Page:The American Cyclopædia (1879) Volume VI.djvu/374

366 EARTHQUAKE vian peninsula, which is now proceeding at the rate on its eastern coasts of five feet per cen- tury. About one half of the islands of the Pa- cific ocean are believed to be rising, as also the island of Spitzbergen, the West Indies, and the whole of the W. coast of South America. Among the most remarkable slow depressions are the case of the southern portion of Greenland, the numerous coral-reefed islands and atolls of the Pacific, and the countries of Holland, Bel- gium, Denmark, and the S. shore of the Baltic sea. In North America the most notable well es- tablished case of depression is that now in pro- gress along the Atlantic coast from Cape Cod to Cape Hatteras, which region is calculated to be sinking at the rate of about two feet per century. Earthquake Waves. This term is commonly applied to the great oceanic waves that attend such earthquakes as originate under the ocean. These waves while in deep water are merely very long but decided swells; in traversing shoal water they become more marked, and on nearing the shores often break most disas- trously upon them, being in this case preceded by a rapid fall of the water, as is the case in ordinary breakers. The numerous cases that have been cited in a former section of this article suffice to show how prominent a feature of a disastrous earthquake are these sea waves. On account of their sudden definite origin and their vast size, these waves have on several oc- casions afforded a reasonable basis, in connec- tion with the received principles of wave mo- tion, for an approximate computation of the average depth of the sea over which they may have passed. A fine example of such a com- putation was given by Prof. Bache in the case of the waves that rolled into the harbors of Simoda, Osaka, Peel's island, San Diego, and San Francisco, in December, 1854. These waves apparently started from a point near Simoda, and on reaching the American coast were recorded on the self-registering tide gauges of the coast survey. The crest of the highest wave experienced at Simoda was 30 ft. above the average sea level at 9h. 30m. A. M., local time ; the highest at San Francisco was 0-45 of a foot above the normal height of the water, and occurred about nine hours after the wave at Simoda. From these and other data Prof. Bache concluded the mean depth of the intermediate Pacific ocean to be between 2,100 and 2,500 fathoms. The wave attending the Arica earthquake of Aug. 13, 1868, also ex- tended over the entire Pacific ocean ; from the records at four tidal stations of the United States coast survey, and the observations made at eight other points, including Australia, Hilgard (1872) concluded that the eastern equatorial portions of the Pacific are the deepest, while the depth of the northern portions seems quite small. Nature of the Earthquake Shock. The discovery of the true nature of the earthquake, to say nothing of its ultimate origin or cause, for a long time baffled the labors of mankind ; the application of a purely inductive train of reasoning has always been difficult, because of our own inability to experiment on the earth on so large a scale as was necessary, and be- cause of the absence of exact observations and records. The principal results arrived at pre- vious to or independently of Mallet's dynamical studies have been already given in the general- izations under the head of geographical distri- bution, &c. The history of the numerous hy- potheses and crude theories that have been advanced from time to time in reference to the nature of the earthquake shock may be thus briefly summarized. Up to the beginning of the 17th century no views other than the most indefinite and superstitious notions seem to have been propounded and accepted. In 1679 Travagini suggested that the successive shocks are pulses of force. Flamsteed in 1693 con- sidered that vibrating air caused the earth to tremble, as in ordinary thunder. During the 18th century Buffon and others used the term concussion in speaking of the movement of the earth ; and an anonymous French author in 1756 maintained that chains of mountains are long levers, so that a slight movement at one end will be felt as a blow or earthquake shock at the other. Since this period all students have recognized the phenomena as those of a concussion, except perhaps Michell, who in 1760 announced his theory that the surface of the earth heaved like the swell of the ocean, or even like a wave ; a view adopted in some respects in 1843 by H. D. and W. B. Eogers to explain the plication of the stra- ta of the Alleghanies. In 1807 Dr. Thomas Young suggested the probability that the mo- tion of the earth at any point is a vibratory one, and that it is propagated through the earth in a manner analogous to that of waves of sound ; but he appears to have made no clear distinction between the subterraneous waves of shock and sound. Similar views were adopted in 1823 by Gay-Lussac. In 1846 Mallet published his work on " The Dynamics of Earthquakes," in which, without specially indicating the ultimate origin of the earth- quake, he very clearly presents views that are now widely accepted as to the true nature of the phenomena observed at the surface of the earth. These views are substantially as fol- lows : An earthquake is the passage past the observer of a wave of elastic compression, in any direction from vertically upward to hori- zontally through the crust and along the sur- face of the earth, from any centre of impulse or from more than one, and which may be attended with sound and tidal waves depend- ing upon the circumstances of the original impulse. "When the wave of compression is passing through a solid stratum, each particle of the earth performs a vibratory movement similar to that made on the passage of a wave of sound, moving forward and returning in an elliptical or a more complicated curve. When the shock reaches the earth's surface the vibration of the particles at any point