338 Mr. G. Johnstone Stoney [Feb. 19, 



whenever that epoch was, that these meteors entered the solar 

 system ; and in the former lecture the reasons were given which led 

 the late Professor Le Verrier to fix upon the spring of the year 

 A.D. 126 as the date of this remarkable event, when the swarm, 

 which had up to that time been an independent cluster, became 

 a member of the solar system. The cluster at that time seems 

 to have been travelling inwards from open space towards the sun, 

 past which it would, if unimpeded, have made a single sweep, 

 and would then have receded from the sun's neighbourhood to the 

 same immensity of distance from which it came. But while advan- 

 cing towards the sun, the great planet Uranus seems to have crossed 

 its path. The cluster of meteors must have nearly collided with 

 that great planet ; in fact, passed so close that the planet was able 

 to drag the group quite out of its previous path, after which the 

 planet advanced along its own orbit, and left the individual meteors 

 to pursue whatever orbits round the sua corresponded to the speed 

 and direction of motion which the planet had impressed upon each of 

 them. Previous to their encounter with the planet the great meteoric 

 cluster seems to have had sufficient coherence from mutual attraction 

 to be able to maintain itself as a compact group. But in sweej)ing 

 past so great a planet the difference of force acting on the members 

 of the group would probably be too great for their feeble attraction 

 towards one another. They got a little scattered, and when aban- 

 doned by the planet, found themselves too far asunder to admit of 

 their assembling again into a compact body ; a? id since then each 

 meteor has had to pursue independently its own orbit round the sun. 

 These orbits, though very close to one another, are not quite the 

 same ; they differ a little in every respect, and amongst the rest, in 

 their periodic times. The average period of traversing the orbit is 

 nearly 33} years. For some of the meteors it seems to be a week 

 longer, and for others a week shorter than their mean period. 

 Hence, at the end of their first revolution, the meteors with the 

 shortest periodic time came to their starting point a fortnight sooner 

 than the greatest laggards. At the end of two revolutions they were 

 a mouth asunder, and so on until now, at the end of 63 revolutions, 

 the foremost of the procession comes round two jears in advance of 

 the hindermost. 



Astronomers already know much which seems to support this re- 

 markable hypothesis of Le Verrier's ; but it is most desirable that 

 probability shall be changed into certainty one way or the other ; and 

 the lecturer urged that a great effort ought to be made on the occasion 

 of the approaching return of the great swarm, to secure observations, 

 so full and so accurate as will enable either ourselves or our posterity 

 to trace back with precision the history of the Leonids in the past, 

 and so ascertain with certainty whether it was, or was not, within a 

 few days of the end of February in the year a.d. 126, that these 

 innumerable minute bodies began their present career within the 

 solar system. 



