You have often seen a fountain playing. How beautifully the stream rises and spreads out, dropping in a shower all around! Now why is it that the water rises? It is because the spring from which the water comes is so much higher than the pipe of the fountain. The water in the pipe tries, as we may say, to get on a level with the water in the spring. This I will make plain to you by two figures. In the first figure you see represented a vessel of water, with a pipe extending from its lower part up at its side. The water stands at the same level in the pipe that it does in the vessel, as in the case of the coffee-pot. Now suppose, as represented in the second figure, the pipe is quite short. If the vessel be filled with water, the water in the pipe, seeking to get to the same level as that in the vessel, will be thrown up in a stream, as you see. The reason that the stream spreads out and drops in a shower is, that the air resists the stream, and so divides it up, because water is so easily separated into parts.
Questions.—What is said about water in a bowl? What is said about the particles of water? Give the comparison about shot. Why will not shot run as easily as water from one vessel into another? What is said about the smallness of the particles of water? How do we know that they are round or smooth? If we could see the particles, how would water look to us? What is said about water’s being in motion? What makes it run? Tell about water in a trough. Give the comparison about a trough and a river. What is said about the power of running water? What is said about dams? Tell about the level of water in a coffee-pot. Tell about the man’s contrivance for perpetual motion. What is said about the pipes of an aqueduct? Why will water sometimes come only to the lower story of a building, and not to the upper? Tell about the playing of water from a fountain. Why does the water come down in a shower of drops?
CHAPTER XVII.
THE PRESSURE OF WATER.
The pressure of the particles of water upon each other.
Any thing that is solid presses only one way, directly down; but water or any fluid presses all ways. It presses just as much sidewise, or even upward, as it does down. The reason is, that the particles of water move about among each other, and are not fastened tight together as they are in a solid. When water freezes, its particles become all fastened together, and then the pressure is all downward.
To see how this pressure of the particles of water operates, look at some shot lying together. One shot does not lie right upon another shot below it in this way, a, but they lie in this way, b. You see that each shot presses down between those that are underneath it. Each shot is trying, as we may say, to get down between its neighbors below; and if there was nothing to prevent it, it would press them apart.
The pile of balls.
You can see that this is so by trying a little experiment. Put some shot close together on a very smooth surface. Now put another shot on top of them, and you will see that it will press them all apart. If the shot should be rough, and the surface on which you lay them should be rough also, your experiment will not succeed, because the shot will not roll easily. It is for this reason that cannon balls, as you see them piled up in an ordnance-yard, as represented in the annexed figure, do not roll away. If they were smooth, and the place which they were piled on were smooth, they would all be pressed apart, and the pile would thus be spoiled.