24 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1915. 



1.81 and 1.99 calories. In 1913 the radiation of the sun was 2.5 per 

 cent below the mean, and 1 per cent above the mean in 1914. A high 

 average value is said to be indicated for 1915. In concluding his re- 

 port for the year Dr. Abbot says : 



Short-period fluctuations of solar radiation were large in 1913, but small in 

 1914. Associated with these quick, irregular fluctuations are found variations 

 of contrast of brightness between the center and edges of the solar disk. Curi- 

 ously enough, while greater contrast is associated with greater radiation and 

 with numerous sun spots in the general march of the sun's activity, lesser con- 

 trast is associated with greater solar radiation in the march of the quick, irregu- 

 lar fluctuations of the sun's emission. This paradox points to two causes of 

 solar radiation — the long period changes may probably be caused by changes yf 

 the sun's eifective temperature attending the march of solar activity ; the quick 

 fluctuations may be ascribed to changes of the transparency of the outer solar 

 envelope. 



INTERNATIONAL EXCHANGES. 



The operations of the International Exchange Service have been 

 necessarily curtailed for some months because of the European war. 

 The total number of packages handled during the year was 275,756, 

 or 65,911 less than the year before, and their weight was 367,854 

 pounds, a decrease of 199,131 pounds. There has accumulated, more- 

 over, a large number of packages awaiting opportunity of shipment, 

 particularly to Austria, Belgium, Bulgaria, Germany, Hungary, 

 Montenegro, Eoumania, Russia, Servia, and Turkey, which were en- 

 tirely shut out of the service at the close of the j^ear, although cor- 

 respondence is in progress to secure renewal of shipment with some 

 of those countries. 



One of the important functions of this service is the interchange 

 of official Government documents with various countries, resulting 

 in the building up of a most important division of the Library of 

 Congress. During the past year there was received in this connection 

 from the Chinese Government a set of the Imperial Institute of the 

 Ching Dynasty and other valuable records aggregating 684 volumes. 



Fifty-six full sets and 36 partial sets of United States official publi- 

 cations are now sent regularly to depositories abroad, in accordance 

 with treaty stipulations and congressional resolutions. A list of these 

 depositories and detailed statistics of the service are given in the 

 appendix to this report. 



INTERNATIONAL CATALOGUE OF SCIENTIFIC 

 LITERATURE. 



The Smithsonian Institution has administered the United States 

 Bureau of the International Catalogue since its organization in 1901. 

 There are 33 of these regional bureaus located in the principal coun- 



