GRAVITATION. 75 



the apple, which should bring the latter down. His ob- 

 ject was to learn what other facts were analogous to this, 

 and what other phenomena were produced in the same 

 way, expecting, when all these should be ascertained, 

 that he would be able to express the whole in general 

 terms, and deduce from them a general law. His in- 

 quiries were completely successful. He obtained the 

 following general law : 



All material objects, have, under all circumstances, a 

 tendency to approach each other. 



That this is a universal law, or in other words, that it 

 is universally true, has been proved by observation in a 

 great variety of particulars. These we shall proceed to 

 describe. 



1. GRAVITATION BETWEEN THE HEAVENLY BODIES. 



For a long time the motions of the heavenly bodies 

 defied all attempts at accurate calculation. The various 

 theories advanced would not account for the facts. The 

 variations from circular orbits, the occasional slight ir- 

 regularities in some cases returning at regular intervals, 

 and sometimes entirely unexpected, received no solution. 

 But the supposition that every material body has a ten- 

 dency to move towards every other, perfectly explains the 

 whole. Not only are the great regular motions of the 

 various primary and secondary planets about their cen- 

 tres fully explained, but all the exact forms of the curves 

 in which they move, and every irregularity in the motion 

 is fully accounted for by this simple supposition. 



The manner in which the revolutions of the planets 

 about their centres is produced, may be thus illustrated. 



Suspend by a string from the wall, a small leaden 

 weight. If left to itself, it hangs perpendicularly, and is 

 at rest. If now it is drawn up from the centre, it tends 

 to return to the centre, and will, if liberated, swing to 

 and fro like a pendulum. If, however, instead of simply 

 letting it fall, it is thrown off upon one side, it will pass 

 round in an eliptical or circular orbit. It will be very 



