Now we are not to suppose, that because the swell or top of the wave runs at the rate of twenty miles an hour, that therefore the current, or water itself of which the wave is composed, runs at that rate. Far from it. To conceive this motion of a wave, make a small experiment or two. Fasten one end of a cord in a window near the top of a house, and let the other end come down to the ground; take this end in your hand, and you may, by a sudden motion, occasion a wave in the cord that will run quite up to the window; but though the wave is progressive from your hand to the window, the parts of the rope do not proceed with the wave, but remain where they were, except only that kind of motion that produces the wave. So if you throw a stone into a pond of water when the surface is still and smooth, you will see a circular wave proceed from the stone as its centre, quite to the sides of the pond; but the water does not proceed with the wave, it only rises and falls to form it in the different parts of its course; and the waves that follow the first, all make use of the same water with their predecessors.
But a wave in water is not indeed in all circumstances exactly like that in a cord; for water being a fluid, and gravitating to the earth, it naturally runs from a higher place to a lower; therefore the parts of the wave in water do actually run a little both ways from its top towards its lower sides, which the parts of the wave in the cord cannot do. Thus, when it is high and standing water at Gravesend, the water twenty miles below has been running ebb, or towards the sea for an hour, or ever since it was high water there; but the water at London Bridge will run flood, or from the sea yet another hour, till it is high water, or the top of the wave arrives at that bridge, and then it will have run ebb an hour at Gravesend, &c. &c. Now this motion of the water, occasioned only by its gravity, or tendency to run from a higher place to a lower, is by no means so swift as the motion of the wave. It scarce exceeds perhaps two miles in an hour.
If it went as the wave does twenty miles an hour, no ships could ride at anchor in such a stream, nor boats row against it.
In common speech, indeed, this current of the water both ways from the top of the wave is called _the tide_; thus we say, _the tide runs strong_, _the tide runs at the rate of one, two, or three miles an hour, &c._ and when we are at a part of the river behind the top of the wave, and find the water lower than high-water mark, and running towards the sea, we say, _the tide runs ebb_; and when we are before the top of the wave, and find the water higher than low-water mark, and running from the sea, we say, _the tide runs flood_; but these expressions are only locally proper; for a tide, strictly speaking, is _one whole wave_, including all its parts higher and lower, and these waves succeed one another about twice in twenty-four hours.