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 If it is desired to consider this question mathematically, the equation of the 215-degree curve of the Gold pin radiator is approximately $$H = 8.28 A^0.79$$, whereas the equation of the similar curve of the Whittier radiator is $$H = 11.8 A^0.68$$, in which H represents the British thermal units per square foot and A the cubic feet of air per square foot. In other words, with the Gold pin radiator the heat is proportional to the 79/100 power of the number of cubic feet of air per square foot of radiation (nearly equal to the fourth root of the cube), while with the Whittier it is proportional to the 0.68 power (nearly the cube root of the square); and the nearer this exponent approaches unity the more effective will the radiator be, unless there is a large variation in the coefficient.

As yet there is not very much practical data on indirect radiators. It would be of special value if some tests were made of the more modern forms of indirects to establish corresponding diagrams for them. Such tests should be made preferably in cold weather and with a constant difference of temperature between the steam and entering air of about 215 or 220 degrees. The setting of the radiator and air ducts should approximate practical conditions and the velocity (as cubic feet of air per square foot of radiator) could be varied by changing the resistance to air flow by means of dampers. In this connection some experiments of much practical value could also be made on the air resistance by determining the velocity in cubic feet of air per square foot of radiator attained for different temperatures in the hot-air duct, D, in Figure 26.

Indirect radiators are seldom installed except for rooms on the first or second floors; and in the former case the duct, D, is very short, and in the latter it is usually from 12 to 16 feet long. It should be stated in this connection that indirects of large size should be spread out as much as possible so as to give a large area against the current of air. If they are made of several radiators, one above the other, as is sometimes the case, by the time the air reaches the upper ones it is of so high a temperature that they have but little effect in comparison with the lower section.

Direct-indirect radiators.—In regard to direct-indirect radiators, their action is much the same as that of the indirect; but they have the added effect of radiation, whereas with the indirect all heat is conveyed by convection. Furthermore, with the