152 



EAETHQUAKE OE LISBON. 



[Ch. XXX. 



ocean, have never been inundated, thougli severely shaken 

 bj earthquakes/"^ 



More recently (February, 1846), Mr. Mallet, in his memoir 

 above cited (p. 137), has endeavoured to bring to bear on this 

 difficult subject the more advanced knowledge obtained of 

 late years respecting the true theory of waves. He conceives 

 that when the origin of the shock is beneath the deep ocean 

 one wave is propagated through the land, and another 

 moving with inferior velocity is formed on the surface of the 

 ocean. This last rolls in upon the land long after the earth- 

 wave has arrived and spent itself. However irreconcilable 

 it may be to our common notions of solid bodies, to imao*ine 

 them capable of transmitting, with such extreme velocity 

 motions analogous to tidal waves, it seems nevertheless cer- 

 tain that sucli "undulations are produced, and it is supposed 

 that when the shock passes a given point, each particle of 

 the solid earth describes an ellipse in sj)ace. The facility 

 with which all the particles of a solid mass can be made to 

 vibrate maybe illustrated, says Gay-Lussac, by many familiar 

 examples. If we apply the ear to one end of a long wooden 

 beam, and listen attentively when the other end is struck by 

 a pin's head, we hear the shock distinctly ; which shows that 

 every fibre throughout the whole length has been made to 

 vibrate. The rattling of carriages on the pavement shakes 

 the largest edifices ; and in the quarries underneath some 

 quarters in Paris, it is found that the movement is communi- 

 cated through a considerable thickness of rock.f 



The great sea-wave originating directly over the centre of 

 disturbance is propagated, as Michell correctly stated, in 

 every direction, like the circle upon a pond when a pebble is 

 dropped into it, the different rates at which it moves depend- 

 ing (as he also suggested) on variations in the depth of the 

 water. This wave of the sea, says Mr. Mallet, is raised by 

 the impulse of the shock immediately below it, which in 

 great earthquakes lifts up the ground 2 or 3 feet perpendicu- 

 larly. The velocity of the shock, or earth-wave, is greater 



* Darwiu's Travels in So\itli Ame- 



rica, &c. 1832 to 18S6. Voyage of p. 428. 

 H.M.S, Boagle, vol. iii. p, 377. 



t Ann. de. Cli. et de Ph. torn. xxii. 





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