26 SCIENCE PRIMERS. [MATERIAL 



distance travelled over. It would appear as if the 

 large body were perfectly still and drew the small body 

 to itself. 



This is just what happens when a single drop of 

 water falls from a cloud, say through a distance 

 of a mile, to the earth. The earth really moves 

 towards it, just as it moves towards the earth, on 

 the straight line which joins the centres of the two. 

 But the length of this line which each travels over 

 is inversely proportional to the quantity of matter 

 in each, that is to say is the less the bigger the quan- 

 tity. So that we "have a rule-of- three sum. As the 

 quantity of matter in the earth is to that in a rain- 

 drop, so is a mile to the distance travelled over by 

 the earth. And if any one worked out this sum, he 

 would find that the fourth term of the proportion 

 would be an inconceivably minute fraction of an 

 inch. For all practical purposes, therefore, we may 

 consider the earth to be at rest in relation to all 

 falling bodies, inasmuch as the quantity of matter in 

 any falling body is insignificant, in comparison with 

 that contained in the earth. 



What is true of water is true, so far as we know, of 

 all kinds of matter, and we therefore say that it is a 

 law of nature that all kinds of matter possess gravity ; 

 that is to say, that of any two, each tends to move 

 towards the other, at a speed which is the slower the 

 greater the quantity of matter it contains in propor- 

 tion to that which the other contains ; and this speed 

 gradually becomes quicker as the two bodies approach. 



What is usually called the law of gravitation is 

 a statement of the same observed facts in another and 

 more complete fashion. (See Physics Primer'.) 



