APPARENT IMMOBILITY. 289 



the cloud away, and it dissolves again, or vanishes 

 when the air is dry. The apparent immobility of a 

 cloud on the top of a mountain is caused by its being 

 constantly re-lornied. Its variations are really inces- 

 sant and rapid. They pass unobserved by us, and 

 thus, fur a long period together, mountain-clouds 

 app:^ar as if they had settled down immutable. Very 

 slow changes, as we have remarked above, present 

 the ai)pearance of nnchangeableness, no less than 

 very rapid ones. 



The vault of heaven is thickly besprent with stars, 

 the greater number of which we call " fixed," while a 

 few, called " planets," constantly change their places. 

 The fixed stars, while preserving the same relative 

 position unchanged, seem, as a whole, to make a 

 daily revolution round a point marked by the polar 

 star, and the latter, or centre of rotation, does not at 

 first sight appear to shift its position. A long and 

 careful study of the heavens, however, has shown 

 that this is not the case. The apparent revolution 

 of the whole concave of stars is accounted for by the 

 fact that the earth moves on its axis, as well as 

 describes an orbit round the sun. But the stars are 

 at such an enormous distance from us that the axis 

 of our globe remains, practically, parallel to itself 

 (luring the whole period of the earth's annual revo- 

 lution, so that, at any given period of the year, the 



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