I 4 8 SEVENTEENTH CENTURY. FT. in. 



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. 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 ourselves, from flying into space 

 at the slightest impulse ; but they are all held to the earth by 

 gravity, and if any object could pass freely through the ground, 

 it would be drawn down with increasing velocity till it reached 

 the earth's centre of gravity. Then its velocity would decrease, 

 till, on reaching the opposite surface, it would be drawn again 

 towards the centre, and move thus to and fro, unless friction 

 at last brought it to rest at the earth's centre of gravity. 



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 planets ; so 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, and the 

 planets pull the sun ; but our sun has 700 times more atoms 



