Page:Popular Science Monthly Volume 27.djvu/423

Rh resembles the pure original sunlight less than the electric beam which has come to us through reddish-colored glasses resembles the original brightness. With this visible heat was included the large amount of invisible heat, and, if there was any law observable in this 'capricious' action of the atmosphere, it was found to be this, that, throughout the whole range of the known heat-spectrum the large wave-lengths passed with greater facility than the shorter ones."

The effect of this selective absorption on the visible rays is to cut out the shorter wave-lengths proportionally more; so that to an eye outside of the earth's atmosphere the sun would be far bluer than to us. On the heat-rays taken together, the total amount of the absorption is very great, far greater than had been previously supposed. Professor Langley's experiments give a very great increase in the amount of solar heat reaching the earth over previous determinations, so that for example, according to him, the solar radiation is sufficient to annually melt an ice-shell one hundred and seventy-nine feet thick all round the earth. According to previous determinations, one hundred and ten feet in thickness could be melted. But while Professor Langley finds a vastly greater amount of heat supplied by the sun, his law of the selective absorption comes in to profoundly modify its terrestrial manifestations. Were there no such selective absorption, the temperature of the soil in the tropics, under a vertical sun, would probably not rise to that of the freezing-point of mercury. "The temperature of this planet, and with it the existence, not only of the human race but of all organic life on the globe, appears, from the results of the Mount Whitney expedition, to depend far less on the direct solar heat" than on the hitherto neglected quality of selective absorption.

The bearing of the observations at Mount Whitney on a great number of important questions, the temperature of the sun, the radiation from the sky, etc., etc., can not be here considered for want of space. The solar spectrum previously known was but half of that mapped out by the expedition, and there is good reason to believe that Professor Langley's observations have now revealed the whole of it to us.

The partial results of these investigations, published from time to time in foreign periodicals, have done much to make Professor Langley honored in other countries than his own. In 1882 he was invited to address the British Association for the Advancement of Science at Southampton, and did so. His paper on that occasion reminds one of that of February, 1874, in the astonishing fullness of experiment, thought, and judgment which seems to lie just back of the sentences. It comes from a full mind. In the spring of 1885 Professor Langley goes to England at the invitation of the Royal Institution to lecture before it.

There are many other most interesting researches of Professor