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 further. All bodies attract each other at sensible distances. I shewed you the electric attraction on the last occasion (though I did not call it so); that attracts at a distance: and in order to make our progress a little more gradual, suppose I take a few iron particles [dropping some small fragments of iron on the table]. There, I have already told you that in all cases where bodies fall, it is the _particles_ that are attracted. You may consider these then as separate particles magnified, so as to be evident to your sight; they are loose from each other--they all gravitate--they all fall to the earth--for the force of gravitation _never_ fails. Now, I have here a centre of power which I will not name at present, and when these particles are placed upon it, see what an attraction they have for each other. [Illustration: Fig. 12.]