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square foot of radiator per hour, which he takes as 250 for a two-column radiator (bronzed) set under the window, but this factor varies within wide limits, as before described, according to the kind of surface used and the nature of the setting.

Indirect radiation.—With indirect radiation the heat lost from the glass and wall surface must be made up by the heated air coming in from the indirect radiator, and to accomplish this the entering air must, in cold weather, have a temperature considerably above the mean temperature desired in the room. The total heat lost by the room is $(t-t_{1})h\,$ where $h\,$ is the expression $(1.3G+0.25W+0.008C)\,$ and the volume of air required in cubic feet per hour is $V=(t-t_{1})hX58\div (T-t)\,$ where $T\,$ is the temperature of the air leaving the radiator. Now it is necessary for the indirect radiator to heat all of this air from the outside temperature to the temperature $T\,$, and the total heat required to be given off by the radiator is

$U=V(T-t_{1})\div 58=(t-t_{1})(T-t_{1})h\div (T-t)\,$

It will be seen that both $U\,$ and $V\,$ vary rapidly with a change in $T$, decreasing as $T\,$ is increased. If $t=70\,$ degrees and $t_{1}=-10\,$ degrees for extreme conditions, and if $T=150\,$, $V\,$ will be one-half and $U\,$ two-thirds of what they would be if $T\,$ were taken at 110. It is this fact that makes the indirect radiator quite a flexible device, for in extreme weather it is possible, by partially shutting off the air supply, to maintain easily the required inside temperature at the sacrifice of a small amount of ventilation. If $T=120\,$, $U=208h\,$, and $V=93h\,$; and with $T130\,$, $U=186h\,$, and $V=77h\,$. As a rule, it is safe to assume from 450 to 500 British thermal units per hour per square foot of surface for an indirect radiator, as will be seen by reference to the tests as described in the last chapter, and taking the former figure, with $T=120\,$, $R=0.46h\,$, and with $T=130\,$, $R=0.415h\,$.

Inasmuch as it has been found that for the same conditions of inside and outside temperatures $R\,$ for direct radiation $=0.325h\,$, it will be seen that according to this calculation from 28 to 40 per cent. more heating surface is required for indirect heating than for direct. The author has in his practice used these proportions for indirect radiators, usually installing about 30 per cent, more than for direct; although in some cases, where an exceptional degree of ventilation is desired and the room has a comparatively