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exactly equal to its own size; but now let me take this 

 card, E, which is equal to the 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 two, which is the square of the distance, and, if I 

 put the screen at one-third of the distance from the lamp, 

 the shadow on the large screen would be nine times the size. 

 Again, if I hold this screen here, at B F, a certain amount 

 of light falls on it; and if I hold it nearer the lamp at E, 

 more light shines upon it. And you see at once how much 

 exactly the quantity which I have shut off from the part 

 of this screen, B D, now in shadow; moreover, you see that 

 if I put a single screen here, at G, by the side of the shadow, 

 it can only receive one-fourth of the proportion of light 

 which is obstructed. That, then, is what is meant by the 

 inverse of the square of the distance. This screen E is the 

 brightest because it is the nearest, and there is the whole 

 secret of this curious expression, inversely as the square of 

 the distance. Now if you can not perfectly recollect this 

 when you go home, get a candle and throw a shadow of 

 something your profile, if you like on the wall and then 

 recede or advance, and you will find that your shadow is 

 exactly in proportion to the square of the distance you are 

 off the wall ; and then, if you consider how much light shines 

 on you at one distance, and how much at another, you get 

 the inverse accordingly. So it is as regards the attraction 

 of these two balls; they attract according to the square of 

 the distance, inversely. I want you to try and remember 

 these words, and then you will be able to go into all the 

 calculations of astronomers as to the planets and other 

 bodies, and tell why they move so fast, and why they go 

 round the sun without falling into it and be prepared to 

 enter upon many other interesting inquiries of the like 

 nature. 



Let us now leave this subject which I have written upon 

 the board under the word FORCE GRAVITATION and 

 go a step farther. All bodies attract each other at sensible 



