[CHAPTER V.]
GRAVITATION.
73. Attraction Between Masses.—I have thus far treated of attraction as existing between the atoms or particles of matter when they are brought very near together, which is called the attraction of cohesion. But it exists also between any portions of matter that are separate from each other. Thus if two cork balls be placed on the surface of water near to each other, their attraction will soon bring them together. To have the experiment striking, the balls must be varnished, that they may glide easily over the water. Bubbles of glass will exhibit the same attraction. So, also, floating pieces of wood are apt to be found together; and when a ship is wrecked, as soon as the sea becomes calm the parts of the wreck are in collections here and there. Now when a stone falls to the ground, it does so for precisely the same reason that the two cork balls come together in the water. The idea of all who have not been informed on such subjects is, that the stone comes to the ground because the ground is down and the stone is up, and there is nothing to support the stone in the air. They have no idea that some power makes the stone come down. There is such a power, and it is the attraction which the earth and the stone have for each other. If you hold the stone in your hand, and thus prevent its falling, you simply resist a power which is pulling it down. If you could in any way suspend the attraction of the earth and the stone for each other, you could let go of the stone, and it would remain just there in the air, and would not come down until the attraction is restored.
74. Attraction Mutual.—The cork balls move toward each other because their attraction is mutual. So do the earth and stone really move toward each other for the same reason. As the stone is drawn toward the earth, so is the earth drawn toward the stone. But the earth is so large a thing to be drawn that its motion is exceedingly small—so small that practically it may be considered as nothing.
75. Illustration.—This may be clearly illustrated if we compare the force of attraction to the force of muscular action. Suppose a man in a boat pulls on a rope which is made fast to a ship lying loose at the wharf, and in this way draws his boat toward it. He does not dream that he moves the ship at all; but he in reality does, for if instead of one boat a hundred or more pull upon the ship, they will move it so much as to make the motion apparent. In the case of the single boat, the ship as really moves as when a hundred boats are pulling on it, but it is only the one hundredth part as much. Now let the ship represent the earth, and the little boat some body, as a stone, attracted by it. The earth and the stone move toward each other, just as the ship and the boat do. And if, as we multiplied the number of boats, we should multiply the bulk of the stone till it is of an immense size, it would have by its attraction a perceptible influence upon the earth's motion.
Observe in regard to the illustration, that it makes no difference whether the man be in the boat or in the ship as he pulls. In either case he exerts an equal force on the ship and the boat, making them to approach each other. So it is with the attraction between the earth and the stone. It is a force exerted equally upon both. Its effect on the earth is not manifest, because it is so much larger than the stone; just as the effect of the man's pulling is not manifest upon the ship, because it is so much larger than the boat.
76. Proportion of the Mutual Motions of Attraction.—Let us pursue the illustration a little farther. If a man stand in a boat, and pull a rope made fast to another boat of the same size and weight, both boats, in coming together, will move over the same space. Just so it is with the attraction between two bodies having the same quantities of matter or equal weights—they attract each other equally, and therefore meet each other half way. Let now one boat be ten times as great and as heavy as the other. The small boat would move ten times as much as the large one when the man brings them together by pulling the rope. In like manner, if a body one-tenth as large as the earth should approach it, they would attract each other, but in coming together the body would move ten times as far as the earth would. In the case of falling bodies, even though they may be of great size, the earth moves so slightly to meet them that its motion is wholly imperceptible. It has been calculated that if a ball of earth the tenth part of a mile in diameter were placed at the distance of a tenth part of a mile from the earth, as the earth and this body would be moved by their attraction to meet each other, the motion of the earth would be only the eighty thousandth of a millionth ( 1 80,000,000,000 ) of an inch.
77. Attraction Universal.—The attraction of which I have been speaking exists between all bodies, however distant they may be from each other. Sun, earth, moon, and stars attract each other; and in obedience to this attraction they have a tendency to come together in one great mass, and would do so if another force acting in opposition to this did not prevent it. This force will be treated of in another part of this book.
78. The Tides.—One effect of the attraction between the earth and the moon is quite familiar. I refer to the tides. When the tide rises it is because the water of the ocean feels the attracting force of the moon. The moon actually lifts the water toward itself. The attraction of the sun sometimes increases and sometimes diminishes the tides, according to its position in relation to the moon and the earth. If the land were as movable as the water, or, in other words, if its particles were held together by no stronger attraction than those of water, there would be the same motion that there is in the ocean over the surface of the earth, as in its revolution successive portions of it present themselves toward the moon.
79. Meaning of the Word Gravitation.—The attraction thus existing between different bodies of matter separated from each other is called the attraction of gravitation or gravity, in distinction from the attraction of cohesion treated of in the previous chapter. This name was given to it because we have such common examples of its influence in the fall of bodies to the earth. They are said to gravitate toward the earth. And they are said to do so by the force or attraction of gravitation or gravity. The term terrestrial gravitation is sometimes used in speaking of the earth's attraction, in distinction from the same thing in operation in other planets.
