102 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1916. 



ranometer and have devised a new method which seems very free 

 from error for making its determination. The apparatus has been 

 constructed and is now set up practically ready for use. 



2. AT MOUNT WILSON. 



Messrs. Abbot and i^ldrich continued observations at Mount Wil- 

 son of the solar constant of radiation from July 1 to October 22, 

 1915, and renewed the expedition early in June, 1916. Besides con- 

 ducting solar-constant observations and determinations of the dis- 

 tribution of light over the sun's disk in seven different wave lengths 

 on each favorable day, comparisons of the pyrheliometers used or- 

 dinarily on Mount Wilson were made in both 1915 and 1916 with 

 standard water-flow pyrheliometer No. 3. The comparisons showed 

 no change to have occurred in the sensitiveness of secondary pyrheli- 

 ometers Nos. IV and VII, on whose readings rest the solar-constant 

 determinations made at Mount Wilson since 1906. 



A good deal of attention was also given to the installation and 

 trial of a solar cooking apparatus comprising ovens heated by oil 

 under gravity circulation maintained by heat collected by a concave 

 cylindric mirror of about 100 square feet surface. The apparatus 

 seems highly promising, but owing to a couple of defects was not 

 in satisfactory operation until after the close of the period covered 

 by this report. 



3. PROPOSED SOLAE-CONSTANT EXPEDITION. 



On recommendation of the writer an allotment was made from 

 the Hodgkins fund of the Smithsonian Institution for the purpose 

 of duplicating the solar-constant work of Mount Wilson at the most 

 favorable station on the earth. The expedition is being prepared 

 and will go forward, probably to South America, in the summer of 

 1917. It is intended to continue solar-constant determinations by the 

 spectro-bolometric method on every favorable day in every month 

 of the year for several years at both Mount Wilson and the station 

 in South America, with a view to determining the dependence of 

 the earth's climatic conditions on the sun's variations of radiation. 



SUMMARY. 



Observations of several kinds have been made, reduced, and pub- 

 lished which support one another in confirming the variability of 

 the sun, and some of which tend to indicate dual causes of it. An 

 expedition is proposed to occupy the most favorable station in South 

 America for several years, beginning in 1917, for the purpose of 

 making, in connection with the Mount Wilson observations, a full 



