402 THE BEAUTIES OF 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, but 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 
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