HOW THE SUN WARMS THE EARTH ABBOT 



169 



H. Schwabe, of Dessau, Germany, kept daily watch upon the sun 

 spots and discovered about 1843 that their numbers wax and wane 

 in a period of about 11 years. His observations were continued by 

 inan,y observers, and old records were consulted which showed that 

 the 11-year periodicity in sun spots has continued for several cen- 



JAN.JUIY JAN. JJLY JAN. JUUf JAN. JUDT JAN. JUtf JAN. J(JIY JAN. JULf JAN. JULY JAM. JULY JAN. JULY JAN JULY JAN. 

 192/ 1925 (926 1927 1920 I92<^, '930 1931 1932 1933 1934 1935 



FiGURB 12. 



-Analysis of apparently irregular solar variation indicates that it Is composed 

 of seven regular periodicities. 



turics (fig. 13). Recently Douglass, by tree-ring measurements, and 

 Baron de Geer, Antevs, and others, by measuring clay deposits, have 

 found evidences of the sun-spot period for many centuries. About 

 25 years ago Dr. Hale at Mount Wilson Observatory discovered that 

 sun spots are magnetic. This fell in nicely with the previously 

 observed fact that the earth's magnetism and the aurora borealis are 

 both closely associated with the sun-spot periodicity. Hale soon 

 found that magnetic polarities are opposite in adjacent spots, and he 

 showed later that the magnetic polarity of sun spots passes through 



