354 NINETEENTH CENTURY. pt. hi. 



first to suggest in 1793 that they are caused by the opening 

 of bright luminous clouds which float round the sun, and 

 break away sometimes in one place and sometimes in an- 

 other, allowing us to see down through the gap into the 

 body of the sun itself, which thus has the appearance of a 

 dark spot. This is the explanation now received by astro- 

 nomers as most probable, and it accounts for the constant 

 appearance and disappearance of the spots. 



In the year 1826, a well-known German astronomer, 

 Herr Schwabe, of Dessau (who died in 1874), determined to 

 take regular notes of the periods when there were most spots 

 to be seen on the face of the sun. Every day during twelve 

 years, when the sky was clear enough for him to observe the 

 sun, he examined it through his telescope, and noted how 

 many spots he could see. 



In this way he discovered that there was a regular de- 

 crease in the number of spots for about five years and a 

 half, and then during the next five and a half years a gradual 

 increase, till they were very numerous indeed. This led him 

 to think that the spots went through a complete round 01 

 cycle of changes in about eleven years ; but as he found it 

 difficult to persuade other astronomers of the fact, he actually 

 carried on his daily observations for twenty years longer, and 

 then, at the end of thirty-four years of daily observation, he 

 was able to assert boldly that he had established the truth of 

 his theory. 



He had now kept an account of three periods of elevenj 

 years. At the beginning of each of these periods the sui 

 was for some time smooth and almost free from spots ; thei 

 from year to year they increased, till, at the end of five anc 

 a half years, as many as fifty or sixty could be seen at one 

 time. Then they decreased again till, at the end of another 



