794. KEPORT— 1898. 



meetings of the Britiah Association. He there pointed out that if a body of small 

 mass crosses in front of a planet it draws the planet forwards, increases the planet's 

 kinetic energy, and necessarily loses itself an equal amount of kinetic energy. 

 This manifests itself by a slackening of its speed. The opposite effect is produced 

 when a meteor passes behind a planet. Its speed is thereby increased. Accordingly, 

 if a meteor from sub-stellar space — the space which lies between the solar system 

 and the nearest stars — approaclies the sun along a parabolic orbit, and happens to 

 traverse Laplace's sphere and to pass in front of the planet, it will find itself, 

 when it emerges from the sphere, moving with a speed too slow for parabolic 

 motion, and accordingly its orbit round the sun is thenceforth elliptic and it 

 becomes a permanent member of the solar system. When this happens to a group 

 of meteors from cosmic space, instead of to an isolated meteor, a new periodic 

 stream of meteors is added to the solar system. 



Thus the great Leonid swarm may have been drawn into the solar system by 

 having, while a nearly compact cluster, passed in front of the great planet Uranus 

 — an event which probably happened (as was pointed out by Le Verrier) at the 

 end of P'ebruary or beginning of March in the year 126 of the Christian era. 



This great meteor stream consists conspicuously of two classes of meteors, one 

 or both of which are found in every meteor stream. These we may call ortho- 

 meteors and clino-meteors. Ortho-meteors, when they are present, are those 

 which form a stream the individual members of which pursue very nearl}' the 

 same path. Accordingly those of them which are intercepted by the earth 

 enter the earth's atmosphere on some definite day and from one direction. 

 In the case of the Leonids they take 33^ years to traverse their immense orbit, 

 with deviations in the case of individuals from the mean periodic time which are 

 so small that they all seem to be less than a week — that is, less thaTi the l,700tli 

 part of the mean periodic time. Yet these very small deviations have enabled the 

 swarm of ortho-Leonids slowly to draw itself out along an arc of its orbit, until 

 now, at the end of seventeen centuries, the stream is long enough to take more 

 than two years to pass the earth's orbit when it comes round, which it does three 

 times every century. It thus happens that on each return of the ortho-Leonids, 

 the earth has time to come round twice to the place where the earth's orbit 

 intersects the orbit of the meteors. The earth, therefore, now pierces the stream 

 six times in a century, and, although in early times the number of its transits was 

 fewer, this event cannot have occurred less than some 70 or 80 times. 



On each such occasion the earth intercepts a vast number of meteors, but those 

 that pass close enough to have their orbit sensibly changed must be much, pro- 

 bably 100 times, more numerous. All the ortho-Leonids thus aflected become 

 clino-Leonids. They are one class of clino-Leonids — namely, those which have 

 been deflected by the earth into orbits which sensibly differ from the ortho-orbit. 

 At the end of each revolution in their new paths they would, if there were no 

 perturbations, return to the position close to the earth's orbit from which they 

 started, so that the earth does not on account ofwhat has happened lose its chance 

 of encountering them on future occasions. But other parts of their new orbits will 

 in general lie farther from the ortho-orbit, and the meteors moving in them will, 

 moreover, have sensibly different periodic times. The variety of their periodic times 

 will cause these clino-Leonids gradually to spread themselves round the whole ring, 

 so that the earth encounters some of them every year and not only in the years 

 when the ortho-Leonids return. Again, these clino-orbits having deviated from the 

 ortho-orbit, perturbations will act differently upon them and will cause their nodes 

 to advance, some faster and some slower than the node of the ortho-Leonids. 

 This will enable some of them to encounter the earth for some days before and for 

 some days after the ortho-date. By the ortho-date is meant that day in the 

 middle of November when the earth reaches the node of the ortho-orbit. 



So far, everything agrees with observation ; but there is one other respect in 

 which these earth-born clino-Leonids do not behave in the way observed. The 

 shifting of the node would be accompanied in them by a corresponding shift of the 

 radiant of about the same amount and in the same direction. This would involve 

 an advance of the radiant which would amount to about 14' in longitude during 



