1897.] on the Approaching Return of the November Meteors. 341 



It will be observed that each of the elements of the orbit of 

 Tempel's comet diifers, but only differs a little, from the correspond- 

 ing element of the orbit of the meteors. Differences of this kind have 

 established themselves in the case of every one of the sporadic 

 meteors which have got separated from the main swarm by the earth. 

 And this gives rise to the suspicion, almost amounting to belief, 

 that the comet was at one time a member of the swarm, and was 

 drawn a little aside on one of the occasions when the earth passed 

 through the stream. Since that event Jupiter and Saturn have 

 been incessantly perturbing its orbit and that of the meteors a little 

 differently, and have thus increased the divergence. Now, if we can 

 determine the orbit of the comet with great accuracy, it will become 

 possible to ascertain with precision what these perturbations have 

 been in the last few centuries, and thus to trace back the path which 

 the comet has pursued in space. If this can be done satisfactorily, 

 we shall be able to find when it was that the comet was so close to 

 the earth that the earth was able to alter its whole future history. 

 This is another problem which the lecturer invited astronomers to 

 set before them, and in order to prepare for it, to make the most 

 exact observations that are practicable upon the comet on the occasion 

 of its approaching return. 



The Main Swarm. 



We may next turn to the main swarm. The inclination of the 

 orbit of the meteors to the planes in which Jupiter and Saturn travel 

 has been referred to above. The meteors, on account of this inclined 

 position of their orbit, glide at a distance of many millions of miles 

 over and under the orbits of those planets, and the planets, as they 

 pass through the inclined orbit of the meteors, are favourably situated 

 for modifying that orbit by their attraction. One of the principal 

 effects that they thus occasion is to make the meteoric orbit, instead 

 of standing out from the sun in one fixed direction, to shift slowly 

 round in the same direction in which the planets travel round the 

 sun. This shifting of the orbit of the meteors has caused the time 

 when the earth encounters the swarm to have gradually advanced 

 from October 12th (Old Style), when the earth encountered the swarm 

 in A.D. 902 (this being the first visit of the meteors of which we 

 possess a record) until November 13th (New Style), when the great 

 shower of 1866 was discharged upon the earth. The point on the 

 earth's orbit where the meteors' orbit intersects is called the node of 

 the meteors' orbit. Accordingly, the facts are usually described by 

 saying that the node of the meteoric orbit has shifted forwards along 

 the earth's orbit from the place which the earth reaches each Octo- 

 ber 19th, which is equivalent in the new style to the date which was 

 called October 12th in a.d. 902, until November 13th or 14tb, which 

 is the present date. Thus the shift forwards in a thousand years of 

 the date on which the showers occur has been about three weeks and a 

 half, and we know that a similar shift must have been going on before 



Vol. XV. (No. 91.) 2 a 



