WHY AN APPLE FALLS.
“Papa,” said Lucy, “I have been reading to-day, that Sir Isaac Newton was led to make some of his great discoveries by seeing an apple fall from a tree. What was there extraordinary in that?”
Papa.There was nothing extraordinary; but it happened to catch his attention, and set him to thinking.
Lucy.And what did he think about?
Pa.He thought by what means the apple was brought to the ground.
Lu.Why, I could have told him that—because the stalk gave way, and there was nothing to support it.
Pa.And what then?
Lu.Why, then it must fall, you know.
Pa.But whymustit fall?—that is the point.
Lu.Because it could not help it.
Pa.But why could it not help it?
Lu.I don’t know—that is an odd question. Because there was nothing to keep it up.
Pa.Suppose there was not—does it follow that it must come to the ground?
Lu.Yes, surely!
Pa.Is an apple animate or inanimate?
Lu.Inanimate, to be sure!
Pa.And can inanimate things move of themselves?
Lu.No—I think not—but the apple falls because it is forced to fall.
Pa.Right! Some force out of itself acts upon it, otherwise it would remain for ever where it was, notwithstanding it were loosened from the tree.
Lu.Would it?
Pa.Undoubtedly! for there only two ways in which it could be moved; by its own power of motion, or the power of something else moving it. Now the first you acknowledge it has not; the cause of its motion must therefore be the second. And what that is was the subject of the philosopher’s inquiry.
Lu.But everything falls to the ground as well as an apple, when there is nothing to keep it up.
Pa.True—there must therefore be a universal cause of this tendency to fall.
Lu.And what is it?
Pa.Why, if things out of the earth cannot move themselves to it, there can be no other cause of their coming together than that the earth pulls them.
Lu.But the earth is no more animate than they are: so how can it pull?
Pa.Well objected! This will bring us to the point. Sir Isaac Newton, after deep meditation, discovered, that there was a law in nature calledattraction, by virtue of which every particle of matter, that is, everything of which the world is composed, draws toward it every other particle of matter, with a force proportioned to its size and distance. Laytwo marbles on the table. They have a tendency to come together, and if there were nothing else in the world they would come together, but they are also attracted by the table, by the ground, and by everything besides in the room; and these different attractions pull against each other. Now, the globe of the earth is a prodigious mass of matter, to which nothing near it can bear any comparison. It draws, therefore, with mighty force, everything within its reach, which is the cause of their falling: and this is called thegravitationof bodies, or what gives themweight. When I lift anything, I act contrary to this force, for which reason it seemsheavyto me, and the heavier the more matter it contains, since that increases the attraction of the earth for it. Do you understand this?
Lu.I think I do. It is like a loadstone drawing a needle.
Pa.Yes; that is an attraction, but of a particular kind, only taking place between the magnet and iron. But gravitation, or the attraction of the earth, acts upon everything alike.
Lu.Then it is pulling you and me at this moment.
Pa.It is.
Lu.But why do not we stick to the ground, then?
Pa.Because, as we are alive, we have a power of self-motion, which can, to a certain degree, overcome the attraction of the earth. But the reason you cannot jump a mile high as well as a foot, is this attraction, which limits the force of your jump, and brings you down again after that force is spent.
Lu.I think, then, I begin to understand what I have heard of people living on the other side of the world. I believe they are calledantipodes, who have their feet turned toward ours, and their heads in the air. I used to wonder how it could be that they did not fall off; but I suppose the earth pulls them to it.
Pa.Very true. And whither should they fall? What have they over their heads?
Lu.I don’t know; sky, I suppose.
Pa.They have. This earth is a vast ball, hung in the air, and continually spinning round, and that is the cause why the sun and stars seem to rise and set. At noon we have the sun over our heads, when the antipodes have the stars over theirs; and at midnight the stars are over our heads, and the sun over theirs. So whither should they fall to more than we?—to the stars or the sun?
Lu.But we are up, and they are down.
Pa.What is up, butfromthe earth andtowardthe sky? Their feet touch the earth, and their heads point to the sky, as well as ours; and we are under their feet, as much as they are under ours. If a hole were dug quite through the earth, what would you see through it?
Lu.Sky, with the sun or the stars; and now I see the whole matter plainly. But pray what supports the earth in the air?
Pa.Why, whither should it go?
Lu.I don’t know—I suppose where there was most to draw it. I have heard that the sun is a great many times bigger than the earth. Would it not go to that?
Pa.You have thought very justly on the matter, I perceive. But I shall take another opportunity of showing you how this is, and why the earth does not fall into the sun, of which, I confess, there seems to be some danger. Meanwhile, think how far the falling of an apple has carried us?
Lu.To the antipodes, and I know not where.
Pa.You may see thence what use may be made of the commonest fact by a thinking mind.