COMETS. 129 



be periodic, with a period of between six and 

 seven years. The comet returned in 1832. In 

 1828 Olbers had published certain calculations 

 showing that portions of the comet would sweep 

 over the part of the Earth's orbit a month 

 later than the Earth itself. This gave rise 

 to a panic that the comet would destroy the 

 Earth, which did not subside till it was an- 

 nounced by Arago that the Earth and the 

 comet would at no time approach to within 

 fifty million miles of each other. The comet 

 returned again in the end of 1845. It was 

 kept well in view by astronomers in Europe 

 and America. On December 19, 1846, Hind 

 noticed that the comet was pear-shaped, and 

 ten days later it had divided in two. The 

 two comets returned again in 1852 and were 

 well observed ; but they were never seen again, 

 at least as comets. Their subsequent history 

 belongs to meteoric astronomy. 



A comet discovered by Faye at Paris in 1843 

 was found to have a period of seven and a 

 half years. It has returned regularly since its 

 discovery, true to astronomical prediction. Its 

 motion was particularly investigated for traces 

 of a resisting medium, by Didrik Magnus Axel 

 Holler (1830-1896), director of the Lund Ob- 

 servatory, who reached a negative conclusion. 



I 



