86 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1912. 



on the ordinary vicissitudes of all experimental work, the smallness 

 of this accidental error seems remarkable. 



Expeditions of 1912. — While the simultaneous observations made 

 in 1911 at Bassour and Mount Wilson seemed justly interpretable as 

 confirming the variability of the sun, yet it was felt that a result of 

 such uncommon interest ought to be put beyond the smallest war- 

 rantable doubt. Accordingly, in May, 1912, Mr. and Mrs. Abbot 

 again returned to Bassour, where they were joined on May 20 by Mr. 

 Anders Knutson Angstrom^ as temporary assistant. Observations 

 were begun on June 2. Observations on Mount Wilson had already 

 been begun by Mr. Fowle in April. June yielded 17 days of meas- 

 urement at Bassour and 25 days on Mount Wilson. It is expected 

 that the two expeditions will continue observing until about Septem- 

 ber 10, 1912. There can hardly be any question that this work, com- 

 bined with that of 1911, will thoroughly prove or disprove the exist- 

 ence of the suspected short-period variations of the sun. 



2. ON THE DISSEMINATION OP STANDARDS OP PYRHELIOMETRY. 



The Smithsonian Institution having undertaken to furnish silver 

 disk pyrheliometers at cost when useful solar researches seemed likely 

 to be promoted thereby, the assembling of the completed instruments, 

 their standardization, and their packing for shipment have been done 

 at the Astrophysical Observatory. During the past year about 10 

 such instruments have been prepared and sent out, mostly to foreign 

 governmental meteorological services. When returning from Al- 

 geria Mr. Abbot compared silver disk pyrheliometer A. P. O. No. 

 IX at Naples and Potsdam with similar instruments furnished by 

 the Institution. In neither case was there found any change of read- 

 ings of the instruments compared. It was hoped to make compari- 

 sons also at London and Paris, but the weather prevented. 



3. ON THE ABSORPTION OP RADIATION BY ATMOSPHERIC WATER VAPOR. 



Mr. Fowle has continued the research on the absorption of radia- 

 tion by water vapor, and has devised and published * a method for 

 determining spectroscopically the total quantity of water vapor in- 

 cluded between the observer and the sun. The method is based on 

 spectrobolometric observations made with the long absorption tube 

 mentioned in the last two reports, and is applicable to all bolometric 

 observations of the sun's infra-red spectrum. It seems probably to 

 be accurate to within 1 or 2 per cent. Heretofore there has been no 

 method of estimating atmospheric water vapor excepting from ob- 

 servations of the humidity prevailing at the surface of the earth 

 or near kites, balloons, and mountains. From such psychrometric 



1 Astrophysical Journal, vol. 35, 1912, p. 149. 



