270 Mr D. Milne on Earthquake-Shocks in Great Britain, 

 ment, on more than a mere point of the earth's surface. Let 



'iX^r\ xf-x-^^^.. f ^^^Pv^ 



E E represent a portion of the earth's surface, to which a vi- 

 bration is transmitted from C, a point at any depth short of 

 the earth's centre. It is evident that the vibratory impulse will 

 advance in circles, just as on the surface of water when a stone 

 is dropped into it. This vibratory impulse, in its advances 

 upwards, may, at different successive stages, be represented 

 by the circular arcs A A A A, A' A' A' A' A', A'' A" A'^ A", 

 and A'" A"', &c. When the arc of vibration first reaches the 

 earth's surface, it, of course, there produces a vertical upheav- 

 ing of the outer cuticle, which resembles a wave in outline. 

 As it continues to advance, the middle portion of the arc is no 

 longer below the earth's surface ; — the undulation then divides, 

 following the broken fragments of the arc, as represented in 

 the foregoing figure. When the arc of vibration reaches the 

 position A" A" A" A", it is evident that the impulse given to 

 the earth's surface is no longer vertically upwards. It is in 

 an oblique direction, and the more so as the distance from 

 the central point increases. Thus not only the wave as- 

 sumes a different outline, but loose articles and objects will 

 be impelled in a direction more or less inclined to the hori- 

 zon. If instruments could be invented which at different 

 places would indicate, not merely the relative intensity of the 



