REPORT OF THE SECRETARY. 21 



warmed earth is cooled partly by convection currents playing over its surface 

 and partly by direct and indirect radiation through the constituents of its 

 atmosphere. Of these the principal hindrances to free radiation are aqueous 

 vapor and carbonic acid gas. 



Mr. Fowle's investigations have fixed the dependence of the trans- 

 mission of the atmosphere on humidity for all wave lengths up to 17 

 microns. This covers a region of spectrum about fifty times as long as 

 that which is visible to the eye. At about 17 microns rock salt, which 

 is used in preference to glass for optical work on long-wave rays 

 because glass is opaque, itself becomes opaque. Further progress in 

 the important region between 17 and 50 microns depends on finding 

 a new transparent medium. Experiments by Mr. Aldrich have 

 shown that potassium iodide is suitable. But hitherto this substance 

 has yielded no crystals bigger than buckshot. Fortunately, new 

 methods devised for war purposes seem likely to furnish large crys- 

 tals of this substance and there is great hope that the investigation 

 of atmospheric transparency may soon be carried further. 



The total solar eclipse of June 8, 1918, was observed at Lakin, 

 Ivans., by Mr. Aldrich, of the Observatory, with two assistants. Some 

 good photographs of the solar corona and other phenomena were 

 secured. Throughout the afternoon and early night hours of June 

 8 and 9 observations were made with the pyranometer. The results 

 "measure the gradual diminution of the radiation of the sun and 

 of the brightness of the sky as the eclipse progressed, the outgoing 

 radiation of the earth's surface during totality, the gradual increase 

 of sun and sky radiation afterwards, their decline toward sunset, 

 and the outgoing radiation from the earth's surface after nightfall." 



Investigations at Mount Wilson of the variability of the sun have 

 been continued and improved. Observations were also made at 

 Hump Mountain, N. C, but that station was abandoned as too 

 cloudy, and in June, 1918, a station believed to be exceptionally well 

 located was established near Calama in Chile at an altitude of 2,250 

 meters where meteorological records indicate 300 days per year 

 favorable for solar constant work. This station is supported by a 

 grant from the Hodgkins fund. It is in charge of Mr. A. F. Moore 

 and is exceptionally well equipped. 



INTERNATIONAL EXCHANGES. 



The total number of packages handled by the International Ex- 

 change Service during the year was 266,916, weighing 182,825 pounds, 

 as compared with 399,695 pounds in 1917, the decrease being due 

 almost entirely to war conditions. 



The operations of the exchange service have been somewhat cur- 

 tailed during the past year by the impossibility at times of obtaining 

 136650°— 20 3 



