of the mountain WB sliall find all are hotter; »o lliat we must up there make all our lines higher, but in very different prapoT- Clona. At 00, for inetance, the heat (and light) may have grown from 2" to 3°, or increwed one-half, vrhlle above 40 the heat (and liglit) may have grown from 1° to 1°, or Increased five timea. These moun- tain measurement a give another spectrum, the ener- gies in each part of which are defined by the middle dotted line, which we see Indicates very much greater energy, whether heat or light. In the blue end than below. Next, the light or heat which would be ob- served nt the surface of the atmosphere is found in this way. If the mountain top rises through one-half the absorbing mass of this terrestrial atmosphere (it does not qult« do so, In fact), aiidhy getting rid of that lower half the ray SO has grown in brightness from '2 to 3, or balf as much again, in going up to the top It would gain liaif as much more, or become 4}; while the ray near 40, which has already increased to five times what It was, would increase five times more, or to 26, Each separate ray locreaiing thus nearly in some geocentric progression (though the heat, as a whole, does not), you see how we are able, by repeat- ing this process at every point, to build up our outer or highest curve, which representa the light and beat at the surface of the atmosphere. These have grown out of all proportion at the blue end, as you see by the outer dotted curve, and now we have attained by actual measurement that evidence which we sought; atid by thus reproducing the spectrum outside the atmosphere, and then recombining the colors by like methods to those you have seen on the screen, we finally get the true color of the sun, which tends, broadly speaking, to blue.
It Is so seldom that the physical Investigator meets any novel fact quite unawares, or finds any thing except that in the field where he is seeking, that he must count it an unusual experience to come unex- pectedly on even the smallest discovery. This expe- rience I had on one of the last days of work on the spectrum on the mountain. I was engaged in ex- ploring that great invisible heat-region still but so partially known, or, rather, I was mapping in that great 'dark continent' of the apectrutn, and by the aid of the ext|uisite sky and the new instrument (the bolometer) found I could carry the survey farther than any bad been before. I sulratttuted the prism for the grating, and measured on in that unknown region till I had pasaed the Ultima Thule of previous travellers, and finally came to what seemed the very end of the invisible heat-spectrum, beyond what had previously been known. This was In itself a return for much trouble, and I was about rising from my task, when it occurred to me to advance the bolometer still farther; and I shall not forget the surprise and emotion with which I found new and yet unrecog- nlied regions below, — a new Invisible spectrum be- yond the farthest limits of the old one.
t will anticipate here by saying, that, after we got down to lower earth again, the explorations aud map- ping of this new region was continued. The amount of solar energy included in this new cxienslon of llie
��invisible region is much less than thai of the visible spectrum; while lis length upon the wave-length scale is eqtuil to all that previously known, visible and In- visible, as yon will see better by this view, having the same thing on the normal as well as the prismatic scale. If It l>e ashed which of these is correct, the answer is. Both of them. Both, rightly interpreted, mean just the same thing; but in the lower one we can more conveniently compare the ground of the researches of others with these. These great gaps I was at first in doubt about; but more recent re- searches at Alleghany make it probable that they are caused by absorption in our own atmosphere, and not in lliat of the sun.
We would gladly have staid longer, in spite of physical discomfort; but the formidable descent and the ensuing desert journey were lietore us, and cer- tainly the reign of perpetual winter around us grew as hard to liear as the heals of the desert summer hail been. On Sept. 10 we sent our Instruments and the escort back by the former route, and, ourselves unen- cumbered, started on the adventurous descent of the eastern precipices by a downward climb, which, if successful, would carry us to the plains in a single day. I at least shall never foi^et that day, nor the scenery of more than Alpine grandeur which we passed In our descent, after first climbing by frozen lakes In the northern shadow of the great peak, till wecrossed the eastern ridges, through a door so nar- row that only one could pass It at a time, by eiingiiig with hands and feet as he swung round the shoulder of the rocks — to find that he had passed In a sin- gle minute from the view of winter to summer, the prospect of the snowy peaks behind shut out, and instantly exchanged for that lielow of the glowing val- ley and the little oaais, where the tents of the lower camp were still pitclied. the tents themselves Invisi- ble, but the oasis looking like a green scarf dropped on the broad Boor of the desert. We climbed still downward by scenery unique In my recollection. This view of the ravitie on the screen is little more than a memorandum made by one of the party In a tew minutes' halt part way down, as we followed the Ice-stream between the tremendous walls o( the defile which rose two thousand feet, and between which we still descended, till, toward night, the ice- brook had grown into a mountain torrent, and. look- ing up the long vista of our day's descent, we saw It terminated by the peak of Whitney, once more lonely in the fading light of the upper sky.
This site. In some respects unequalled tor a physi- cal observatory. Is likely, I am glad to say, to be utilized; the president of the United States having, on the proper representation of its value to science, ordered the reservation, for such purposes, of an area of a hundred square miles about and Inclusive of Mount Whitney.
There is little more to add about the journey back to civilization, where we began to gather the results of our observation, and to reduce them; to smell, so to speak, the metal from the ore we had brought home, — a slow hut necessary process, which has oc- cupied a large part of two years. The results, stated
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