

Parr Secr. ii] EARTHQUAKES. © | - 267 
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up to severe shocks, by which houses are levelled, rocks dislocated, 
landslips precipitated, and many human lives destroyed. ‘The 
phenomena are analogous to the shock communicated to the ground 
by explosions of mines or powder-works. ‘They may be most 
intelligibly considered as wave-like undulations propagated through 
the solid crust of the earth, In Mr. Mallet’s language an earth- 
quake may be defined as “ the transit of a wave of elastic compression, 
or of a succession of these, in parallel or intersecting lines through 
the solid substance and surface of the disturbed country.” The 
passage of this wave of shock constitutes the real earthquake. 
Besides the wave of shock transmitted through the solid crust, 
waves are also propagated through the air, and, where the site of 
the impulse is not too remote, through the ccean. Harthquakes 
originating under the sea, but not far from land, are sometimes 
specially destructive in their effects. They illustrate well the three 
kinds of waves associated with the progress of an earthquake. ‘These 
are, Ist, The true earth-wave through the earth’s crust ; 2nd, a wave 
propagated through the air to which the characteristic sounds of 
rolling waggons, distant thunder, bellowing oxen, &c., are due; 35rd, 
Two sea-waves, one of which travels on the back of the earth-wave 
and reaches the land with it, producing no sensible effect on shore ; 
the other an enormous low swell, caused by the first sudden blow of 
the earth-wave, but travelling at a much slower rate, and reaching 
land often several hours after the earthquake has arrived. 
Velocity.—Experiments have been made to determine the 
_velocity of the earth-wave, and its variation with the nature of the 
material through which it is propagated. Mr. Mallet found that the 
shock produced by the explosion of gunpowder at Holyhead travelled 
at the rate per second of 951 feet in wet sand, 1283 feet in friable 
granite, and 1640 feet in solid granite. Observations of the time at 
which an earthquake has successively visited the different places on 
its track have shown similar variations in the rate of movement. 
Thus in the Calabrian earthquake of 1857, the wave of shock varied 
from 658 to 989 feet per second, the mean rate being 789 feet. The 
earthquake at Viege in 1855 was estimated to have travelled 
northwards towards Strasburg at a rate of 2861 feet per second, and 
southwards towards Turin at a rate of 1398 feet, or less than half the 
northern speed. The rate of the central European earthquake of 
1850, p. 1; 1851, p. 272; 1852, p. 1; 1858, p. 1; 1861, p. 201. “The Great Neapolitan 
’ Earthquake of 1857,” 2 vols., 1862. D. Milne, Edin. New Phil. Journ. xxxi.-xxxvi. A. 
Perrey, Mém. Couronn. Bruxelles, xviii. 1844) Comptes rendus, lii. p. 146. Otto Volger, 
«© Untersuchungen tiber die Pha:omene der Erdbeben in der Schweiz,” Gotha, 1857-8 : 
Z. Deutsch. Geol. Ges. xiii. p. 667. K. von Scebach, ‘“ Das Mitteldeutsche Erdbeben 
yon 6 Marz, 1872,” Leipzig, 1873. R. Falb, “ Grundziige einer Theorie der Erdbeben 
und Vulkanensausbriiche,”’ Graz, 1871; “Gedanken und Studien tiber den Vulkanismus, 
&e.,” 1874. Pfaif, “Allgemeine Geologie als exacte Wissenschaft,” Leipzig, 1873, p. 224. 
- Records of observed earthquakes will be found in the memoirs of Mallet and Perrey ; 
also in papers by Fuchs in Newes Jahrb. 1865-1871 and in Tschermak’s Mineralog. 
Mittheilungen, 1873 and subsequent years. Other papers are quoted in the following 
pages. : 
