138 EARTHQUAKES IN THE EIGHTEENTH CENTURY. [Ch XXIX 



principles bearing on this subject. With this obioot lia 



visited part of the Neapolitan territo 

 great earthquake of December^ 1857* 

 violently shaken at that period was al 



The 



N 



district convulsed in 1783. 



shortly after the 



region most 

 miles east of 

 north of the 



many 



laid in ruins^ and there was much loss of life^ the destruction 

 was by no 



means 



wrought in the river-courses were not on so grand a scale. 



To obtain the seismic vertical Mr. Mallet observed the 

 direction in ivhich chimneys^ urns, and statues had been 



the tops of high buildings. Such bodies 

 in consequence of their inertia usually fall backwards in the 



from 



direction 



om which the shock comes 



ty 



Fig. 122. 



C 



are thrown forwards. In either case they indicate the 

 direction of the shocks, and two or more such lines of 

 direction prolonged to their point of intersection give the 

 seismic vertical. That point being found, the next step is to 

 ascertain the angle at which the wave emer^jed at different 

 points at the surface. 



Suppose a rectangular building d, e,f, g (fig. 122), to stand 

 with its principal walls in the direction of the shock, and the 



earthquake-wave to emerge in 

 the direction A, C. The shock 

 will tend to produce fissures 

 h h\ i i\ at right angles to its 

 o^vn path. The inclination of 

 A, C, to the horizon, or the 

 angle of emergence, being thus 



known by reference to these 

 fissures, we obtain the position 

 of the focus A 

 the line c, h', to be prolonged 

 till it meets the vertical line 



B,A. 



By referring to the former 



A 



diagram, fig. 120, the reader ..... ^. ^..^^ ..^ ...^ ^-^ 



of emergence of the wave at any given distance from the 

 seismic vertical, B, will depend upon the depth of the focus; 



n 



( 



