22 FARADAY 



out telling you something about its laws and regularity; 

 and, first as regards its power with respect to the distance 

 that bodies are apart. If I take one of these 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 

 attract 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 discovered 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, Nep- 

 tune, by reason of the effects of its attraction at this over- 

 whelming distance. Now I want you clearly to understand 

 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. n) a lamp, A, shining most intensely 

 upon this disc, B, C, D, and this light acts as a sun by which 



FIG. 



I can get a shadow from this little screen B F (merely a 

 square piece of card), which, as you know, when I place it 

 close to the large screen, just shadows as much of it as is 



