274 THE WORLD MACHINE 



permanent part of the comet at all, but that, as Kepler had 

 partly guessed, it is simply a stream of matter driven off from 

 the head, or nucleus, by the pressure of the sunlight. It would 

 be more tenuous than any mist or vapour we may conceive. 

 It would bear perhaps the same relation to the comet proper 

 as the thin line of smoke and naming cinders thrown out by a 

 flying locomotive to the locomotive itself. This inference seems 

 fairly substantiated by the fact that the tail only appears as the 

 comet approaches the sun ; when at a great distance the comet 

 represents only an indistinct patch of nebulous light. Finally, 

 if this view be correct, we should expect to find that the short- 

 period comets which sweep around the sun every few years or 

 so would, through a long series of observations, show a diminu- 

 tion in their volume, owing to the quantity of matter driven 

 off into the tail and left in space. This is precisely what has 

 been found. 



It remains, then, merely to find out the nature of the 

 nucleus, or head, and the mystery of these fiery portents 

 is completely solved. The question has a double interest from 

 the 'fact that it may shed some light upon the stuff of which 

 the universe is made. While the great majority of the comets 

 appear to describe closed orbits, some certainly do not. They 

 are simply drawn into our solar system by the chance of having 

 come within the clutches of solar attraction in their flight through 

 space. They appear but once, and then are gone for ever. 

 Could we but know their chemic composition, we should know 

 a little more than we do of worlds beyond our own. It is only 

 within the last thirty or forty years that the clue has come. 



It was unexpected in its simplicity. Watching the heavens 

 by night, the eye is often thrilled by the spectacle of a gorgeous 

 flash of light which we are wont to call a shooting-star. Some- 

 times, as we know, these shooting-stars come in a kind of a shoal ; 

 we speak of these as meteoric showers. As a rule the headlong 

 dash of these bodies through the earth's atmosphere generates 

 so fierce a heat that they are dissipated into vapour, and their 

 substance is left to float about in the air as a part of the dust 

 which gives the sky its wonderful hue of blue. Eventually it 

 settles down to the earth ; and this, it seems likely, is the origin 

 of the fine metallic dust which will cover snow-fields even of far 

 northern climes, where the atmosphere is of the purest and where 

 smoke and dust seem almost excluded. This surmise has ap- 



