Page:Popular Science Monthly Volume 26.djvu/413

Rh blue —not merely bluish, but a real azure just tinted with green, like the hue of a mountain-lake fed with a glacier-stream.

Moreover, the further consequence ensues, that the sun is hotter than had been supposed; for, the higher the temperature of a glowing body, the more copiously it emits rays from the violet end of the spectrum. The blueness of its light is, in fact, a measure of the intensity of its incandescence. Professor Langley has not yet ventured (that we are aware of) on an estimate of what is called the "effective temperature" of the sun—the temperature, that is, which it would be necessary to attribute to a surface of the radiating power of lamp-black to enable it to send us just the quantity of heat that the sun does actually send us. Indeed, the present state of knowledge still leaves an important hiatus—only to be filled by more or less probable guessing—in the reasoning by which inferences on this subject must be formed; while the startling discrepancies between the figures adopted by different and equally respectable authorities sufficiently show that none are entitled to any confidence. The amount of heat received in a given interval of time by the earth from the sun is, however, another matter, and one falling well within the scope of observation. This, Professor Langley's experiments (when completely worked out) will, by their unequaled precision, enable him to determine with some approach to finality. Pouillet valued the "solar constant" at 1·7 "calories"; in other words, he calculated that, our atmosphere being supposed removed, vertical sunbeams would have power to heat in each minute of time, by one degree centigrade, 1·7 gramme of water for each square centimetre of the earth's surface. This estimate was raised by Crova to 2·3, and by Violle in 1877 to 2·5; Professor Langley's new data bring it up (approximately as yet) to three calories per square centimetre per minute. This result alone would, by its supreme importance to meteorology, amply repay the labors of the Mount Whitney expedition.

Still more unexpected is the answer supplied to the question. Were the earth wholly denuded of its aëriform covering, what would be the temperature of its surface? We are informed in reply that it would be at the outside 50° of Fahrenheit below zero, or 82° of frost. So that mercury would remain solid even when exposed to the rays—undiminished by atmospheric absorption—of a tropical sun at noon. The paradoxical aspect of this conclusion—a perfectly legitimate and reliable one—disappears when it is remembered that under the imagined circumstances there would be absolutely nothing to hinder radiation into the frigid depths of space, and that the solar rays