64 



ANNUAL KEPORT SMITHSONIAN INSTITUTION, 1911. 



day iu its output of radiation within limts of from 5 to 10 per cent in quantity 

 and in irregular periods of from 5 to 10 days. This conclusion I stale tenta- 

 tively. Before it can be accepted without question it must be confirmed by 

 Showing that the results obtained day after day at another equally pood station. 

 at a great distance, confirm those obtained simultaneously at Mount Wilson. 

 Such a final tost, it is now expected, will he made during the coming fiscal year. 



Summary of solar-constant values. 





Wash- 

 ington. 





Mount Wilson. 





Mount Whitney. 





1902-1907 



1905 



1900 



1988 



1909 



1910 



1909 



1910 



Times observed 



Mean value 



44 

 1.960 



59 

 1.925 



02 

 1.921 



113 

 1.929 



95 

 1.896 



128 

 1.914 



1 



1.959 



' 3 

 1.956 



1 Other days of observation not yet ready. 



General mean, 1.922 calories (15° C.) per square centimeter per minute. 



Number of determinations, 405. 



Other observations made on Mount Whitney. — Although the main purpose 

 of the Mount Whitney expedition of 1910 was served by proving that the 

 determinations of the solar constant of radiation are independent of the altitude 

 of the observing station, advantage was taken of the unusual opportunity to 

 make several other kinds of observations. Kapteyn's sky photometer was 

 employed there on two successive nights to measure the relative brightness of 

 the different regions of the night sky and to estimate the total quantity of sky 

 illumination per square degree compared with that of a first-magnitude star. 

 Tntema had employed similar apparatus in Holland. He found the average 

 brightness of the Milky Way about two or three times that of nongalactic 

 regions of the sky, such as the north polar region, hut that the sky near the 

 horizon was of about the same brightness as the Milky Way. He concluded 

 that the sky at night is illuminated more by some terrestrial sources of light 

 than by the stars. 



The results obtained' on Mount Whitney at nearly 3 miles elevation agreed 

 in general with those of Yntema. The following is a summary of the prin- 

 cipal points. Mean values are given: 



Brightness of night slcy. 

 [Polar 1. Mount w hitney 1-09-1910.] 





Galactic latitude. 



Near hori- 





0° to ±5° 



1 

 ±15° to ±30° ±45° to±00° 



±60° to ±75° 



zon. 





2.10 



1.25 1.19 



1.17 



1.40 







The total illumination from 1 square degree of polar sky was found to be 

 0.074G that of one first-magnitude star in the zenith. It is possible that the 

 fraction just given may be a little too small, owing to a source of error discov- 

 ered after the observations were ended. 



