CH. XVIII. THE LAW OF GRAVITATION. 151 



Imagine his satisfaction when it came out perfectly right ! 

 It is said that he was so agitated when he saw that it was 

 going to succeed, that he was obHged to ask a friend to 

 finish working out the calculation for him. His patience was 

 rewarded ; the attraction of the earth exactly agreed with the 

 rate of movement of the moon, and he knew now that he 

 had discovered the law which governed the motions of the 

 heavenly bodies. 



This law of Newton's is called the * Law of Gravitation^ 

 and we must now try to understand what it is. Gravitation 

 means the drawing of one thing towards another, or towards 

 a centre. All the objects upon our earth are held there by 

 gravity, which pulls or attracts them towards the centre of 

 the earth. If there were no such thing as gravity there would 

 be nothing to prevent our chairs and tables, and even our- 

 selves, from flying into space ; but they are all held to the 

 earth by gravity, and if you dig a hole under them they 

 fall directly nearer to the centre. 



Now let us see how this attraction of gravitation affects 

 the planets. Every one of the bodies in the heavens pulls or 

 attracts all the other bodies, just in the same way as the earth 

 attracts the apple on the tree. But as they are all moving 

 rapidly along (like the ball swung round your head) they do 

 not fall into each other, but the smaller bodies move round 

 the larger ones which are near them, just as if they were 

 fastened to them by invisible elastic threads. The smaller 

 ones move round the larger one, because it is not only 

 each body as a whole which pulls the other bodies, but 

 every tiny atom of matter in each planet is pulling at all 

 the atoms in all the other planeti ; so that the bigger a 

 body is, and the more atoms it has in it, the more it will 

 draw other bodies towards it. Our sun pulls the planets. 



