Page:Popular Science Monthly Volume 23.djvu/509

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in fact, if we carefully examine a wave, we shall find that it is covered with very fine ripples, that correspond to the atmospheric vibrations. The ripples give rise to wavelets, which correspond to the undulations of the air, and are seen on the upper part of the waves. The wave proper appears to consist of a series of wavelets. A number of waves constitute a billow; a series of billows gives rise to a heavy sea (paquet de mer); a series of heavy seas produces the great swell or tidal wave of the storm.

From the nautical point of view, the ripples are of no importance, for they are seldom more than a few millimetres in diameter; but from the scientific point of view they may be considered as the origin of the swing of the liquid element, for they engender the wavelets. The last are still of no interest to the sailor, but are important in their relation to works of art, which are disintegrated by their blows, apparently insignificant, but infinitely multiplied. The wavelets are from ten to thirty centimetres in diameter and not very long. A very heavy wind breaks them up and contributes to the formation of a fine dust of salt water or salt spray, which is destructive to vegetation on exposed coasts. The wave proper may, in the English Channel, be about ten feet high, thirty feet or more broad, and eighty feet long; its proportions do not disturb large ships, but it is destructive, in the long run, to port works, and is dangerous to small craft when it breaks. We may estimate that ten waves make a billow. The first of the ten may be relatively small, but the others go on increasing to the last.

The heavy seas are the terror of sailors. They represent an enormous volume of water in motion. A gust of wind can not possibly raise up such a mass, and it can only be the result of the combined efforts of the tempest. A heavy sea may reach a height equal to twenty-five or thirty or more feet. It is massive, and strikes like a battering-ram. On the land it causes great damage, and makes breaches in works of earth and stone; at sea, it can send a trans-atlantic packet to the bottom with a single blow.

The great tidal wave is produced by two causes. On one side, it is the general resultant of the billows and the great seas; and, on the other hand, it is produced by barometrical depressions causing the waters of the ocean to rise. In cyclones, the rise of the water in the center would be neutralized by the centrifugal force, and it is therefore probable that the former cause acts alone. The tidal wave has but small amplitude, but, when inclosed by parallel coasts, it may rise to a height of several metres. It then causes inundations of low shores.

The singular fact has been remarked at Havre that in a storm the swell almost always comes after the tide. The sea rises to its normal high-water mark at the prescribed hour, and then begins to retire as usual; all at once it rises again, to a height generally much greater