KEPOET OF THE SECRETARY 103 



A compilation of all results on the solar constant of radiation, 

 from 1918 to November, 1924, was published as No. 3 of Volume 77, 

 Smithsonian Miscellaneous Collections. 



The investigations hitherto made having indicated that a higher 

 degree of accuracy in our solar measurements is needed to supply 

 proper data for forecasting purposes, a very great deal of attention 

 has been given to the elimination of small sources of error in the 

 observations and reductions of solar radiation. Already the average 

 deviation of individual days' results between Chile and Arizona is 

 but one-half per cent. It follows that in order to attain higher ac- 

 curacy we shall be obliged to regard sources of error which formerly 

 we supposed would always be negligible. 



This has led to the designing and construction of new apparatus 

 for use in pyrheliometry, which eliminates the employment of the 

 observer's watch altogether. It has also required the investigation 

 of the infra-red and ultra-violet portions of the solar spectrum, be- 

 yond the usual limits of our daily spectrum observations. Still 

 more important, it has led to a complete revision of the methods 

 of measuring and reducing solar energy spectra. With these new 

 modifications in mind, a complete re-reduction of all solar radiation 

 work since the beginning of the year 1922 has been undertaken, and 

 occupies the whole force at Washington. 



Removal of Mount Harq^va Hala station. — The station at Mount 

 Harqua Hala, Ariz., first occupied in 1920, proves to be too far to 

 the east, so that the summer months there are unsuitable for ob- 

 serving, because of the atmospheric conditions which go to bring 

 about the severe thunderstorms of Arizona. Very few days of June, 

 July, and August have been suitable for our exacting work, and even 

 some of the spring months have been marred by long-continued 

 haziness. Had weather conditions there been first-rate, the ob- 

 servers would gladly have suffered the excessive isolation of the 

 place, which is almost wholly cut off from relaxations, but to make 

 such a sacrifice fruitlessly is indeed very depressing. 



Accordingly, investigations have been made which have fixed on a 

 better site, both as regards weather conditions and comfort. This is 

 chosen on Table Mountain, within the bounds of the Los Angeles 

 County Park, about 30 miles northeast of Mount Wilson. Lying on 

 the edge of the Mojave Desert, at 7,500 feet elevation, the weather 

 observations indicate very decided improvement over Harqua Hala 

 for our purpose. Add to this the convenience of access and pleas- 

 ant surroundings and we have combined there great advantages. 



Mr. John A. Roebling has added to his already great gifts suffi- 

 cient means to enable necessary buildings to be erected on Table 

 Mountain, and to remove the observing outfit thence from Harqua 



