In all its motions the water is always trying to be level; and this is the only reason that water ever runs. Water that is level will not run; it will be still. But, when you disturb this level, it will run till it finds its level again.
I will make this plain to you. Suppose that you have a trough stopped at both ends. Put some water in it as it lies on level ground. The water is level in it, and is quiet. Now raise up one end of the trough a little. The water is at once in motion. Why? Because you have disturbed the level. The water runs from the end that you raise toward the other end. Now hold the trough still a little time with the end raised, and as soon as the water gets its level again, it will be as still as it was before.
Brooks and rivers.
Suppose the trough is open at both ends, and water is running in all the time at the raised end. It will keep running toward the lower end. It will be all the time trying to get on a level, but never can. You see here the reason that water runs in a brook or river. You can think of a brook or a river as a trough with one end a little raised; and the water in it is always, as we may say, running after a level, but never finds it. The sea is to a river as a tub would be to the trough that pours its water into it.
The power of running water.
There is often great power in the water of a running stream. It works a great deal of machinery in mills of various kinds; and, if the stream be swollen with heavy rains, the water carries away bridges, houses, etc. It does all this in trying to get on a level. If it all could be made level in some way, as you see it in a bowl or a pond, it would do no such violence.
Dams.
Sometimes men build a dam across a river. This is for the purpose of turning the water off one side into a canal. The dam stops some of the water running in the river, sometimes all of it. In doing this the water is made about level just above the dam, and so is much more quiet than it is any where else in the river.
Children often build mud dams, and the water that they stop is very still because it is level. When the dams give way, how briskly the water runs to try to get on a level again.
Pouring from a coffee-pot.