THE CENTRE OF GRAVITY. 15 



myself, sensible that these balls did attract one 

 another ; and yet we know that such is the case, 

 because if, instead of taking a small ivory ball, 

 we take a mountain, and put a ball like this 

 near it, we find that, owing to the vast size of 

 the mountain, as compared with the billiard 

 ball, the latter is drawn slightly towards it ; 

 showing clearly that an attraction does exist, 

 just as it did between the shell-lac which I 

 rubbed and the piece of paper which was over- 

 turned by it. 



Now, it is not very easy to make these things 

 quite clear at the outset, and I must take care 

 not to leave anything unexplained as I proceed, 

 and, therefore, I must make' you clearly under- 

 stand that all bodies are attracted to the earth, 

 or, to use a more learned term, gravitate. You 

 will not mind my using this word, for when I 

 say that this penny-piece gravitates, I mean 

 nothing more nor less than that it falls towards 

 the earth, and if not intercepted, it would go on 

 falling, falling, until it arrived at what we call 

 the centre of gravity of the earth, which I will 

 explain to you by and by. 



I want you to understand that this property 

 of gravitation is never lost, that every substance 



