To give an idea of the propagation of the waves or billows of the sea agitated by the wind, they have been compared to a field of corn under the action of the air. It seems as if the ears of corn had an impulsive swiftness, which however does not exist, since they do not quit their places. Farther, if the wind is feeble, it is only the ears which waver without the stalks being shaken; but, in proportion as the wind rises, the stalks take part in the movement to a greater and greater depth down to the root. The waves have been again compared to the movements of a cord, which is made to undulate by shaking one of its extremities in the hand. It seems as if the cord was going at a rapid rate, while in reality it does not quit the hand that shakes it, only each point of it rises and falls alternately, and this movement is the greater according as the impulse is stronger; if the extremity of the cord opposite to that which receives this impulse encounters an obstacle, as the surface of a wall for instance, it will strike it at each movement of the hand. It is exactly the same with the waves of the sea; every fluid molecule placed at the surface of the billow experiences an oscillatory movement nearly vertical, so that if a body floating on the surface of a wave is watched it will be seen to remain in the same place, sometimes in the hollow of the wave, sometimes on its summit, and if at length it changes its place, that depends upon other circumstances, such as the force of the wind or the direction of the currents. This oscillatory movement which is perceived on the surface of the sea, is necessarily developed to a certain depth, which will be greater in proportion as the undulations are stronger at the surface. This fact has been confirmed by experiment; it has been ascertained in effect that the agitation of the sea caused by the wind, is communicated to a certain depth, variable according to the wind, according to the sea, and the places where the observations were made, and that beyond that depth the sea is perfectly calm. Thus it may be admitted as an observed and well proved fact, that the waves require a certain depth for their free developement; if an obstacle is presented to this developement, there will be a forcible re-action of the wave against this obstacle which will be carried off, if it is moveable, and will enter into the system of the wave. This action of the waves against the deeps is what is called the _ground swell_.