It has now been decided that when stormy winds prevail long in one direction, another and peculiar force is given to the waves on which they operate. For the wind, by pressing long _on the side_ of a wave, changes its form from that of an upright ridge (_i.e._ with _vertical_ axis) to one which has a stoop or bend, sometimes of great inclination. Such a wave will come in upon the shore with far greater momentum, for its _velocity_ has been enormously increased, while its _bulk_ is nowise diminished. The speed which the water acquires under the influence of a prolonged sou’-wester on the coasts of Sussex and Hampshire, although there the gale has only traversed the breadth of the Channel, is very notable at times. At Lowestoft and at Peterhead, easterly points of land, the danger of the winds from the North Sea is proportionally far greater, as a larger body of water has been continuously acted upon. For it must always be remembered that a wave at sea is simply an _oscillation_ of the water which rises and falls in that place. The identical wave does not pass on to the shore, though it appears to do so; it is the _motion_ which is propagated, and, as it were, handed on, like an electric shock through the successive plates of a battery. Now this motion _increases_ with the onward career of the gale. “Vires acquirit eundo.” And when the mass of the fluid over which the wind is blowing has once been agitated, and the equilibrium of its upper layers been thoroughly disturbed, if the current of air not only continue, but wax stronger and stronger, there scarcely seems any limit to the momentum which the rolling wave may thus acquire. The arithmetic of a few facts upon this subject will, however, give us some idea of what that momentum must be on certain occasions.