l6o LIGHT SCIENCE FOR LEISURE HOURS. 



a long- extended flight of cosmical bodies travelling in 

 the track of Tempel's comet. 



Now it appears clear that this flight of cosmical 

 bodies may be looked upon as constituting an ex- 

 tension of the comet an invisible train as it were. 

 But for the accident that the comet's track intersects 

 the earth's path in space, we should have remained 

 for ever ignorant of the fact that the comet has 

 any other extent than that which is indicated by 

 its telescopic figure. Now, however, that we know 

 otherwise, we recognise the probability that other 

 comets which have been looked upon as tailless may 

 have invisible extensions reaching far behind them into 

 space, or even completely around their orbit. 



But the members of the November shooting-star 

 system have been subjected to spectroscopic analysis. 

 We know that they contain several terrestrial elements ; 

 and we recognise the probability that if we could 

 examine one of them before its destruction (in tra- 

 versing our own atmosphere) we should find a close 

 resemblance in its constitution to that of those aero- 

 lites or meteorites which have reached the surface of 

 the earth. 



But here we encounter a new difficulty. One theory 

 respecting the tails of comets has accounted for them 

 by the supposition of a propulsive effect exerted by the 

 solar rays ; and another theory has ascribed them to the 

 action of vapours ascending in the solar atmosphere. 

 But if the tails of comets really consist of solid matter 

 very widely dispersed, it must be quite evident that 



