A totally different kind of solar research is that in aid of which the Mount Whitney expedition was organized in 1881. Professor S. P. Langley, director of the Alleghany observatory in Pennsylvania, has long been engaged in the detailed study of the radiations emitted by the sun; inventing, for the purpose of its prosecution, the “bolometer,”[17] an instrument twenty times as sensitive to changes of temperature as the thermopile. But the solar spectrum as it is exhibited at the surface of the earth, is a very different thing from the solar spectrum as it would appear could it be formed of sunbeams, so to speak, _fresh from space_, unmodified by atmospheric action. For not only does our air deprive each ray of a considerable share of its energy (the total loss may be taken at 20 to 25 per cent. when the sky is clear and the sun in the zenith), but it deals unequally with them, robbing some more than others, and thus materially altering their relative importance. Now it was Professor Langley’s object to reconstruct the original state of things, and he saw that this could be done most effectually by means of simultaneous observations at the summit and base of a high mountain. For the effect upon each separate ray of transmission through a known proportion of the atmosphere being (with the aid of the bolometer) once ascertained, a very simple calculation would suffice to eliminate the remaining effects, and thus virtually secure an extra-atmospheric post of observation. The 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.