THE SEMI-CENTENNIAL ANNIVERSARY 41 



Here is a photograph of the sun, as it appears in the telescope 

 (Fig. 4*). Scattered over its surface are sun-spots, which in- 

 crease and decrease in number in a period of about 1 1.3 years. It 

 is well known that a curve, showing the number of spots on the 

 sun, is closely similar to a curve representing the variations of 

 intensity of the earth's magnetism. The time of maximum sun- 

 spots corresponds, as Dr. Bauer found, with that of reduced 

 intensity of magnetization of the earth, and the parallelism of the 

 two curves is too close to be the result of accident. We may 

 therefore conclude that there is some connection between the 

 spotted area of the sun and the magnetic field of the earth. 



We shall consider a little later the nature of sun-spots, but for 

 the present we may regard them simply as solar storms. When 

 spots are numerous the entire sun is disturbed, and eruptive 

 phenomena, far transcending our most violent volcanic outbursts, 

 are frequently visible. In the atmosphere of the sun, gaseous 

 prominences rise to great heights. This one (shown on the 

 screen), reaching an elevation of 85,000 miles, is of the quiescent 

 type, which changes gradually in form and is abundantly found 

 at all phases of the sun's activity. But such eruptions as the one 

 of March 25, 1895, photographed with the spectroheliograph of 

 the Kenwood Observatory, are clearly of an explosive nature. 

 As these photographs show, it shot upward through a distance of 

 146,000 miles in 24 minutes, after which it faded away. 



When great and rapidly changing spots, usually accompanied 

 by eruptive prominences, are observed on the sun, brilliant dis- 

 plays of the aurora (Fig. 7) and violent magnetic storms are 

 often reported. The magnetic needle, which would record a 

 smooth straight line on the photographic film if it were at rest, 

 trembles and vibrates, drawing a broken and irregular curve. 

 Simultaneously, the aurora flashes and pulsates, sometimes light- 

 ing up the sky with the most brilliant display of red and green 

 discharges. 



Birkeland and Stormer have worked out a theory which ac- 

 counts in a very satisfactory way for these phenomena. They 



* Figs. 4, 5 and 6 represent the same region of the sun, photographed at successively 

 higher levels. 



