REPORT OF COMMITTEE ON OBSERVATORIES 53 



The absorption of the atmosphere for heat and light radiations 

 depends mainly upon the length of the air path which must be trav- 

 ersed by the rays. At an altitude of 1 5,000 feet the most fluctuating 

 part of the atmosphere has been left below, and the rare atmosphere 

 above is subject to comparatively little variation in its absorptive 

 power. If, then, an observatory for the study of solar radiation 

 should be established at some such height above the earth's surface, 

 in a region where but little water vapor is present, the difficulties 

 hitherto experienced in the measurement of the solar heat would in 

 large measure disappear. 



But it by no means follows that such a site would be suitable for 

 that department of solar research which requires a perfectly defined 

 image of the sun. As a matter of fact, the sharpness of definition 

 experienced during the day on mountain tops is frequently much 

 inferior to that which may be found at lower levels. At the summit 

 of Pikes Peak (14,147 feet), for example, although the transparency 

 of the atmosphere is very marked, the sun's image is usually not well 

 defined. The same is true at Mount ^tna (9,650 feet), except in 

 the early morning when the low sun has not yet greatly heated the 

 mountain slopes. At the lyick Observatory, on Mount Hamilton 

 (4,208 feet), the day conditions are better, and are probably similar 

 to those which are found at the Yerkes Observatory ( i , 100 feet) . 



Until recently no mountain peak has been known on which the 

 sun could be observed to advantage throughout the day. This is 

 presumablj' due in large part to the fact that the mountain slopes, 

 if not thoroughly covered with foliage, become greatly heated by 

 the sun's rays, producing ascending columns of warm air, which 

 rise toward the summit and mingle with the cooler currents brought 

 by the wind. Under such circumstances bad definition would be 

 inevitable. At Mount Lowe (5,650 feet), near Pasadena, California, 

 in a region where the remarkable uniformity of temperature and 

 pressure would lead one to expect good definition, the solar image is 

 frequently disturbed by currents of warm air rising from the un- 

 protected slopes. Separated from Mount Lowe only by the width of 

 a single canyon is Mount Wilson (5,886 feet). This mountain is 

 well covered to the very summit with foliage, and thus stands in 

 marked contrast with many of the mountains in southern Calif ornia.^-' 

 In consequence of this fact, all of the advantages to be expected 

 from the exceptional qualit)' of the atmosphere are experienced, 

 without the disadvantages due to warm air rising from the heated 



*See Professor Hussey's report, Appeuuix A. 



