EARTHQUAKES. 729 



The latter is far the most violent, as the simple impulse of 

 vibration has an additional onward progression equivalent to the 

 uplift or displacement. 



Besides these wave-movements, there are also, in most cases, 

 the very rapid wave which gives sound to the ear. The sound- 

 wave may be felt before the translation-wave, and may travel 

 farther. At the shock of St. Vincent, in 1812, sounds like thun- 

 der were heard over several thousand square miles in the Caraccas. 

 the plains of Calaboso, and on the banks of the Eio Apure. At 

 the Lima earthquake, in 1746, a subterranean noise, like a thun- 

 der-clap, was heard at Truxillo, where the earthquake did not 

 reach. 



The rate will vary with the elasticity of the rock, and somewhat, 

 also, with the elevations over the surface. 



2. Regular progression in earthquakes. — Eegular progression 

 may be a usual fact, although not generally observed. Professor 

 Eogers has shown that an earthquake, on the 4th of January, 

 1843, traversed the United States from its northwestern military 

 posts, beyond the Mississippi, to Georgia and South Carolina, along 

 an east-southeast course, Natchez lying on the southern border 

 and Iowa about the northern. The rate of travel ascertained was 

 thirty-two to thirty-four miles a minute. 



3. Phenomena attending earthquakes. — (1) Fractures of the 

 earth, sometimes of great extent; (2) subsidences or elevations 

 of extended regions, and draining of lakes ; (3) displacements of 

 loose rocks,, and, where a mass overlies another and is not attached 

 to it by its precise centre, a partial revolution, resulting from an 

 onward impulse ; (4) destruction of life in the sea, on the same 

 principle that a blow on the ice of a pond will stun or kill the fish 

 in the waters beneath ; (5) production of forced waves in the ocean; 

 (6) destruction of life on the land. Destructions of cities and of 

 human life have been too often recounted to need special illustra- 

 tion in this place. 



The elevations that take place are sometimes spoken of as 

 effects of an earthquake, although not properly so. Vibration may 

 be attended by fractures and uplifts ; but these effects result from 

 the cause that produces the shaking. 



Some of the elevations and subsidences that have attended 

 earthquakes are mentioned on page 588. 



4. Earthquake oceanic waves have been alluded to on page 

 655. One or two additional examples of their effects may here be 

 added. In 1755, accompanying the Lisbon earthquake, the sea 

 came in in a wave 40 feet high in the Tagus, 60 at Cadiz, 18 on the 



