402 THE BEAUTIES OE NATURE chap. 



the Sun they pass away again into space, 

 never to return. 



The appearance which is generally regarded 

 as characteristic of a Comet is that of a 

 head with a central nucleus and a long tail. 

 Many, however, of the smaller ones possess 

 no tail, and in fact Comets present almost 

 innumerable differences. Moreover the same 

 Comet changes rapidly, so that when they 

 return, they are identified not in any way by 

 their appearance, hut by the path they 

 pursue. 



Comets may almost be regarded as the 

 ghosts of heavenly bodies. The heads, in 

 some cases, may consist of separate solid 

 fragments, though on this astronomers are 

 by no means agreed, but the tails at any rate 

 are in fact of almost inconceivable tenuity. 

 We know that a cloud a few hundred feet 

 thick is sufficient to hide, not only the stars, 

 but even the Sun himself. A Comet is 

 thousands of miles in thickness, and yet even 

 extremely minute stars can be seen through 

 it with no appreciable diminution of bright- 

 ness. This extreme tenuity of comets is 



