192 POPULAR LECTURES AND ADDRESSES. 



extends over a portion of the orbit so great as to 

 occupy about one-tenth or one-fifteenth of the 

 periodic time in passing any particular point, and 

 gave a choice of five different periods for the 

 revolution of this meteoric stream round the sun, 

 any one of which would satisfy his statistical result. 

 He further concluded that the line of nodes, that is 

 to say, the line in which the plane of the meteoric 

 belt cuts the plane of the Earth's orbit, has a 

 progressive sidereal motion of about S 2 ^'4 P cr 

 annum. Here, then, was a splendid problem for 

 the physical astronomer; and, happily, one well 

 qualified for the task, took it up. Adams, by the 

 application of a beautiful method invented by 

 (iauss, found that of the five periods allowed by 

 Newton just one permitted the motion of the line 

 of nodes to be explained by the disturbing influence 

 of Jupiter, Saturn, and other planets. The period 

 chosen on these grounds is 33', years. The inves- 

 tigation showed further that the form of the orbit 

 is a long ellipse, giving for shortest distance from 

 the Sun 145 million kilometres, and for longest 

 distance 2895 million kilometres. Adams also 

 worked out the longitude of the perihelion and 



