RATE AT WHICH WAVES TRAVEL.
Professor Bache states, as one of the effects of an earthquake at Simoda, on the island of Niphon, in Japan, that the harbour was first emptied of water, and then came in an enormous wave, which again receded and left the harbour dry. This occurred several times. The United-States self-acting tide-gauge at San Francisco, which records the rise of the tide upon cylinders turned by clocks, showed that at San Francisco, 4800 miles from the scene of the earthquake, the first wave arrived twelve hours and sixteen minutes after it had receded from the harbour of Simoda. It had travelled across the broad bosom of the Pacific Ocean at the rate of six miles and a half a minute, and arrived on the shores of California: the first wave being seven-tenths of a foot in height, and lasting for about half an hour, followed by seven lesser waves, at intervals of half an hour each.
The velocity with which a wave travels depends on the depth of the ocean. The latest calculations for the Pacific Ocean give a depth of from 14,000 to 18,000 fathoms. It is remarkable how the estimates of the ocean’s depth have grown less. Laplace assumed it at ten miles, Whewell at 3·5, while the above estimate brings it down to two miles.
Mr. Findlay states, that the dynamic force exerted by Sea-Waves is greatest at the crest of the wave before it breaks; and its power in raising itself is measured by various facts. At Wasburg, in Norway, in 1820, it rose 400 feet; and on the coast of Cornwall, in 1843, 300 feet. The author shows that waves have sometimes raised a column of water equivalent to a pressure of from three to five tons the square foot. He also proves that the velocity of the waves depends on their length, and that waves of from 300 to 400 feet in length from crest to crest travel from twenty to twenty-seven and a half miles an hour. Waves travel great distances, and are often raised by distant hurricanes, having been felt simultaneously at St. Helena and Ascension, though 600 miles apart; and it is probable that ground-swells often originate at the Cape of Good Hope, 3000 miles distant. Dr. Scoresby found the travelling rate of the Atlantic waves to be 32·67 English statute miles per hour.
In the winter of 1856, a heavy ground-swell, brought on by five hours’ gale, scoured away in fourteen hours 3,900,000 tons of pebbles from the coast near Dover; but in three days, without any shift of wind, upwards of 3,000,000 tons were thrown back again. These figures are to a certain extent conjectural; but the quantities have been derived from careful measurement of the profile of the beach.