mon rule for ordinary purposes being three-fourths of a square foot of heating surface to each square foot of glass, or its equivalent." He further states that he has used this rule in preference to any other for several years and found it very satisfactory. Following in Mr. Baldwin's footsteps, the writer has used this method of calculating surface in the design of a large number of heating systems, chiefly for large office buildings in Chicago and elsewhere. He has found, however, for low-pressure systems in office buildings that from 60 to 70 per cent. of the glass-equivalent surface in figuring radiation gives ample and satisfactory results. For such buildings he has counted each square foot of wall surface as being equivalent to one-tenth of a square foot of glass. For brick houses or apartment buildings of ordinary construction it is better to take the wall surface as 15 or 20 per cent. of the glass. For office buildings 65 per cent. of the glass-equivalent surface in radiating surface is ample in most cases, and it may average rather less. It should be greatest on the sides of the building which are exposed to the severest winds in winter, and may be less on the southern exposures.
Mills' rule for direct radiation.—Mr. Baldwin's method of calculating radiation surface calls for the exercise of careful judgment on the part of engineers using his rule, and many authorities have devised rules which are more specific. Most of these take into account the glass surface, the wall surface, and also the cubic contents of the room. The rule recommended by Mr. Mills in his work is
in which is the number of square feet of radiating surface; , the square feet of glass surface; , the square feet of wall surface (exclusive of windows); and , the contents in cubic feet. This rule is recommended where the rooms are to be heated to a temperature of 70 degrees with an outside temperature of 10 to 15 degrees Fahr. If the outside temperature is less or greater, the result should be multiplied by the proportionate factor. This is a very good rule and perfectly safe. The writer knows of a contractor who has had a very wide experience in steam heating who uses this rule universally, except that he multiplies the cubic contents by 0.004.
Willett's rule for direct radiation.—Mr. Jas. R. Willett, an architect of wide experience, who has given much study to heating
