EEPORT OF THE SECRETARY. 95 



repeat the balloon pyrheliometer observations, and this time at 

 Omaha. Ascensions were not made until after July 1, 1914, but it 

 may be said in anticipation that two ascensions by day and one by 

 night were made. All three instruments were recovered. No unex- 

 pected phenomena were disclosed by the night record. One day 

 record appears to be excellent. Fortunately the instrument which 

 recorded it came back uninjured, and further tests and calibrations 

 with it are intended. The instrument reached a very great height, 

 and recorded radiation successfully until after it began to descend. 

 Preliminary reductions show that the values recorded fall below 

 our adopted value of the solar constant of radiation. 



SUMMARY. 



Progress has been made in the measurement of the effects pro- 

 duced by atmospheric water vapor on solar and terrestrial radia- 

 tion. New apparatus for measuring sky radiation has been devised 

 and perfected. Special pyrheliometers have been constructed and 

 caused to record solar radiation with considerable success at great 

 altitudes when attached to free balloons. The results obtained tend 

 to confirm the adopted value of the solar constant of radiation. 

 Further results from balloon pyrheliometry are expected. A tower 

 telescope has been erected and put in operation on Mount Wilson. 

 By means of it the variability of the sun has been independently con- 

 firmed, for it appears that changes of the distribution of radiation 

 over the sun's disk occur in correlation with the changes of the sun's 

 total radiation. 



Respectfully submitted. 



C. G. Abbot, 

 Director Astrophysical Ohservatory. 



Dr. Charles D. Walcott, 



Secretary of the Smithsonian Institution. 



