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honor of rendering this important service to science was adjudged to the highest summit in the United States. The Sierra Nevada culminates in a granite pile, rising, somewhat in the form of a gigantic helmet, fronting eastwards, to a height of 14,887 feet. Mount Whitney is thus entitled to rank as the Mount Blanc of its own continent. In order to reach it, a railway journey of 3,400 miles, from Pittsburg to San Francisco, and from San Francisco to Caliente, was a brief and easy preliminary. The real difficulty began with a march of 120 miles across the arid and glaring Inyo desert, the thermometer standing at 110° in the shade (if shade there were to be found.) Towards the end of July 1881, the party reached the settlement of Lone Pine at the foot of the Sierras, where a camp for low-level observations was pitched (at a height, it is true, of close upon 4,000 feet), and the needful instruments were unpacked and adjusted. Close overhead, as it appeared, but in reality sixteen miles distant, towered the gaunt, and rifted, and seemingly inaccessible pinnacle which was the ultimate goal of their long journey. The illusion of nearness produced by the extraordinary transparency of the air was dispelled when, on examination with a telescope, what had worn the aspect of patches of moss, proved to be extensive forests.
The ascent of such a mountain with a train of mules bearing a delicate and precious freight of scientific apparatus, was a perhaps unexampled enterprise. It was, however, accomplished without the occurrence, though at the frequent and imminent risk, of disaster, after a toilsome climb of seven or eight days through an unexplored and, to less resolute adventurers, impassable waste of rocks, gullies, and precipices. Finally a site was chosen for the upper station on a swampy ledge, 13,000 feet above the sea; and there, notwithstanding extreme discomforts from bitter cold, fierce sunshine, high winds, and, worst of all, “mountain sickness,” with its intolerable attendant debility, observations were determinedly carried on, in combination with those at Lone Pine, and others daily made on the highest crest of the mountain, until September 11. They were well worth the cost. By their means a real extension was given to knowledge, and a satisfactory definiteness introduced into subjects previously involved in very wide uncertainty.
Contrary to the received opinion, it now appeared that the weight of atmospheric absorption falls upon the upper or blue end of the spectrum, and that the obstacles to the transmission of light waves through the air diminish as their length increases, and their refrangibility consequently diminishes. A yellow tinge is thus imparted to the solar rays by the imperfectly transparent medium through which we see them. And, since the sun possesses an atmosphere of its own, exercising an unequal or “selective” absorption of the same character, it follows that, if both these dusky-red veils were withdrawn, the true color of the photosphere would show as a very distinct blue18– 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 the 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 unequalled precision, enable him to determine with some approach to finality. Pouillet valued the “solar constant” at 1·7 “calories”; in other works, had 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;19 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 degrees 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.20 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 would, consequently, find abundant employment in maintaining a difference of 189 degrees21 between the temperature of the mercury and that of its environment. What we may with perfect accuracy call the clothing function of our atmosphere is thus vividly brought home to us; for it protects the teeming surface of our planet against the cold of space exactly in the same way as, and much more effectually than, a lady’s sealskin mantle keeps her warm in frosty weather. That is to say, it impedes radiation. Or, again, to borrow another comparison, the gaseous envelope we breathe in (and chiefly the watery part of it) may be literally described as a “trap for sunbeams.” It permits their entrance (exacting, it is true, a heavy toll), but almost totally bars their exit. It is now easy to understand why it is that on the airless moon no vapors rise to soften the hard shadow-outlines of craters or ridges throughout the fierce blaze of the long lunar day. In immediate contact with space (if we may be allowed the expression) water, should such a substance exist on our enigmatical satellite, must remain frozen, though exposed for endless æons of time to direct sunshine.
Amongst the most noteworthy results of Professor Langley’s observations in the Sierra Nevada was the enormous extension given by them to the solar spectrum in the invisible region below the red. The first to make any detailed acquaintance with their obscure beams was Captain Abney, whose success in obtaining a substance – the so-called “blue bromide” of silver – sensitive to their chemical action, enabled him to derive photographic impressions from rays possessing the relatively great wave-length of 1,200 millionths of a millimetre. This, be it noted, approaches very closely to the theoretical limit set by Cauchy to that end of the spectrum. The information was accordingly received with no small surprise that the bolometer showed entirely unmistakable heating effects from vibrations of the wave-length 2,800. The “dark continent” of the solar spectrum was thus demonstrated to cover an expanse nearly eight times that of the bright or visible part.22 And in this newly discovered region lie three-fifths of the entire energy received from the sun – three-fifths of the vital force imparted to our planet for keeping its atmosphere and ocean in circulation, its streams rippling and running, its forests growing, its grain ripening. Throughout this wide range of vibrations the modifying power of our atmosphere is little felt. It is, indeed, interrupted by great gaps produced by absorption somewhere; but since they show no signs of diminution at high altitudes, they are obviously due to an extra-terrestrial cause. Here a tempting field of inquiry lies open to scientific explorers.
On one other point, earlier ideas have had to give way to better grounded ones derived from this fruitful series of investigation. Professor Langley has effected a redistribution of energy in the solar spectrum. The maximum of heat was placed by former inquirers in the obscure tract of the infra-red; he has promoted it to a position in the orange approximately coincident with the point of greatest luminous intensity. The triple curve, denoting by its three distinct summits the supposed places in the spectrum of the several maxima of heat, light, and “actinism,” must now finally disappear from our text-books, and with it the last vestige of belief in a corresponding threefold distinction of
Defined by the tint of the second hydrogen-line, the bright reversal of Fraunhofer’s F. The sun would also seem – adopting a medium estimate – three or four times as brilliant as he now does.
Annales de Chimie et de Physique, t. x. p. 360.
S. P. Langley, “Nature,” vol. xxvi. p. 316.
Sir J. Herschel’s estimate of the “temperature of space” was 239°F.; Pouillet’s 224°F. below zero. Both are almost certainly much too high. See Taylor, “Bull. Phil. Soc. Washington,” vol. ii. p. 73; and Croll, “Nature,” vol. xxi, p. 521.
This is true only of the “normal spectrum,” formed by reflection from a “grating” on the principle of interference. In the spectrum produced by refraction, the red rays are