Hence oscillations of the water-surface are set up, which go on accumulating. These waves then travel away with a speed depending upon their wave-length, and we may have great disturbances of the sea-surface at places where there is no actual storm-wind. These “echoes of a far-off storm” are known as a “ground swell.” In some localities the inhabitants are able to apprise themselves of the coming of a storm by noticing movements of the sea which indicate the arrival of waves which have travelled more quickly than the storm-centre itself. Every visitor to the seaside will have noticed occasions on which the sea is violently disturbed by waves, and yet the air in the locality is tolerably calm. In this case the waves have been propagated from some point of disturbance at a distance. A study of breaking waves shows us that the cause of their great power to effect damage to coast structures, such as piers, harbour works, and shipping in harbours, is really due to the forward motion of the water as the wave is breaking. Every cubic foot of water weighs 63¹⁄₂ lbs., so that a cubic yard of water weighs about three-quarters of a ton. If this water is moving with a speed of many feet per second in a forward direction, the energy of motion stored up in it is tremendous, and fully sufficient to account for the destructive power of storm waves on a coast. The total volume of water which is comprised in the space occupied by even one sea-storm wave of moderate dimensions may have a mass of many hundreds of tons, and its energy of motion may easily amount to that of an express train in motion. Hence when, in the last stage of its career, this mass of water is hurled forward on the shore, its destructive effects are not a matter for surprise. We must now leave the subject of waves in the open sea on a large level surface, and consider that of waves in narrow channels, such as canals or rivers. The laws which govern water-wave production in a canal can best be studied by placing some water in a long tank with glass sides. If at one end we insert a flat piece of wood and give it a push forward, we shall start what is called a _long wave_ in the tank. The characteristic of this kind of wave is that the oscillatory motion is chiefly to-and-fro, and not up-and-down. This may be very easily seen by placing some _bran_ in the water, or floating in it some glass balls which have been adjusted so as to just float anywhere in the water. When this is done, and a wave started in the tank, it runs up and down, being reflected at each end (see Fig. 13). [Illustration: FIG. 13.—Water-wave produced in a tank.]