REPORT OF THE SECRETARY. 91 



as the old and to have the great advantages: (1) That it avoids the 

 assumption of uniformity of atmospheric transparency during the 

 several hours formerly required for observing, and (2) that it di- 

 minishes the time required for computing the result from about 15 

 hours to about 3 hours. 



PERSONNEL. 



Miss Florence A. Graves, computer, was placed on furlough be- 

 ginning September 5, 1918, in order that she might take up work in 

 ■connection with the Red Cross operations in France. 



Miss Gladys L. Thurlby reported as assistant computer on Decem- 

 ber 2, 1918, and Miss Inez A. Ensign reported as computer on Feb- 

 ruary 1, 1919. 



SUMMARY. 



At Washington, outside of the usual reductions of observations and 

 various pieces of experimental investigation, some connected with 

 "the war, others with the study of radiation, but for which, for one 

 reason or another, no definite result can at present be reported, 

 progress has been made with the preparation of a new optical me- 

 dium, potassium iodide, for the investigation of the rays beyond 

 where rock salt is transmissible, and a new instrument based upon the 

 principle of the perfect radiator or " absolutely black body " has been 

 prepared and is undergoing test for the purpose of measuring noc- 

 turnal radiation such as the earth sends out to space. 



At Mount Wilson the measurements of the solar constant of radia- 

 tion have been continued, and a very neat and excellent piece of 

 w^ork has been done by Mr. Aldrich, in cooperation with the Army 

 Balloon School at Arcadia, on the measurement of the reflection of 

 sun and sky radiation from great sheets of clouds, which lead to the 

 result that a fully clouded earth would reflect to space 78 per cent of 

 the radiation of the sun falling upon it. 



In South America,' a successful expedition by Dr. Abbot observed 

 the total eclipse of the sun on May 29 at La Paz, Bolivia. Good 

 photographs of the phenomenon and also pyranometric observations 

 hy Mr, A. F. Moore of the brightness of the sky were obtained dur- 

 ing the progress of the eclipse. A conference which is likely to prove 

 of great future value was held by Dr. Abbot with the chief and chief 

 foi-ecaster of the Argentine meteorological service with reference to 

 the employment of solar radiation measurements for weather fore- 

 casting. At Calama, Chile,, Dr. Abbot, in cooperation with the 

 Smithsonian observers there, Messrs. Moore and Abbot, devised a 

 new method of reducing solar radiation observations so as to deter- 

 mine the solar constant of radiation with at least equal precision to 

 that obtained by the older method, and the advantages (1) that the 



