84 REPOKT OF THE SECRETARY. 



were iiumeilintely erected on iNIonnt Wilson, and were finished, including all 

 their piers and accessories, in about two weeks. The spectro-liolometer was 

 completely installed and the lirst Itolographic observations were made June G, 

 1905. 



In the meantime observations had been made nearly every day with the mer- 

 cury pyrheliometer, and these had given promise of great transparency and 

 uniformity of sky. From the 1st of June hardly a cloud was ever seen above 

 the level of the horizon for many weeks. Extraordinary calmness adds to the 

 advantages of this site. A few of the earliest holographs were measured at 

 once to see if all appeared to be well, and, as the measurements turned out very 

 satisfactorily, holographic work was continued nearly every day thereafter, 

 leaving the reduction of the observations to a later time. 



Investigations at Washington, as already reported, have given strong grounds 

 for the conclusion that the solar radiation varies considerably and frequently 

 in its amount. It would be most desirable if the Mount Wilson expedition 

 could give ample opportunity to test this conclusion, but in spite of the too 

 short time at present command it has been thought best by you to initiate it, and 

 at the same time to study more closely the manner of the solar variability. To 

 this end the work includes, not only solar-constant measurements, but other 

 studies designed, to add to our knowledge of the whole question. 



The expedition is under the greatest obligations for the continued aid fur- 

 nished by the director and staff of the Carnegie Solar Observatory. The results 

 obtained will doubtless form a principal part of next year's report. 



SUMMARY. 



The principal object of the Observatory during the past year has continued 

 to be the measurement of the solar radiation. The work of this and the two 

 preceding years strongly supports the view that the radiation of the sun is 

 frequently diminished and augmented for periods of a few weeks or months, 

 in consequence of a variability of the transparency of the solar absorbing en- 

 velope, and that this variation of radiation causes and quickly produces 

 changes of several degrees in the mean temperature of the land areas of the 

 earth. It is hoped that the study of the solar radiation will soon prove a val- 

 uable aid in forecasting climate. 



Impoi'tant additions and improvements have been made in the equipment of 

 the Observatory, both in buildings and apparatus. The new apparatus includes 

 a standard pyrheliometer, capable of continuously registering the solar radia- 

 tion and provided with means of certifying the exactness of its measures inde- 

 pendently of the theory of the instrument. 



An expedition to measure and test the variability of solar radiation has 

 been sent to Mount Wilson, in Southern California. 



Respectfuly submitted. 



C G. Abbot, 

 Aid Acting in Charge. 



Mr. S. P. Langley, 



Secretary Smithsonian Institution, Washington, D. C. 



