If a gold leaf, or piece of thread, or any other substance, were hung where this ball is, it would swing to and fro in the same manner, and in the same time too. Do not be startled at this statement: I repeat, in the same manner and in the same time; and you will see by and by how this is. Now, that power which caused the water to descend in the balance--which made the iron weight press upon and flatten the bubble of air--which caused the swinging to and fro of the pendulum,--that power is entirely due to the attraction which there is between the falling body and the earth. Let us be slow and careful to comprehend this. It is not that the earth has any _particular_ attraction towards bodies which fall to it, but, that _all_ these bodies possess an attraction, every one towards the other. It is not that the earth has any special power which these balls themselves have not; for just as much power as the earth has to attract these two balls [dropping two ivory balls], just so much power have they in proportion to their bulks to draw themselves one to the other; and the only reason why they fall so quickly to the earth is owing to its greater size. Now, if I were to place these two balls near together, I should not be able, by the most delicate arrangement of apparatus, to make you, or myself, sensible that these balls did attract one another: and yet we know that such is the case, because, if instead of taking a small ivory ball, we take a mountain, and put a ball like this near it, we find that, owing to the vast size of the mountain, as compared with the billiard ball, the latter is drawn slightly towards it; shewing clearly that an attraction _does_ exist, just as it did between the shell-lac which I rubbed and the piece of paper which was overturned by it. Now, it is not very easy to make these things quite clear at the outset, and I must take care not to leave anything unexplained as I proceed; and, therefore, I must make you clearly understand that all bodies are attracted to the earth, or, to use a more learned term, _gravitate_. You will not mind my using this word; for when I say that this penny-piece _gravitates_, I mean nothing more nor less than that it falls towards the earth, and if not intercepted, it would go on falling, falling, until it arrived at what we call the _centre of gravity_ of the earth, which I will explain to you by and by. [Illustration: Fig. 2.]