70 REPORTS ON INVESTIGATIONS AND PROJECTS. 



accordingly decided to send an expedition to Mount Wilson under 

 his own direction and under the auspices of the Smithsonian Insti- 

 tution. Mr. C. G. Abbot, aid acting in charge of the Smithsonian 

 Astrophysical Observatory, was placed at the head of the party, and 

 Mr. Leonard R. Ingersoll, of the University of Wisconsin, came as 

 his assistant. The usual work at Washington is being continued 

 by Mr. Fowle. Although conducted under the auspices of the 

 Smithsonian Institution, the work of this expedition bears so im- 

 portant a relationship to that of the Solar Observatory that a brief 

 statement regarding it may not be inappropriate. 



As is well known, the direct measurement of the solar radiation 

 received at the surface of the earth is much less difficult than the 

 determination of the amount of heat lost by absorption in our atmos- 

 phere. Although it has attacked both phases of the subject in a most 

 thorough manner, the Smithsonian Observatory enjoys special dis- 

 tinction from the fact that the bolometer used there permits the 

 relative amount of the atmospheric absorption to be automatically 

 recorded for each wave-length of the spectrum. In short, it furnishes 

 the information essential in determining the true value of the solar 

 constant. 



The fine weather experienced by the Smithsonian party since the 

 installation of their instruments on Mount Wilson has permitted a 

 great amount of excellent work to be done. The collection of holo- 

 graphs and pyrheliometer readings is so great that months will be 

 required for their complete discussion. In addition to all of these 

 observations, much time has been devoted to frequent determinations 

 of the absorption in the apparatus, experimental work with a new 

 and promising form of recording pyrheliometer devised by Mr. Abbot, 

 and holographic observations of the radiation of sun-spots and the 

 absorption of the solar atmosphere, made with the 6.7-inch image 

 given by the Snow telescope. As all the results indicate that Mount 

 Wilson is an ideal site for investigations of the solar radiation, Sec- 

 retary Langley has been invited by President Woodward to continue 

 the work of the expedition, at least during another summer. If for 

 any reason he shall not be able to accede to this request, arrange- 

 ments will be made for the Solar Observatory to carry forward 

 the observations in cooperation, it is hoped, with the work of the 

 Smithsonian Astrophysical Observatory in Washington. The Solar 

 Observatory has profited greatly by the visit of Messrs. Abbot and 

 Ingersoll, and all the members of the staff would be glad to see it 

 repeated another year. 



