448 



SCIENCE, 



[Vol. V., No. 121. 



in fact, the heavier apparatus was not carried above 

 the mountain camp. 



The view from the very summit was over number- 

 less peaks on the west to an horizon, fifty miles away, 

 of unknown mountain-tops ; for, with the exception 

 of the vast ridge of Mount Tyndall, and one or two 

 less conspicuous ones, these summits are not known 

 to fame; and, wonderful as the view may be, all the 

 charm of association with human interest which we 

 find in the mountain landscape of older lands is here 

 lacking. It was impossible not to be impressed with 

 the savage solitude of this desert of the upper air, 

 and our remoteness from man and his works ; but I 

 turned to the study of the special 

 things connected with my mis- 

 sion. Down far below, the air 

 / \ seemed filled with reddish dust 



; \ that looked like an ocean. This 



• \ dust is really present everywhere 

 I \ (I have found it in the clear air of 



Etna); and, though we do 

 \ not realize its presence in 



| \ looking up through it, to one 



. ! ..-—^ \ who looks down on it, the 



i / \ t \ dwellers on the earth seem 



| / \ \ indeed like creatures at the 



i / \ \ bottom of a troubled 



• ! \ \ ocean. We had cer- 

 j ; \ \ tainly risen towards the 

 ; [ \ \ surface; for about us 

 S i \ \ the air was of 

 j i \ \ exquisite purity, 



and above us the 

 sky was 

 of such a 



DISTRIBUTION OF SOLAR ENERGY AT SEA-LEVEL AND AT VARIOUS ALTITUDES 



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 parti- 

 cles 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 re- 

 gion. 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 sur- 

 face, 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 moun- 

 tain camp, and found that it had suffered less than 

 might be expected, considering the pathless character 

 of the wilderness. We went to work to build piers, 

 and mount telescopes 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 propor- 

 tions. On the screen is the spectrum 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 repre- 

 sented here by these equal perpendiculars in every 

 color. Fix your attention on these three as types, 

 and you will see better what we found on the moun- 

 tain, 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 following statement, where I use, not the 

 exact figures used in calculation, but round num- 

 bers, to illustrate the process employed. I may pre- 

 mise that the visible spectrum extends 

 ^ from H (in the extreme blue) to A (in 



""^--"-V^--.^ the deepest red), or from 



~ ztI ~~~~---r=rr-» liear 40 ( tlie ra y 



'^^^^SBEMm^S^^^^m of forty hundred- 

 thousandths of a 

 millimetre in 

 wave-length) to 

 near 80. All be- 

 low 80, to the 

 right, is the invisible infra-red spectrum. Now, 

 the shaded curve above the spectrum represents the 

 amount of energy in the sun's rays at the foot of 

 the mountain, and was obtained in this way: Fix 

 your attention for a moment on any single part of 

 the spectrum; for instance, that whose wave-length 

 is 60. If the heat in this ray, as represented by the 

 bolometer at the foot of the mountain, was (let us 

 suppose) 2°, on any arbitrary scale we draw a vertical 

 line, two inches or two feet high, over that part of the 

 spectrum. If the heat at another point, such as 40, 

 were but 5 , a line would be drawn there a quarter 

 of an inch high; and so on, till these vertical lines 

 mark out the shaded parts of the drawing, the gaps 

 and depressions in whose outline correspond to the 

 ' cold bands ' already spoken of. Again : if on top 



