Now see what this sidewise pressure will do in a vessel filled with shot if there should be an opening made in the side. The shot close by the opening will run out, because they are pressed sidewise by the shot lying right above them; and as they go out, those that press them out will be pressed out in their turn by those above them, and so on.
Particles of water compared with shot.
Just so it is with the little fine balls or particles of water. They lie on each other in the same way that shot do. Each particle is pressing always to get down between the particles that are underneath it, as I have showed you it is with the shot. And if you make an opening in the vessel that holds the water, its particles will run, or rather roll out, like the shot, only a great deal easier, because they are so smooth. They are pushed out by this pressing down of each particle between those that are below it.
About water running from openings in different parts of a vessel.
If you make an opening near the top of a vessel filled with water, it does not run out with much force; but if the opening be made near the bottom, it spouts out as if it was in a great hurry to get out of the vessel. What is the reason of this difference? To understand this, observe that all the particles are pushing downward in the way that I have shown. Those particles, therefore, that are near the bottom, have a great deal more pressure on them than those that are near the top; so that when the opening is made near the bottom, the particles there are pushed out with great force. There is a large crowd of particles pushing down to get out at that opening. And observe, as the water in the vessel lessens, the force of the stream from the opening lessens; it does not leap out so straight as it did at first. It is very much as it is with a crowd pressing through a door. When the crowd is very great, those that are pushed through the door are pushed with great force; but as the crowd lessens, the pressure lessens.
It is found that water runs out of a vessel from an opening in the side close to the bottom just as quickly as it does from an opening of the same size in the bottom itself. What is the reason of this? It is because the little round particles of water roll so easily. They roll out just as easily as they drop out.
See the difference between pressing on a fluid and on a solid. If you press on a block of ice, you press it all one way. If you press it down, you press it all down. If you press it sidewise, it all moves sidewise. And it makes no difference whether your hand, or whatever you push with, covers the whole side of the block or not. But it is not so with water. If you press your hand down into a vessel of water, you press down some of the water, but not all of it. Some of it is pressed up; for, as you press down what is right under your hand, this pushes what is below it off each way to the side, and this pushes up the water that is over it. This is because the round, smooth particles roll so easily on each other. When pressure is made upon them, they roll away from it just where they can—downward, or sidewise, or upward.
Water moving in a tube.