THE MOVEMENTS OF THE STARS. 267 



less in motion. For centuries, for untold thousands of years, 

 these stars have been moving, fresh stars have come in 

 from the abyss of space, and have gradually sunk awav 

 again to invisibility. There is no permanence in the 

 heavens. Even in the lapse of geological time the heavens 

 must have worn a succession of aspects, each wholly 

 different from the other. The sky as we see it to-night 

 presents us with an arrangement of stars different from 

 the stars which were visible to the eye of the first man 

 that trod this earth, and the stars which the first man 

 saw must have been widely different from those on which 

 in far earlier times the ichthyosaurus may have looked, 

 while these again must have been totally different from 

 the stars which spangled the sky in those excessively 

 remote times when life began to dawn on the earth. 



In our attempt to understand the nature of things 

 celestial there is nothing of more importance than a clear 

 comprehension of the mode in which the universe of stars 

 is gradually transforming itself. We want to learn how 

 each star is moving, and whither it is moving ; and when 

 we have obtained such knowledge, we are in a position to 

 learn more truly the disposition of that vast array in which 

 our sun is only one of the units. Astronomers had well- 

 nigh despaired of attaining any comprehensive knowledge 

 on this subject within the lifetime of the present genera- 

 tion. Centuries must elapse before the majority of the 

 stars have advanced by proper motion to a sufficient 

 distance from their present places to render the change 

 conspicuous. We want some readier way of determining 

 the pace at which a star is going we want an instrument 

 which shall tell us at once the speed of the body without ' 



