in previous cases, this formula becomes

$R = 0.42 G + 0.08 W + 0.0026 C\,$.

In the following table are given the proportions of four representative rooms of an office building for which the writer was engaged to design the heating system, and for which the heating surface has been figured out according to Mills', Willett's, Carpenter's and the author's formulas, and also according to Mr. Baldwin's formula taking 65 per cent. of the glass equivalent surface.