92 CARNEGIE INSTITUTION 



expected every year, especially in winter ; but they are not numerous 

 and are to be regarded as exceptional. 



Altogether, I spent forty six days on these mountains. In that 

 time there was one severe storm, when the wind was high — proba- 

 bly as much as 60 miles an hour. In addition to this, there were 

 four or five other nights when the wind velocity rose to 25 or 30 

 miles an hour. Otherwise, the days and nights were very quiet. 

 From all that I could learn, it appears that this record is not more 

 favorable than the average. 



Mount Wilson and Mount Dowe are on the southern edge of the 

 San Gabriel mountains, and near the middle of the range from east 

 to west. Here the range has its greatest breadth but not its greatest 

 height. To the northwest of Mount Wilson there is a succession 

 of ridges and peaks that rise to elevations of from 5,000 to 6,000 

 feet. But it is to the north, northeast, and east that the highest 

 peaks of the range are situated. In these directions, beyond the San 

 Gabriel river, numerous ridges may be seen, some more than 7,000 

 feet high, with peaks that tower above them, culminating in San 

 Antonio (Old Baldy), 10,080 feet above the sea. These higher 

 ridges lie directly between Mount Wilson and the Mohave desert. 

 By reason of their situation and height they no doubt greatly pro- 

 tect Mount Wilson from winds that would otherwise blow across 

 this region between the desert and the sea. 



These higher peaks and ridges are from 10 to 25 miles away. 

 Deep, broad canyons lie between, winding in and out among the 

 mountains until they reach the valley on the .south. The advan- 

 tages of this arrangement from an astronomical standpoint will be 

 readily understood. Streams of cold air settling down from these 

 higher peaks cannot flood Mount Wilson. This source of bad see- 

 ing, therefore, appears to be wholly absent. San Gabriel peak is 

 somewhat higher than Mount Wilson, but there is a deep col be- 

 tween them. Flooding with cold air appears to be a fruitful cause 

 of bad seeing at some observatories. Thus Mr. A. E. Douglass 

 speaks of the condition at Arequipa, Peru : 



"The observatory is situated close to a river, down which on 

 clear nights a swift stream of cold air descends. When this cold 

 air reached the telescope, the seeing was immediately ruined. When 

 this current was once established, no more good seeing could be 

 expected for the remainder of the night." * 



At Flagstaff also the temperature often falls suddenly in the latter 



♦American Met. Journal, vol. II, p. 395. 



