132 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



courtesy of the General Staff of the United States Army, all of the 

 paintings and drawings made by the official artists of the American 

 Expeditionary Forces in France, and showing in a graphic and 

 striking manner the activities of the United States Army from the 

 entrance of America into the war until the signing of the armistice, 

 November 11, 1918. Besides these pictures, a large and most im- 

 portant collection of paintings executed by various artists on behalf 

 of Liberty loan work and formerly exhibited in New York City, has 

 been added to the collection. 



The Museum has already received as complete a collection of 

 Liberty loan posters as it is possible to make at this time. 



The numismatic features of the collection have also received many 

 interesting additions, including about 200 medals, issued in the allied 

 and neutral countries during the progress of the war to commemo- 

 rate notable events, and especially to perpetuate allegorical designs 

 in connection with the various stages of the conflict. 



The war collection as a whole has grown so rapidly and has 

 attracted so much material of historical and intrinsic value to the 

 Museum that its continued development is assured. 



The great problem remaining is to supply adequate facilities for 

 the installation of this priceless aggregation of material, which 

 should be installed as a unit in suitable cases amid appropriate sur- 

 roundings in a new building erected exclusively for the purpose. 



RESEARCH WORK OF THE ASTROPHYSICAL OBSERVATORY. 



Astrophysical Observatory. — The work of the Astrophysical Ob- 

 servatory is closely related to meteorology. The researches carried 

 on there relate to the quantity of heat received b}^ the earth from 

 the sun, the effect of the atmosphere to diminish and alter the quality 

 of the solar radiation, and, on the other hand, the determination of 

 the outgoing radiation of the earth and the effect of the atmosphere 

 to hinder it and to alter its composition. 



The work of the observatory from 1905 to 1912 established the 

 standard scale of solar radiation measurements, determined the in- 

 tensity of the sun's radiation as it is outside the atmosphere, and 

 measured the transparency of the atmosphere for solar rays under 

 a great variety of circumstances — at sea level and at various altitudes 

 up to that of Mount Whitney, the highest mountain in the United 

 States. These researches also showed that the sun is probably a 

 variable star, having a variation both of long period associated with 

 the variation of other solar phenomena like sun spots and also a 

 variation in short periods of a few days or weeks. The magnitude 

 of these fluctuations in the solar heat appeared to amount to several 

 per cent and could reasonably be expected to affect the temperature 

 and rainfall of the earth. 



