A CENTURY'S PROGRESS IN PHYSICS 337 



We shall mention but one other achievement of the 

 methods of celestial mechanics. Those visitors of the 

 skies, the comets, which become so prominent only to fade 

 away and vanish perhaps forever, had interested astron- 

 omers from the earliest times. Soon after the discovery 

 of the law of gravitation, Newton had worked out a 

 method by which the elements of a comet's orbit can be 

 computed from observations of its position. It was 

 found that the great majority of these bodies move in 

 nearly parabolic paths and only a few in ellipses. Of the 

 latter the most prominent is the brilliant comet first 

 observed by Halley in 1681. It has reappeared regu- 

 larly at intervals of seventy-six years ; the last appear- 

 ance in the spring of 1910 is no doubt well remembered 

 by the reader. Kant had considered comets to be 

 formed by condensing solar nebulae, whereas Laplace had 

 maintained that they originate in matter which is scat- 

 tered throughout stellar space and has no connection 

 with the solar system. A study of the distribution of 

 inclinations of comet orbits by H. A. Newton (16, 165, 

 1878) of New Haven substantiated Laplace's hypothesis, 

 and led to the conclusion that the periodic comets have 

 been captured by the attraction of those planets near to 

 which they have passed. Of these comets a number 

 have comparatively short periods, and are found to have 

 orbits which are in general only slightly inclined to those 

 of the planets, and are traversed in the same direction. 

 Moreover, the fact that the orbit of each of these comets 

 comes very close to that of Jupiter made it seem probable 

 that they have been attached to the solar system by the 

 attraction of this planet. Further confirmation of this 

 hypothesis was furnished by H. A. Newton's (42, 183 and 

 482, 1891) explanation of the small inclination of their 

 orbits and the scarcity of retrograde motions among 

 them. 



In 1833 occurred one of the greatest meteoric showers 

 of history. Olmstead (26, 132, 1834) and Twining (26, 

 320, 1834) of New Haven noticed that these shooting 

 stars traverse parallel paths, and were the first to sug- 

 gest that they must be moving in swarms in a permanent 

 orbit. From an examination of all accessible records, 

 H. A. Newton (37, 377, 1864; 38, 53, 1864) was able to 



