SUNLIGHT AND THE EARTH'S ATMOSPHERE. 655 



invitingly cool, from the torrid heat of the desert, where we were 

 encamped by a little rivulet that ran down from some unseen ice-lake 

 in that upper air. Here we pitched our tents and fell to work (for 

 you remember we must have two stations, a low and a high one, to com- 

 pare the results), and here we labored three weeks in almost intoler- 

 able heat, the instruments having to be constantly swept clear of the 

 red desert dust which the hot wind brought. Close by these tents a 

 thermometer covered by a single sheet of glass, and surrounded by 

 wool, rose to 237 in the sun, and sometimes in the tent, which was 

 darkened for the study of separate rays, the heat was absolutely beyond 

 human endurance. Finally, our apparatus was taken apart and packed 

 in small pieces on the backs of mules, who were to carry it by a ten 

 days' journey through the mountains to the other side of the rocky 

 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 tantalized us below in the 

 desert with suggestions of delicious, unattainable cold. That desert 

 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 five thousand feet we came to a wretched band of nearly naked 

 savages, crouched around their camp-fire, and at six thousand 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 side- 

 rostat 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 burned the skin quite literally burned 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 recognizable, 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. Radiation here is increased by the absence of 

 water vapor too, and on the whole this intimate personal experience 



