LAW OF GRAVITATION. 33 



balls and place it within an inch of the other, 

 they attract each other with a certain power. 

 If I hold it at a greater distance off, they at- 

 tract with less power, and if I hold it at a 

 greater distance still, their attraction is still less. 

 Now this fact is of the greatest consequence ; 

 for, knowing this law, philosophers have dis- 

 covered most wonderful things. You know 

 that there is a planet, Uranus, revolving round 

 the sun with us, but eighteen hundred millions 

 of miles off; and because there is another 

 planet as far off as three thousand millions of 

 miles, this law of attraction, or gravitation, still 

 holds good, and philosophers actually discovered 

 this latter planet, Neptune, by reason of the 

 effects of its attraction at this overwhelming 

 distance. Now I want you clearly to under- 

 stand what this law is. They say (and they 

 are right) that two bodies attract each other 

 inversely as the square of the distance, a sad 

 jumble of words until you understand them ; 

 but I think we shall soon comprehend what this 

 law is, and what is the meaning of the " inverse 

 square of the distance." 



I have here (fig. 11) a lamp A, shining most 

 intensely upon this disc, B, c, D ; and this light 



D 



