THE BRIGHTNESS OF THE SKY 

 By a. F. MOORE and L. H. ABBOT 



INTRODUCTION 



It was planned to send an expedition from the Smithsonian Insti- 

 tution to Argentina or Chile in the early summer of 191 /, for the 

 purpose of making daily observations on the solar constant of radia- 

 tion similar to those which have been made for fourteen summers 

 past at the station of the Smithsonian Astrophysical Observatory on 

 Mt. Wilson, California. Accordingly, much of the apparatus was 

 boxed ready for shipment to Chile early in 191 7. Owing to war 

 conditions, however, it was thought best to defer sending the expedi- 

 tion to a foreign country, and a location was selected on the slope 

 of -Hump Mountain, near Elk Park, North Carolina. 



From all available reports, this locality, it was thought, might 

 be expected to furnish two hundred clear days per year. It was 

 selected because of this fact, and also because it had a sufficient 

 elevation, was at a great distance from Mt. Wilson (insuring dif- 

 ferent local weather conditions at the two stations), and because of 

 its easy access. 



Bolometric observations were begun at Hump Mountain on June 

 17, 1 91 7. But it soon became evident that there was too much 

 cloudiness and rainfall to render it suitable for a permanent solar 

 constant station. However, the expedition possessed an instrument 

 for which the varying conditions of Hump Mountain were just suited, 

 this instrument being the pyranometer, recently perfected at the 

 Smithsonian Astrophysical Observatory, and described in detail in 

 two former publications of the Institution (Smithsonian Miscel- 

 laneous Collections, Vol. 66, Nos. 7 and 11). Accordingly, a con- 

 siderable amount of work was done with the pyranometer during 

 the expedition's stay at Hump Mountain, which lasted until April, 

 1918, when it prepared to go to Chile. 



The present paper is a description of these various lines of obser- 

 vation. It is thought best not to burden the reader with the many 

 pages of observations which were taken, therefore an effort will 

 be made to condense as much as possible without omitting important 

 details. 



Smithsonian Miscellaneous Collections, Vol. 71, No. 4 



