OBJECTS.] INTRODUCTORY. 25 



18. Gravity and Gravitation. 



The word gravity, when it was first used, had ex- 

 actly the same meaning as weight ; and a body which 

 has weight is said to gravitate towards the centre of 

 the earth. But gravity has now acquired a much 

 wider sense than weight. For an immense number 

 of careful observations and experiments have estab- 

 lished the general rule, or law of nature, that every 

 material substance tends to approach every other 

 material substance, just in the same way as a drop of 

 rain falls towards the earth; and, in fact, that any 

 two portions of matter, whatever the nature of that 

 matter may be, will move towards one another if there 

 is nothing to prevent them from doing so. 



To make this clear, let us suppose that the only 

 material bodies in the universe were two spherical drops 

 of water, each a tenth of an inch in diameter. Each 

 of these drops would have the same bulk as the other, 

 and would be a quantity of matter exactly equivalent 

 to the other. Then, however great the distance 

 which separated these two drops, they would begin 

 to approach one another ; and, each moving with 

 gradually increasing swiftness, they would at length 

 meet in a point exactly half-way between the positions 

 which they at first occupied. But if the bulk 

 of one drop were greater than that of the other 

 drop, then the larger would move more slowly, and 

 the point of meeting would be by so much nearer 

 the larger drop. It follows that, if the one body of 

 water were as big as the earth and the other remained 

 of its original size, no bigger than a rain-drop the 

 motion of the large mass towards the small one would 

 be an inconceivably minute fraction of the total 



