Geology 



have been active in the past and have slowly brought 

 our world into the condition in which we now find it. 



As we shall see later, a knowledge of the mode of 

 origin of the Earth is necessary for many of these ends, 

 and we shall therefore first turn our attention to the 

 study of the Earth as a whole and its relations to the 

 other bodies in its neighbourhood. 



When we wish to indicate the whole assemblage of 

 the heavenly bodies, and the vast space which lies 

 around and between them, we use the term Universe. 

 The Universe is so vast that we can form but a vague 

 conception of its dimensions in fact, every increase 

 in the power of our telescopes, or improvement in the 

 photographic apparatus which we use in connection 

 with them, makes new additions to our knowledge and 

 extends the boundaries of the visible Universe. 



With the actual dimensions of the Universe, even if 

 these were ascertainable, we have little to do, suffice it 

 to say that light, which travels 186,000 miles in one 

 second, takes rather more than three years to travel 

 from a Centauri, the nearest star, to the Earth. We 

 now see the " Dog Star," Sirius, by the light which left 

 it seventeen years ago, and the light now reaching us 

 from the Pole Star started nearly half a century ago. 

 Other stars are perhaps a hundred or even a thousand 

 times more distant. 



There are two main classes of bodies visible through 

 our telescopes stars and nebulae. 



The stars are in many cases similar to the Sun ; 

 they shine by their own light, and for anything we 

 know to the contrary, may be attended by families of 

 planets. 



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