268 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. JO 



prevailing probably along with greater solar radiation at times of 

 high solar activity. Thus in 1913, v^^hen there was a minimum of 

 solar radiation (and also an exceptionally developed minimum of 

 sun spots), the contrast of brightness between the center of the 

 sun's disk and the edge was decidedly less than in the years 1914 

 and 1917, when there was evidently higher solar activity indicated 

 by greater solar radiation (greater "solar constant"). "Besides 

 these long-period changes there appear to be small changes of con- 

 trast from day to day, correlated with the changes of solar radiation 

 heretofore discovered by the authors. For this type of changes 

 increased contrast is associated with decreased solar radiation," 



The authors are thus " led to consider two causes of change 

 existing in the sun. One, going with increased solar activity, they 

 regard to be increased effective solar temperature, which naturally 

 produces increased radiation and increased contrast. The other, 

 altering from day to day, they regard to be increased transparency 

 of the outer solar envelope, which naturally produces increased 

 radiation but decreased contrast. All these changes are greater for 

 shorter wave lengths." 



It may also be of some interest to mention here that according 

 to the observations made during the year 1913 there should have 

 been a sudden change in the solar radiation on September 23, when 

 the solar constant and solar contrast values fell ofif and remained 

 comparatively low during the rest of that season. At the same time 

 " a marked change in the distribution and total amount of the water 

 vapor in the atmosphere took place. The values of precipitable 

 water in the atmosphere were far above the normal until September 

 23, and from then to the end of the period of observation generally 

 about normal or a little below. A similar change is indicated, but 

 not in so great a degree, by the observations with the wet and dry 

 thermometers. The temperature also fell at the same critical time. 



In his paper on " Arequipa Pyrheliometry " Dr. Abbot discusses 

 the observations made with the silver-disk pyrheliometer and nearly 

 simultaneous measurements of atmospheric humidity made from 

 August, 1912, to the end of March, 191 5, at Arequipa, Peru, at the 

 station of the Harvard College Observatory. We find that the 

 Arequipa results fully confirm the variability of the sun both from 

 year to year and from day to day, shown by investigations at Mount 

 Wilson and elsewhere. The monthly mean values of the Arequipa 

 observations also show remarkably close connections between the 

 solar constant (solar radiation) and vapor pressure of the terrestrial 

 atmosphere. 



