1 86 LIGHT SCIENCE FOR LEISURE HOURS. 



comet of short period ; for although it was never seen 

 after its first visit, yet nothing can be more certain 

 than that it did actually return once, and that it 

 went twice round its new orbit. Indeed, if it has 

 not been absorbed by Jupiter a very unlikely con 

 tingency it must still be revolving in space with an 

 orbit which brings it, once in each revolution, to the 

 scene of its former encounters. The figure of its orbit 

 may be altered again and again by encounters with 

 Jupiter ; but each new orbit must traverse this dan 

 gerous point. This follows directly from the laws of 

 orbital motion around an attracting centre. A body 

 will continue to revolve in any orbit along which it 

 has once begun to move, unless it is acted upon by 

 some extraneous force. Accordingly, if at any point 

 of its path an extraneous force suddenly disturb its 

 motion, the disturbed orbit cannot fail to pass through 

 the point of disturbance. Thus the body may again 

 fall under the influence of the disturbing agent, and be 

 caused to move in yet another orbit through the same 

 point. And in the course of millions of years, a body 

 might thus travel in a hundred different orbits, all 

 passing through a common point. There is, indeed, 

 one way in which Lexell s comet might have escaped 

 from Jupiter s control. If after one of its encounters 

 with Jupiter, it happened to pursue a path which 

 brought it very nearly into contact with Saturn or some 

 other large planet, it might be compelled thenceforth 

 to abandon its allegiance to Jupiter. But the proba 

 bility of this happening to a comet which had once got 



