on the Atmosphere of the Rocky Mmntains. 93 



free from clouds and many local thunder storms occurred 

 about the higher peaks, but they seldom extended to the 

 plateaus below. 



September 1st and 2d our camp was 8900 feet above the 

 sea in the vicinity of mountains rising 10,000 and 11,000 feet 

 These peaks seemed to be nearer than they really were, for the 

 transparency of the air causes estimates of distance to be decep- 

 tive. From the top of one I subsequently saw the Seminole 

 mountain which was 150 miles distant; it did not appear to be 

 50 miles away. The night of Sept 1st was quite clear, with 

 very little cloud, and the atmosphere remarkably tranquil. 

 Ankres, when near setting, hardly twinkled at all, and Arc- 

 turus in the telescope exhibited four diffraction rings unbroken 

 by flickering. The central disc was as hard and sharply de- 

 fined as the pinhole in the lamp screen I am accustomed to 

 use in testing specula and lenses. I looked for the companion 

 of Polaris, but partly on account of the nearly full Moon, and 

 partly from the thickness of the diffraction rings, I could not be 

 sure of it. The Moon was perfectly steady ; with a power of 

 60 there was no trace of twinkling at the terminator. I tried 

 to see Titan, the largest satellite of Saturn, but did not succeed. 

 At the time it was not certain whether this failure was due to 

 the position of Titan with relation to Saturn, or whether it 

 arose from the blinding effulgence of the Moon. Capella was 

 perfectly steady though there was a slow change of color 

 from bluish to reddish occupying about a second. 



The succeeding night at nine o'clock, though the sky was 

 mostly covered with cumulus clouds in motion southward, I 

 was surprised to find the terminator of the Moon absolutely free 

 from twinkling, and An-mms down among the tops of the 

 dead aspen trees as steady as possible. The four diffraction 

 rings round the central disc were not perfectly circular, but 

 that was the fault of the lens. Every defect of centering or of 

 surface, and any vein in the glass comes out even more clearly 

 than in the workshop examinations, because while the air is as 

 steady the light is far more intense. 



I am certain if a large telescope could be brought here and 

 maintained is winds great results might be 



attained if there is much of this weather. But this particular 

 place is difficult of access and possibly no better than other 

 situations on the line of the railroad. The sky is not as black 

 as I had expected; it is rather of a light blue, though the full 

 Moon makes much difference. 



On several other nights in both lower and higher places I 

 made observations but never saw the combination of - 

 and transparency again. On the plateaus at the foot of the 

 mountains and away from the groves of aspen trees and pines 



