120 Things not generally Known. 



progress, and their propagation, through rocks of different 

 density and elasticity ; and the causes of the rapidity of pro- 

 pagation, and its diminution by the refraction, reflection, and 

 interference of the oscillations have been mathematically in- 

 vestigated. Air, water, and earth waves follow the same laws 

 which are recognised by the theory of motion, at all events in. 

 space ; but the earth- waves are accompanied in their destruc- 

 tive action by discharges of elastic vapours, and of gases, and 

 mixtures of pyroxene crystals, carbon, and infusorial animal- 

 cules with silicious shields. The more terrific effects are, how- 

 ever, when the earth-waves are accompanied by cleavage ; and, 

 as in the earthquake of Riobamba, when fissures alternately 

 opened and closed again, so that men saved themselves by ex- 

 tending both arms, in order to prevent their sinking. 



As a remarkable example of the closing of a fissure, Hum- 

 boldt mentions that, during the celebrated earthquake in 1851, 

 in the Neapolitan province of Basilicata, a hen was found caught 

 by both feet in the street-pavement of Barile, near Melfi. 



Mr. Hopkins has very correctly shown theoretically that 

 the fissures produced by earthquakes are very instructive as 

 regards the formation of veins and the phenomenon of disloca- 

 tion, the more recent vein displacing the older formation. 



RUMBLINGS OF EARTHQUAKES. 



When the great earthquake of Coseguina, in Nicaragua, 

 took place, January 23, 1835, the subterranean noise the 

 sonorous waves in the earth was heard at the same time on 

 the island of Jamaica and on the plateau of Bogota, 8740 feet 

 above the sea, at a greater distance than from Algiers to Lon- 

 don. In the eruptions of the volcano on the island of St. 

 Vincent, April 30, 1812, at 2 A.M., a noise like the report of 

 cannons was heard, without any sensible concussion of the earth, 

 over a space of 160,000 geographical square miles. There have 

 also been heard subterranean thunderiugs for two years without 

 earthquakes. 



HOW TO MEASURE AN EARTHQUAKE-SHOCK. 



A new instrument (the Seismometer) invented for this pur- 

 pose by M. Kreil, of Vienna, consists of a pendulum oscillating 

 in every direction, but unable to turn round on its point of 

 suspension ; and bearing at its extremity a cylinder, which, by 

 means of mechanism within it, turns on its vertical axis once 

 in twenty-four hours. Next to the pendulum stands a rod bear- 

 ing a narrow elastic arm, which slightly presses the extremity 

 of a lead-pencil against the surface of the cylinder. As long as 

 the pendulum is quiet, the pencil traces an uninterrupted line 



