88 ANNUAL REPOKT SMITHSONIAN INSTITUTION;, 1912. 



variation of 4 per cent was indicated at both stations in the first 

 half of September, 1911. Many values remain to be computed, but 

 it can now hardly be doubted that the outcome will prove conclu- 

 sively the irregular short-period variability of the sun. 



Numerous copies of the silver disk pyrheliometer have been stand- 

 ardized and sent out, mainly to foreign governmental meteorological 

 services. 



Valuable results have been secured in the research on the trans- 

 mission of radiation through atmospheric water vapor. An accurate 

 method of estimating the total water-vapor contents of the atmos- 

 phere between the observer and the sun has been devised by Mr. 

 Fowle. 



Eespectfully submitted. 



C. G. Abbot, 

 Director, Astro physical Observatory. 



Dr. C. D. Walcott, 



Secretary of the Smithsonian Itistitution. 



