188 PROGRESS OF SCIENCE IN THE CENTURY. 



sequent occasions (1872 and 1885), as the earth was 

 crossing the path of the comet when it (if it had 

 persisted) was nearly due at the same place, there 

 was an unusually brilliant shower of meteors. 

 Meteors may be fragments of a broken-up comet, or 

 a comet may be a swarm of meteors. 



In the study of comets the accuracy of the gravi- 

 tational formula has been beautifully illustrated, and, 

 during the latter half of the century, considerable 

 progress was made towards an understanding of their 

 physical nature. 



THE STUDY OF THE STABS. 



Almost until the end of the eighteenth century, 

 it was the general belief, even among astronomers, 

 that the stars were fixed and unchanging. As Miss 

 Clerke says, " their recognised function, in fact, was 

 that of milestones on the great celestial highway 

 traversed by the planets." Gradually, however, it 

 became evident that this emphatically static image 

 was far from being true. What Giordano Bruno 

 had imagined, was confirmed by Halley in 1718, 

 when he showed that Siriua, Aldebaran, Betelgeuse, 

 and Arcturus had changed thoir positions in the sky 

 since Ptolemy marked these out. Many similar facts 

 came to light, and in the last quarter of the eigh- 

 teenth century, sidereal astronomy included " three 

 items of information that the stars have motions, 

 real or apparent ; that they are immeasurably re- 

 mote; and that a few shine with a periodically varia- 

 ble light." * 



William Herschel. It was about the beginning of 



Agnes M. Clerke. A popular Hittory of Astronomy 

 During the Nineteenth Century, 1885, p. 13. 



