34 



GRAVITATION. 



acts as a sun by which 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 exactly equal to its own size ; but now 

 let me take this card E, which is equal to the 



Fig. 11. 



other one in size, and place it- midway between 

 the lamp and the screen ; now look at the size 

 of the shadow B D, it is four times the original 

 size. Here, then, comes the " inverse square of 

 the distance." This distance, A E, is one, and 

 that distance, A B, is two ; but that size E being 

 one, this size B D of shadow is four instead of 



