46 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. /I 



4. The means of the solar radiation values and of the temperature 

 at Buenos Aires following minor maxima of radiation show periods 

 of 6.5 and 13 days which are fractions of the solar rotation period 

 near the equator. 



5. The periodic terms hence appear to be complex and variable, 

 depending" on the latitude in which outbreaks on the sun's surface 

 occur, and are rendered further complex at the earth's surface by the 

 seasonal changes, due to the movements of the sun north and south 

 of the earth's equator. However, the evidences furnished by the 

 various lines of research indicate that there is a distinct tendency to 

 form periods around certain lengths such as 3.5 to 3.7 days, 4.9 to 5.3, 

 6.4 to 7.3, 8.5 to 9.1, 10.5 to 11.4, 13.0 to 14.5, 18 to 21, 26 to 27, 30 

 to 34. These periods are either variable in length or else there are a 

 number of periods near the same length varying rapidly in intensity. 

 Thus the seven-day period varies in length from about 6.5 to 7.3 days, 

 or else there are several periods having lengths of about 6.6, 6.8, and 

 7.3 days. The shorter periods are considered as probable sub- 

 multiples of periods having the length of a solar rotation. 



Changes in the length of the periods could be explained as arising 

 from changes in the latitude of the solar outbreaks. However, as 

 this hypothesis does not yet fully explain all the periods found, and 

 especially the periods longer than 44 days, it is probable that there 

 are other causes for the periodic terms, and solar rotation niay be 

 only a modifying cause. 



6. Studies of the sun's surface in connection with these changes 

 will be necessary for further progress, but already a considerable 

 advance has been made by the studies of Abbot, Arctowski, and 

 Huntington. Abbot has found that in the short period changes of 

 solar radiation, the maximum values occur with minimum contrast in 

 brightness between the center and the edge of the sun (Smithsonian 

 Misc. Coll., Vol. 66, No. 5, by C. G. Abbot, F. E. Fowle, and L. B. 

 Aldrich). 



The lack of solar contrast may be interpreted as indicating an 

 increase of heated matter brought to the surface by increased con- 

 vectional overturning near the edges of the sun. As explained by 

 Abbot, the greatest radiation comes from matter below the outer 

 atmosphere of the sun. In this outer atmosphere, there is absorption 

 of radiation under normal conditions which is greatest near the edges 

 of the sun ; hence, outbreaks of heated matter near the limb of the 

 sun would increase the total radiation and diminish the contrast of 

 brightness between the center and the edge of the sun. Outbreaks of 



