104 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



lamps of different kinds, and also observations on large hollow radia- 

 tors at different constant temperatures. Values of the constant of 

 the fourth power law of radiation differing by only 1 per cent from 

 the best accepted value were readily obtained in this latter work. 

 On the whole the " honeycomb pyranometer " is an instrument of 

 great promise for standard measurements. 



. Experiments on the constant " sigma" — In collaboration with Dr. 

 C. E. Mendenhall, a new attempt was begun to devise means to 

 measure the constant of radiation with greater certainty. Apparatus 

 was devised and constructed in the Observatory shop for this pur- 

 pose. There was not time to try it before the departure of Messrs. 

 Abbot and Aldrich into the field, so that the apparatus was loaned to 

 Dr. Mendenhall for trial at the University of Wisconsin. 



Field work at Mount Wilson. — Mr. Aldrich continued observing on 

 Mount Wilson until October, 1919. As said above, the year was un- 

 favorable both by reason of a defect in equipment and by reason of 

 much haze, cirrus cloud, and streakiness of sky. Also on many days 

 a curious wandering of the galvanometer needle occurred. This 

 phenomenon has been noted at Mount Wilson occasionally in former 

 years, but was unusually pronounced in 1919. By anticipation, it 

 may be remarked that it occurred also very markedly in late July 

 and in August, 1920. The march of the galvanometer spot in these 

 wanderings is relatively slow. A centimeter or two back and forth 

 upon the scale in one to two minutes is the usual magnitude. It 

 occurs with the galvanometer unconnected to- the bolometer. Reasta- 

 ticising of the needle system till it turned in the earth's field at the 

 same rate as the supporting quartz fiber failed to cure the trouble. 

 The Mount Wilson expedition was renewed in June, 1920, by Messrs. 

 Abbot and Aldrich. 



Proposed station in Arizona, — The prevailing cirrus cloudiness 

 and haziness at Mount Wilson in all recent years, greatly exceed- 

 ing that which obtained from 1905 to 1910, when the station was new, 

 has been very discouraging. Furthermore, the station is quite un- 

 suitable for " solar-constant " work in winter and spring months 

 owing to cloudiness. It is urgently desirable to observe the solar 

 radiation daily, as far as possible, in the United States, in order to 

 check the results which are being obtained by Smithsonian observers 

 in Chile. 



Accordingly it seemed best to set up a station in the most cloud- 

 less region of the United States, where the work could go on during 

 the entire year. Chief Marvin, of the Weather Bureau, obligingly 

 caused investigations to be made of various proposed sites in Cali- 

 fornia, Nevada, and Arizona. The one of highest promise appeared 

 to be on the Harqua Hala Mountain (elevation about 5,800 feet) near 

 Wenden, Ariz. Congress was urged to appropriate $25,000 for the 



