It is necessary to have a clear and distinct conception of the difference between the motion of a wave and that of a current.  In the current there is a transfer of water; in the wave the transfer is no more than would be brought about by a particle of water impinging on another where that particle has a motion perpendicular to the surface, and a rising and falling results.  The onward movement of the wave itself is always perceptible enough.  That the water is not moving with the same velocity is also evident from watching the progress of any light body floating on its surface.  This fact may be practically illustrated in the case of a ship at sea, sailing before the wind in the same direction as the waves are moving.  When the crest of a wave is near the stern, drop a piece of wood on it.  Almost instantly the wave will be seen shooting ahead of the vessel, while the wood is scarcely removed from the position where it fell on the water.  The wave has moved onward, preserving its identity as a wave, the water of which it is formed being constantly changed; and thus the motion of the wave is one thing, that of the water in which the waves are formed is quite another thing.

Again, waves are formed by a force acting horizontally; but in the case of the tide wave, that force acts uniformly from the surface to the lowest depths of the ocean, and the breadth of the wave is that curved surface which, commencing at low water, passes over the summit of the tide down to the next low water—­this is a wave of the first order.  In waves of the second order, the force raising them acts only on the surface, and there the effect is greatest (as in the wind waves)—­where one assists in giving to the water oscillating motion which maintains the next, and gradually puts the whole surface in commotion; but at a short distance down that effect entirely disappears.

If the earth presented a uniform globe, with a belt of sea of great and uniform depth encircling it round the equator, the tide wave would be perfectly regular and uniform.  Its velocity, where the water was deep and free to follow the two luminaries, would be 1,000 miles an hour, and the height of tide inconsiderable.  But even the Atlantic is not broad enough for the formation of a powerful tide wave.  The continents, the variation in the direction of the coast line, the different depths of the ocean, the narrowness of channels, all interfere to modify it.  At first it is affected with only a slight current motion toward the west—­a motion which only acquires strength when the wave is heaped up, as it were, by obstacles to its progress, as happens to it over the shallow parts of the sea, on the coasts, in gulfs, and in the mouths of rivers.  Thus the first wave advancing meets in its course with resistance on the two sides of a narrow channel, it is forced to rise by the pressure of the following waves, whose motion is not at all retarded, or certainly less so than that of the first wave.  Thus an actual current of water is produced in straits and narrow channels; and it is always important to distinguish between the tide wave, as bringing high water, and the tidal stream—­between the rise and fall of the tide and the flow and ebb.