A RECENT STAR SHOWER. 203 



certain portion of its velocity, and so taking away 

 (in fact) its power of retreating.* To do this a planet 

 must be peculiarly placed. It must be nearly on the 

 course of the arriving body ; but that is by no means 

 sufficient : indeed the action of a planet so placed would 

 simply be to accelerate the motion of the body as it 

 approached, and it is precisely the contrary effect which 

 is in the long run to be produced. A planet must be so 

 placed that it draws nearer and nearer to the place 

 where the body will cross, but not so near as to exert 

 its most powerful action until the body has passed that 

 place. Then, the body being just inside the planet's 

 course, and travelling sunwards, the planet should be 

 at its very nearest, and so exert its greatest pull in 

 drawing the body back. The whole affair must be 

 managed very neatly, so to speak ; for the body will 

 inevitably go past very quickly, and thus the planet 

 has very little time during which to exert its power. 

 Unless the planet gets a really effective pull, owing to 

 the closeness of the body at the critical moment, the 

 body will escape scot-free. And this applies quite as 



* The case may be thus illustrated : When an elastic ball falls from 

 a great height on a hard surface, it rebounds nearly to the height from 

 which it had fallen. But suppose a netting to be interposed (not far 

 from the hard surface), and that though the body bursts its way 

 through the netting, it only does so at the expense of the best part of 

 its velocity ; then the body will not rebound to a great height, even 

 though the netting be removed. A planet may, in like manner, inter- 

 fere with a body approaching the sun from outer space ; and though 

 the body may get past the planet, it may only do so at the expense of 

 a large portion of its velocity ; and then instead of returning to the 

 interstellar depths it will pass away only to a moderate distance from the 

 sun (after its perihelion swoop). 



