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



of the planet perturbing tbeir motions. The most conspicuous of 

 these relations is that 14 revolutions of Jupiter in his orbit occupy 

 almost exactly the same time as five revolutions of the Leonids — 

 probably exactly the same time as five revolutions of those meteors 

 which occupy the foremost position in the procession. This re- 

 markable cycle has, therefore, been repeated as many as ten times 

 since the year a.d. 126, when it is supposed that the meteors entered 

 the solar system. Similar relations exist between the periodic time 

 of the Leonids and those of the planets Saturn and Uranus. Now 

 students of what is known as the " Planetary Theory " are aware 

 that numerical relations of this kind produce a very marked effect 

 on the perturbations, tending to make the perturbations in a short 

 limited time conspicuously different from their mean values, and ren- 

 dering it all the more necessary in the interests of physical astronomy 

 that such observations shall be made and such data collected when 

 the great stream returns to us, as will enable the computations to be 

 made for each revolution separately. 



At present we can only predict the return of a shower from our 

 knowledge of the average amount of the shift of the node, and the 

 time so determined is, as we see from the diagram, usually several 

 hours before or after the actual time. If we could calculate the per- 

 turbations in a single revolution we should be in a position to compute 

 the actual time. Even making use of the elements of the orbit as 

 already determined by Professor Adams from imperfect data, it V70uld 

 probably be possible to make a moderate approximation to the amount 

 of the perturbations between 1866 and 1899, so as to be able to come 

 nearer to ascertaining the hour at which the next meteoric shower 

 will commence than we can at present. It is to be hoped that this 

 eminently useful computation will be made before November 1898, 

 since it is possible that the head of the swarm will have reached the 

 earth's orbit by that time. 



But still more important information may emerge if we can 

 calculate with sufficient accuracy the perturbations in individual 

 revolutions. It will become possible to explore the past, to trace 

 back the history not only of the meteoric procession as a whole, but 

 of each part of it, and so ascertain with certainty when and through 

 what instrumentality it was that these foreigners annexed themselves 

 to the solar system. Similar information may be won in reference to 

 Tempel's comet. We may discover when and on what occasion this 

 body broke away from the main stream. These, if they can be 

 effected, will be great achievements, and will show the observers and 

 mathematicians of the present generation to be worthy successors of 

 the great men — Professors Adams, Hubert Newton, Le Verrier and 

 Schiaparelli — who made careful preparation before the return of the 

 meteors in 1866, so that the most instructive observations might then 

 be attempted, or who afterwards made use of the materials so collected 

 to splendid effect 



