CH. XXXI. RETURNING COMETS. 291 



of importance because after Professor Encke, of Berlin, had 

 calculated that it returned regularly every three years and a 

 quarter, he found that it arrived two hours and a quarter 

 earlier each time. Why it should come earlier is a question 

 which is still very perplexing to astronomers, though several 

 explanations have been suggested. In order to find out 

 how fast this comet moved, Encke was obliged to calculate 

 very accurately how much the different planets attract it ; 

 and this led him to discover that Jupiter is larger than the 

 earlier astronomers had supposed, while Mercury is much 

 smaller. 



Biela's Comet, 1826. — In 1826 another remarkable 

 comet was observed by an Austrian officer named Biela, 

 and on that account called * Biela's comet.' M. Clausen, a 

 German astronomer, computed that it revolved in an elliptic 

 orbit in a period of six years and eight months, and it was 

 then shown to be the same comet which had been observed 

 in 1772, 1805, and 1818. This comet has had a very curious 

 history. In the year 1832 great alarm was excited because 

 the astronomers calculated that it would cross the orbit of 

 our earth on October 29. People who did not understand 

 the question thought this meant that it would run into us 

 and perhaps destroy our earth ; and many even sold their 

 houses and land because they thought the end of the world 

 was at hand. The people in Paris especially were so 

 frightened about it that the Academy of Sciences were 

 obliged to ask Arago, the French astronomer, to quiet their 

 fears, and he wrote a popular essay showing that though the 

 comet crossed the path of our earth, yet on that day we 

 should be fifty-five millions of miles away from the spot. 



But it was in 1845 that Biela's comet proved most 

 interesting. On November 26 in that year it came at the 



