381. But if the _attentive spectator_, instead of trusting to a vague impression, or listening to some crude theory of undulations, reflects on one of the most simple facts respecting the ebbing and flowing of the tides, he will be very little disposed to acquiesce in the above conclusion. He has only to consider, that the flowing of the tide requires just six hours, and the ebbing of it likewise six hours; so that the same body of water flows in upon the shore, and retreats from it, in the same time. The quantity of matter moved, therefore, and the velocity with which it is moved, are in both cases the same; and it remains for Mr Kirwan to show in what the difference of their force can possibly consist. The force with which the waves usually break upon our shores, does not arise from the velocity of the tide being greater in one direction than in another. In the main ocean, the waves have no progressive motion, and the columns of water alternately rise and fall, without any other than a reciprocating motion: a kind of equilibrium takes place among the undulations, and each wave being equally acted upon by those on opposite sides, remains fixed in its place. Near the shore this cannot happen; the water on the land side from its shallowness being incapable of rising to the height necessary to balance the great undulations which are without. The water runs, therefore, as it were, from a higher to a lower level, spreading itself towards the land side. This produces the breakers on our shores, and the surf of the tropical seas. A rock or a sandbank coming within a certain distance of the surface, is sufficient, in any part of the ocean, to obstruct the natural succession of undulations; and, by destroying the mutual reaction of the waves, to give them a progressive instead of a reciprocating motion. 382. It is, however, but from a small distance, that the waves are impelled against the shore with a progressive motion. The border of breakers that surrounds any coast is narrow, compared with the distance to which the _detritus_ from the land is confessedly carried; the water, while it advances at the surface, flows back at the bottom; and these contrary motions are so nearly equal, that it is but a very momentary accumulation of the water that is ever produced on any shore. If it were otherwise, and if it were true that the sea throws out every thing, and carries away nothing, we should have a constant accumulation of earth and sand along all shores whatsoever, at least wherever a stream ran into the sea. This, as is abundantly evident, is quite contrary to the fact.