88 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1918. 



PERSONNEL. 



R. Eisinger resigned from our service in June, 1918, and after 

 service in the Treasury Department enlisted in the Army. 



SUMMARY. 



During the year covered by this report, great advance has been 

 made in the study of very long wave-length rays and their trans- 

 missibility in our atmosphere. Solar constant work at Mount Wil- 

 son has been continued and improved. An expedition under the 

 auspices of the Hodgkins Fund of the Smithsonian Institution, but 

 equipped and directed from the Astrophysical Observatory, has ob- 

 served the solar constant at Hump Mountain, N. C, and now is lo- 

 cated for a term of years in exceptionally favorable circumstances at 

 Calama, Chile. The total solar eclipse of June 8, 1918, was success- 

 fully observed. The variability of the sun is shown to have vestiges 

 of periodicity, though predominently irregular. A great deal of 

 attention has been given to war problems. 



Respectfully submitted. 



C. G. Abbot, 



Director Astroyhysical Observatory. 



Dr. C. D. Walcott, 



Secretary Smithsonian Institution. 



