SOLAR RESEARCH AT MOUNT WILSON, CALIFORNIA. l6l 



conditions wliich exist at Washington. Nevertheless, it by no means 

 follows that Dr. Langley's purpose could be accomplished at such 

 a point. The humidity of our atmosphere is a most serious obstacle in 

 this particular work, since the solar heat is very subject to absorption 

 by water vapor. It is therefore desirable to establish the instruments 

 at least a mile above the dense and disturbed layers of the atmos- 

 phere which lie near the sea-level. Certain problems connected with 

 the investigation may render it desirable to make some of the obser- 

 vations at a hig-her altitude, reaching from 12,000 to 15,000 feet. 

 We conclude, therefore, that the principal work should be done 

 at a station having an elevation of 5,000 to 6,000 feet, in a dry 

 climate, where the weather is continuously clear over long periods 

 of time. The work at higher altitudes, if needed at all, could in all 

 probability be completed in two or three summers by expeditions 

 equipped with a portable outfit erected at an altitude of from 12,000 

 to 15,000 feet. It would thus be convenient to have the principal 

 station at a lower altitude, not far removed from accessible moun- 

 tains of this considerable elevation. It would be inadvisable, for 

 reasons which it is hardly necessary to specify, to establish the 

 principal station at an altitude much greater than 6,000 feet. 



POSITION AND NATURAL RESOURCES OF MOUNT WILSON. 



From a meteorological standpoint, the State of California may 

 naturally be divided into three parts. In the northern region the 

 rainfall is very considerable, much cloudiness prevails, and dn 

 almost all respects the conditions are very unfavorable for astro- 

 nomical work. The central region, which may be considered to 

 extend as far south as Point Concepcion, is favored with much 

 better weather conditions, best exemplified at the Lick Observa- 

 tory, on Mount Hamilton, where a high average of night-seeing 

 is maintained during a large part of the year. Except for the 

 frequent winds at night, which interfere with some classes of work, 

 Mount Hamilton might be regarded as an almost ideal observa- 

 tory site, at least for night observations. For solar work it may 

 not be superior to certain stations in the eastern part of the United 

 States, because of the excessive radiation from the heated slopes of 

 the mountain, which is almost devoid of trees near the summit. 



In the southern part of California the climatic conditions are 

 decidedly different from those which prevail in the two other 

 sections of the State. The much lighter rainfall is naturally asso- 

 ciated with fewer clouds, a remarkably steady barometer, and very 

 light winds. During a part of the year the fog rolls in from the 



