44 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL.. 'Jl 



I 



in the solar radiation is found when the spot crosses the meridian of 

 the sun, and maxima when the spot is near the east and west Hmb of 

 the sun, at the points marked A and B in the diagram. In addition 

 there are six other maxima, making eight in all during the interval 

 of the solar rotation, thus giving a mean period of 3.4 days. Taking 

 the maxima a, B, C, D in the curve of the large spots, there is found 



Table 13. — Mean Solar Radiation in Relation to Sun Spots, 1921-24- 



Days before 



Large spots . 

 Small spots . 



Groups 



Single spots. 



Increasing. . 

 Decreasing . 



Spots north 

 Spots south. 



Large spots 



Small spots 



Groups 



Single spots 



Increasing 



Decreasing 



Spots north 



Spots south 



Spot 



cen- 

 ter 



Davs after 



Days after 



Note: The values in the table are to be added to 1.900 so that the first value for example becomes 

 1.929. The number of cases of large spots is 37 and of small spots 47. The number of groups was 

 48 and of single spots 36. The number of cases of increasing spots was 34 and of decreasing spots 

 26. The number of cases of spots 10° or more north of the equator was 31 and 10° or more south of 

 the equator 19. 



an exact interval of 7 days and. if these be combined with A', which 

 is a repetition of A, there is found a mean interval of 6.8 days. At 

 least from the days of Prof. Joseph Henry down to the present this 

 interval has been noted from time to time in weather changes, and 

 has excited curiosity and comment. That it is related to solar changes 

 is probable, but the reason for these solar changes is not yet evident. 



