KEPOKT OF THE SECRETARY. 105 



the director, prepared the sensitive parts of a galvanometer and 

 a vacuum bolometer of usual types for solar work, and also of a 

 vacuum galvanometer and vacuum bolometer of very unusual design 

 suited to observing the energy distribution in the spectra of the 

 stars. These extremely delicate and sensitive instruments required 

 extraordinary skill and patience for their construction and testing. 

 Acknowledgments are due the Director of the Bureau of Standards, 

 the Director of the Nela Research Laboratory, and also Dr. Elihu 

 Thomson, of Lynn, for aiding these preparations. 



The instrument making for these new pieces and others required 

 in the expedition to Mount Wilson, including a special spectrometer, 

 plate carrier, and other apparatus, was done by the instrument 

 maker, Mr. A. Kramer. 



A great many of the " solar constant " observations made at Mount 

 Harqua Hala, Ariz., were reduced by Mr. Fowle and Mrs. Bond in 

 consultation with the director. Despite our long experience in 

 solar-radiation work, new problems and difficulties still crop up. 

 The publication of the Mount Harqua Hala results has hitherto been 

 withheld so that a comprehensive discussion of them might be made 

 to reveal and correct any systematic errors. 



Expedition to Chile. — It became necessary for the director to un- 

 dertake a visit to Chile to inspect the observing station at Monte- 

 zuma maintained by the Hodgkins fund for the study of the solar 

 variations, in cooperation with the stations in California and Ari- 

 zona. Leaving Washington near the end of October, 1921, he spent 

 the month. November 15 to December 15, at the station and returned 

 to Washington early in Januai-y, 1922. During the month at Mon- 

 tezuma he revised all the adjustments of apparatus and some of the 

 methods employed there, besides assisting in the daily observations 

 and reductions on 26 days. Silver disk pyrheliometer S. I. No. 5, 

 loaned by the Department of Agriculture for the purpose, was com- 

 pared with instruments at Montezuma, and before and afterwards 

 with instruments at Washington. No change in the scale of pyr- 

 heliometry was disclosed by these comparisons. 



Expedition to Mount Wilson. — In June an expedition, including 

 the director and Mr. L. B. Aldrich, went out to Mount Wilson. 

 Four objects were in view. First, to inspect the station at Mount 

 Harqua Hala and compare pyrheliometers there with silver disk 

 pyrheliometer S. I. No. 5, above mentioned, so as to connect the 

 fundamental scales of pyrheliometry in Arizona and Chile. Second, 

 to repeat with all possible precautions and variations of method the 

 determination of the form of the solar spectrum energy curve out- 

 side the atmosphere. Third, to undertake preliminary measurements 

 of the distribution of energy in the spectra of the brighter stars. 



