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.