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 that the tests under discussion were made on radiators of different sizes, and several years apart, by different men, and under very different conditions of setting, the uniformity of the curves is very striking, and the few points which are evidently out of place are doubtless due either to some error of observation or in some marked difference in the way of taking measurements.

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Figure 28.—Tests of Whittier Indirect Radiator.

Inasmuch as in practice we are most concerned with extreme conditions, the curves for the difference of temperature of 215 degrees are of most value, as they may be taken to represent an initial air temperature of 0 degree and low-pressure steam at 215 degrees. It will be noticed that the 215-degree curve for the Gold pin radiator is quite different from that for the Whittier. As these curves show for a constant difference of temperature, the relation between the cubic feet of air per square foot of radiator, which is a measure of the velocity of air flow, and the heat given off per square foot, they take into consideration all variations in setting; and the similar curves for any two radiators indicate precisely the relative values of the two radiators. For example, the 215-degree curve of the Gold pin radiator shows a uniformly higher ratio between the British thermal units per square foot and the cubic feet of air per square foot than the similar curve of the Whittier. The former is, by just so much, therefore, the more effective radiator. Mr. Mills states of the Gold pin radiator, which was first introduced by Mr. Samuel Gold in 1862, that it "has proved the most efficient indirect-heating surface ever produced." It is still extensively used, although some modern makes are largely supplanting it. It is to be regretted tests have not been made on some of the more modern types in comparison with it.