148 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1920. 



Starting from this hypothesis, the two lands of observations were 

 found to have come into complete accord, so that 1 per cent change 

 in the sun corresponded with 1 per cent change in the brightness 

 of Saturn, just as it ought to do. We may then look upon the sun's 

 variation as of twofold origin. First, the long-period changes, as- 

 sociated with sun-spot activity, depend upon increased general tem- 

 perature of the sun's surface, due to the increased circulation of 

 the hot, dense gases of which the sun is composed. On the other 

 hand, the short-period, irregular fluctuations are to be regarded as due 

 to the inequality of the sun's radiation in different directions, per- 

 haps caused by the presence of an obscuring atmosphere of different 

 thicknesses from place to place. 



MOUNT WHITNEY. 



It is now almost 40 years since the late Doctor S. P. Langley, third 

 Secretary of the Smithsonian Institution, made his picturesque and 

 famous expedition to Mount Whitney, California, to observe the 

 radiation of the sun. The expedition was made possible by the gen- 

 erous aid of the late William Thaw, of Pittsburgh. It went forward 

 in a special car, carrying the observers and the whole equipment. 

 Mount Whitney, 14,500 feet elevation, is by a few feet the highest 

 mountain in the United States, exclusive of Alaska. In 1881 the 

 region about it was but little settled and Indians were frequently met 

 with. Accordingly, a detail of soldiers accompanied the expedition 

 through the desert from the stopping place of the car to the little 

 town of Lone Pine, where the experiments were begun. 



Doctor Langley has often told me of the tremendous heat en- 

 countered in the small tent where the spectrobolometer was set up 

 at Lone Pine. The indications of the bolometer, that electrical ther- 

 mometer capable of detecting temperature changes of the millionth 

 of a degree, are recorded by a sensitive galvanometer. In the very 

 unfavorable conditions of the little tent Doctor Langley told me 

 that the spot of light from the galvanometer mirrer used to rush 

 off the scale a meter long in a single minute of time, so that the ob- 

 server there read and called out the position of the spot on the scale 

 as fast as he could do so without knowing what sun rays, or if any, 

 were being observed by the bolometer. All of these thousands of 

 readings were graphically plotted and reduced with almost infinite 

 labor to obtain the results of the solar observations. 



A little later the whole apparatus was moved nearly up to the top 

 of Mount Whitney, where, on the shore of a beautiful mountain 

 lake, at more than 12,000 feet elevation, the work was repeated. 

 A few partial observations were made by the expedition on the very 

 summit of Mount Whitney, but conditions there were found to be 



