At the equator the tide-wave follows the moon at the rate of 1000 miles an hour; it moves very slowly in the northern seas on account of the shallowness of the water; but the tides are so retarded by the form of the coasts and irregularities of the bottom of the sea, that a tide is sometimes impeded by an obstacle till a second tide reaches the same point by a different course, and the water rises to double the height it would otherwise have attained. A complete extinction of the tide takes place when a high-water interferes in the same manner with a low-water, as in the centre of the German Ocean—a circumstance predicted by theory, and confirmed by Captain Hewett, who was not aware that such an interference existed. When two unequal tides of contrary phases meet, the greater overpowers the lesser, and the resulting height is equal to their difference; such varieties occur chiefly in channels among islands and at the estuaries of rivers. When the tide flows suddenly up a river encumbered with shoals, it checks the descent of the stream; the water spreads over the sands, and a high crested wave, called a bore, is driven with force up the channel. This occurs in the Ganges; in the Amazon, at the equinoxes, where, during three successive days, five of these destructive waves, from 12 to 15 feet high, follow one another up that river daily; and in a lesser degree in some of our own rivers. There may be some small flow of _stream_ with the oceanic tide; but that does not necessarily follow, since the tide in the open ocean is merely an alternate rise and fall of the surface: so that the wave, not the stream, follows the moon. A bird resting on the sea is not carried forward as the waves rise and fall; indeed, if so heavy a body as water were to move at the rate of 1000 miles in an hour, it would cause universal destruction, since in the most violent hurricanes the velocity of the wind hardly exceeds 100 miles an hour. During the passage of the great tidal wave in deep water, the particles of the fluid glide for the moment over each other into a new arrangement, and then retire to their places; but this motion is extremely limited and momentary. Over shallows, however, and near the land, both the water and the waves advance during the flow of the tide, and roll on the beach.[111]