Masterpieces of Science 



are required for a revolution. Sometimes, how- 

 ever, a comet passing near to Jupiter is so fasci- 

 nated by that planet that, in its vain attempt to 

 follow it, it loses so much of its primitive velocity 

 as to circulate around the sun in a period of a 

 few years, and thus to become, apparently, a new 

 member of our system. If the orbit of such a 

 comet, or, in fact, of any comet, chances to inter- 

 sect that of the earth, the latter in passing the? 

 point of intersection encounters minute particles 

 which cause a meteoric shower. The great 

 showers of November, which occur three times in 

 a century and were well known in the years 

 1866-67, may be expected to reappear about 

 1900, after the passage of a comet which, since 

 1866, has been visiting the confines of our system, 

 and is expected to return about two years henc^. 



But all this does not tell us much about the 

 nature and make-up of a comet. Does it con- 

 sist of nothing but isolated particles, or is there 

 a solid nucleus, the attraction of which tends 

 to keep the mass together? No one yet knows. 

 The spectroscope, if we interpret its indications 

 in the usual way, tells us that a comet is simply 

 a mass of hydro-carbon vapour, shining by its 

 own light. But there must be something wrong 

 in this interpretation. That the light is reflected 

 sunlight seems to follow necessarily from the 

 increased brilliancy of the comet as it approaches 

 the sun and its disappearance as it passes away. 



Great attention has recently been bestowed 

 upon the physical constitution of the planets 



