Proceedings. ' 65 



say that the shocks pass under them at such a distance below 

 that they are unaffected by them, but that beyond or outside 

 those exempted districts the causes of the shocks rise nearer 

 to the surface of the earth, and break through the thinner 

 crust of the earth. 



Earthquake-shocks are propelled often to a great distance 

 from the centre of convulsion. Humboldt estimates that the 

 concussion travels at the rate of between twenty and thirty 

 miles a minute, losing force on its journey. The convulsion 

 is propagated very much after the mode of propagation of a 

 wave caused by throwing a stone into a lake : the nature of 

 the crust of the earth causing some variation in the direction 

 of the wave and in the force of the convulsion, the force 

 diminishing as the distance from the centre increases. 



When the distance is so great that the solid surface of the 

 earth is unaffected, the surface of the ocean and of inland 

 lakes is not unfrequently affected. Water will rise or fall 

 above or below the ordinary level. Springs are stopped or in 

 some cases burst forth with increased energy. In the case of 

 a great earthquake in Peru, in 1868, the ocean-waves pro- 

 duced by the convulsion were propagated so far as Australia 

 and New Zealand. 



Tlie earthquake which convulsed Calabria in so frightful a 

 manner did not extend its influence to a great distance from 

 its centre, but the earthquake of Lisbon was traced over a 

 space of about fifteen millions of square miles, equal to one- 

 twelfth of the area of the globe. The effect of this earthquake 

 crossed the Atlantic, and influenced several of the Windward 

 Islands of the West Indies, and Trinidad. It was also felt in 

 the North-eastern States of America, and in Canada. 



Although a subterranean noise usually precedes or accom- 

 panies a convulsion, it does not always do so. 



On the occasion of some earthquakes, longitudinal chasn.s 

 have been formed in the earth, usually of no great width, but 

 often many yards in length ; when numerous, they occur in 

 parallel lines. On an occasion of an earthquake in Italy, the 

 inhabitants, seeing this character of the chasms, felled trees 

 and threw them across the openings ; thus many persons 



