CH. xxxii. RETURNING COMETS. 301 



mainly 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 the mass of 

 Jupiter is greater than the earlier astronomers had supposed, 

 while that of Mercury is much less. 



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 had 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 

 thought it advisable 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 in- 

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



