1885.] on Sunlight and the Earth's Atmosphere. 277 



wall which, though only ten or twelve miles distant, arose miles 

 above our heads ; and, leaving these mule trains to go with the escort 

 by this longer route, I started with a guide by a nearer way to those 

 white gleams in the upper skies, that had daily tantalised us below 

 in the desert with suggestions of delicious, unattainable cold. That 

 desei't sun had tanned our faces to a leather-like brown, and the 

 change to the cooler air as we ascended was at first delightful. At 

 an altitude of 5000 feet we came to a wretched band of nearly naked 

 savages, crouched around their camp fire, and at 6000 found the first 

 scattered trees ; and here the feeble suggestion of a path stopped, and 

 we descended a ravine to the bed of a mountain stream, up which we 

 forced our way, cutting through the fallen trees with an axe, fighting 

 for every foot of advance, and finally passing what seemed impassable. 

 It was interesting to speculate as to the fate of our siderostat mirrors 

 and other precious freight, now somewhere on a similar road, but 

 quite useless. We were committed now, and had to make the best 

 of it — and, besides, I had begun to have my attention directed to a 

 more personal subject. This was, that the colder it grew the more 

 the sun burnt the skin — quite literally burnt, I may say, so that by 

 the end of the third day my face and hands, case-hardened, as I 

 thought, in the desert, began to look as if they had been seared with 

 red-hot irons, here in the cold where the thermometer had fallen to 

 freezing at night; and still as we ascended the paradoxical effect 

 increased ; the colder it grew about us, the hotter the sun blazed 

 above. 



We have all heard probably of this curious effect of burning in 

 the midst of cold, and some of us may have experienced it in the 

 Alps, where it may be aided by reflection from the snow, which we 

 did not have about us at any time except in scattered patches, but 

 here by the end of the fourth day my face was scarcely recognisable, 

 and it almost seemed as though sunbeams up here were different 

 things, and contained something which the air filters out before they 

 reach us in our customary abodes. Eadiation here is increased by 

 the absence of water vapour too, and on the whole this intimate 

 personal experience fell in almost too well with our anticipations 

 that the air is an even more elaborate trap to catch the sunbeams 

 than had been surmised, and that this effect of selective absorption 

 and radiation was intimately connected with that change of the 

 primal energies and primal colour of the sun which we had climbed 

 towards it to study. 



On the fourth day, after break-neck ascents and descents, we 

 finally ascended by a ravine, down which leaped a cataract, till, at 

 nightfall, we reached our upper camp, which was pitched by a little 

 lake, one of the sources of the waterfall, at a height of about 

 12,000 feet, but where we seemed in the bottom of a valley, nearly 

 surrounded as we were by an amphitheatre of rocky walls which 

 rose perpendicularly to the height of Gibraltar from the sea, and cut 

 off all view of the desert below or even of the peak above us. 



