136. Action of Gravity on Solids in a Liquid.—The reason that a very heavy substance, as a stone, sinks in water is simply that the earth attracts it more strongly than it does the water, and so drags the stone down through it. If the stone lay upon a bladder filled with water, it would press upon it with the force with which it is attracted by the earth. But where water is not thus confined, the stone thrusts its particles to the one side and the other till it gets to the bottom.
It is the attraction of gravity, also, that makes light substances, as wood and cork, rise in water. In this case the water is attracted by the earth more strongly than the wood or cork, and so gets below it, and in so doing pushes the lighter substance up above itself.
Fig. 87.
But you will observe that the wood, on rising in the water, does not come completely out of it and lie upon the surface, but a part of it remains immersed in the water. The explanation of this will furnish you with the key to the understanding of many very interesting facts. Suppose that half of a block of wood, A, Fig. 87, weighing a pound, is above the surface of water. As it is attracted to the earth with the force of a pound, it has pushed to the one side and the other just a pound of water, and taken its place. It is drawn down toward the earth with the same force with the pound of water on either side of it, b or c. If it were attracted any more than with the force of a pound, that is, if it weighed more than a pound, it would displace more than a pound of water. If it were of just the same weight with the same volume of water, it would displace a volume of water equal to itself; it would be wholly immersed, and would stay any where in the water, wherever you placed it, because it is attracted by the earth with the same force that the same bulk of water is.
Fig. 88.
137. Farther Explanation.—Suppose water in a vessel divided into equal portions of a pound each, as represented in Fig. 88. Now suppose that the portion a should at once change into solid ice without at all altering its bulk or weight. It would not move from its position, because it is attracted by the earth precisely as much as when it was water, and as much as is each of the equal portions of water around it. But as water on becoming ice does really increase in bulk, and therefore become lighter, this block of ice would rise so that a part of it would be above the surface.
Fig. 89.