512 POPULAR SCIENCE MONTHLY. 



■unique feature was its situation between two clouds. Below us to the 

 west was the coast cloud, which reached a varying distance inland 

 according to the topography. Up the Eimac Valley it flowed like a 

 great river; occasionally it filled not only the valley, but the barren 

 ravines that branched from it north and south, rising till it covered 

 even the ridges at our feet and, flowing around us, formed islands of 

 Mount Harvard and the other more lofty points. The upper surface of 

 this cloud was very sharply defined, but of wave-like form, so that its re- 

 semblance to water was at times so perfect that we could with difficulty 

 persuade ourselves that far beneath its under surface all the varied 

 activities of ordinary life were going on cheerfully. If from this view 

 of 'clouds wrong side up' we turned our eyes upward, we at times saw 

 another cloud system far above us ; so that frequently we were between 

 two clouds in a wide but shallow world, ourselves, perhaps, the sole 

 inhabitants. 



Fortunately for our work, the clouds above us appeared but seldom 

 during the first months of our residence on Mount Harvard. During 

 this time the extension of the Harvard photometry to the southern sky 

 was begun. This is a determination of the brightness of all stars 

 visible to the naked eye, a work begun by Professor Pickering in 1879. 

 Photographic work was also carried on. The climatic conditions from 

 April to September were excellent, but later clouds became trouble- 

 some. This condition of the sky growing worse as the cloudy season 

 approached, it was decided to devote the following months to a meteoro- 

 logical study of different localities along the coast, and incidentally 

 to extend the work of the meridian photometer in some region more 

 free from clouds. Of the clearness and steadiness of the atmosphere 

 in these different localities there was no certain knowledge, and the 

 only way was to investigate it for ourselves. We left the Mount Har- 

 vard Station early in November in charge of a Peruvian assistant. 

 During the next four months a personal examination was made by 

 Dr. M. H. Bailey and myself of what appeared to be the most desirable 

 localities along the coast, including Arequipa, the region about Lake 

 Titicaca, both in Peru and in Bolivia, the Desert of Atacama, Valpa- 

 raiso, Santiago and various other places in Chili. Perhaps no spot in all 

 America offers a clearer sky than the Desert of Atacama. More than 

 a month was passed at Pampa Central, near the center of this desert, 

 and a study was made of the meteorological conditions, while the work 

 of the meridian photometer was rapidly extended. There is a striking 

 difference during the Peruvian cloudy season, between the cloudiness 

 at Mount Harvard or at Arequipa, where the conditions are similar, 

 and that on the Desert of Atacama. This is well shown in the follow- 

 ing brief table, where A represents a perfectly clear sky during the 

 whole night ; B, a clear sky for a portion of the night ; G, sky partially 

 cloudy all night, and D, sky completely cloudy all night. 



