SUNLIGHT AND THE EARTH'S ATMOSPHERE. 657 



troubled ocean. We had certainly risen toward the surface, for about 

 us the air was of exquisite purity, and above us the sky was of such a 

 deep violet blue as I have never seen in Egypt or Sicily, and yet even 

 this was not absolutely pure, for, separately invisible, the existence of 

 fine particles could yet be inferred from their action on the light near 

 the sun's edge, so that even here we had not got absolutely above that 

 dust-shell which seems to encircle our whole planet. But we certainly 

 felt ourselves not only in an upper, but a different region. We were 

 on the ridge of the continent, and the winds which tore by had little 

 in common with the air below, and were bearing past us (according 

 to the geologists) dust which had once formed part of the soil of 

 China, and been carried across the Pacific Ocean ; for here we were 

 lifted into the great encircling currents of the globe, and, " near to 

 the sun in lonely lands," were in the right conditions to study the 

 differences between his rays at the surface and at the bottom of that 

 turbid sea where we had left the rest of mankind. We descended the 

 peak and hailed with joy the first arrival of our mule-trains with the 

 requisite apparatus at the mountain-camp, and found that it had suf- 

 fered less than might be expected, considering the pathless character 

 of the wilderness. We went to work to build piers and mount tele- 

 scopes and siderostats, in the scene shown by the next illustration on 

 the screen, taken from a sketch of my own, where these rocks in the 

 immediate foreground rise to thrice the height of St. Paul's. We 

 suffered from cold (the ice forming three inches deep in the tents at 

 night) and from mountain-sickness, but we were too busy to pay much 

 attention to bodily comfort, and worked with desperate energy to 

 utilize the remaining autumn days, which were all too short. 



Here, as below, the sunlight entered a darkened tent, and was spread 

 into a spectrum, which was explored throughout by the bolometer, 

 measuring, on the same separate rays which we had studied below 

 in the desert, all of which were different up here, all having grown 

 stronger, but in very different proportions. On the screen is the spec- 

 trum as seen in the desert, drawn on a conventional scale, neither pris- 

 matic nor normal, but such that the intensity of the energy shall be 

 the same in each part, as it is represented here by these equal perpen- 

 diculars in every color. Fix your attention on these three as types, 

 and you will see better what we found on the mountain, and what we 

 inferred as to the state of things still higher up, at the surface of the 

 aerial sea. 



You will obtain, perhaps, a clearer idea, however, from the follow- 

 ing statement, where I use, not the exact figures used in calculation, 

 but round numbers, to illustrate the process employed. I may premise 

 that the visible spectrum extends from H (in the extreme blue) to A 

 (in the deepest red), or from near 40 (the ray of yjnf.ow of a millimetre 

 in wave-length) to near 80. All below 80, to the right, is the invisible 

 infra-red spectrum. 



VOL. XXVII. 42 



