120. EAETHQUAKES IN THE EIGHTEENTH CENTURY. [Ch. XXIX 



Mr 



displacement may be due to a vibratory rather tban a wbirlinj^ 

 motion f and more lately Mr. Mallet, in the paper already 

 cited, has offered a very ingenious solution of the problem. 

 He refers the twisting simply to an elastic wave, which has 

 moved the pedestal forwards and back again, by an alternate 

 horizontal motion within narrow limits ; and he has succeeded 

 in showing that a rectilinear movement in the ground mav 

 have sufficed to cause an incumbent body to turn partiallv 

 round upon its bed, provided a certain relation exist between 

 the position of the centre of gravity of the body and its centre 



t 



movement 



emicians call the 



singularly illustrated by what the Aca( 

 'sbalzo,' or bounding into the air, to the heightof several yards, 

 of masses slightly adhering to the surface. In some towns 

 a great part of the pavement stones were thrown up, and found 

 lying with their lower sides uppermost. In these cases, we 

 must suppose that they were propelled upwards by the mo- 

 mentum which they had acquired ; and that the adhesion of 



greater than that of the other, a 



mass 



motion had been communicated 



sufficient 



When the 



moi 



ition in the air, it pitched 

 down on its edge, and fell with its lower side ujDpermost. 



Neiv fissures and changes of leveL—I shall now consider, in 



the first place, those changes which are connected with the 

 rending and Assuring of rocks or with alterations in the 

 relative level of the different parts of the land : and afterwards 



describe those whicli are more 

 the deranprement 



immediateh 



where the force of running water cooperated with that of 

 the earthquake. 



In regard to alterations of relative level, none of the 

 accounts establish that they were on a considerable scale ; 



emembered that, in proportion to the 



must 



V ^ X X — 



area moved is the difficulty of proving that the general level 



^ Journal of a Naturalist, p. 376, 

 and ii. ib. 308, 



t Proceedings Koy. Irish Acad. 1846, 

 pp. 14-16. 



*. 



