' 
ON THE FACTS AND THEORY OF EARTHQUAKE PHENOMENA. 101: 
to overthrow buildings, &c. depends not only upon its intensity, but upon 
the direction of its movement with respect to the horizon. A shock per- 
fectly vertical has no tendency to overturn the walls of a house, though it 
may bring down the roof or floors. Now it is obvious from the figure, 
that as the wave passes outwards from the origin, A, it reaches the earth’s 
surface vertically at B, the point in the prime vertical, pA, directly over 
the same; and that as it travels outwards, it emerges at the surface with 
angles more and more nearly horizontal; the angle of emergence being the. 
same at all points of any coseismal line, all such lines being, on the as- 
sumption of homogeneity, concentric circles round B (like those upon a 
pond into which a stone has been thrown). 
So far as the direction of wave-motion is concerned, therefore, its power to 
overturn buildings is greater the further it has travelled, or the greater the 
radius of the coseismal cirele from B ; but its‘erergy has been shown to be 
inversely as the square of the distance (not upon the earth’s surface, but in 
the normal). Hence it follows that there must be some given distance upon 
the surface around B at which the combined effect, of most advantageous 
direction and lessened energy, shall produce the most destructive effects 
upon buildings, &c., or a point, C, intermediate to B and Z, or Z! supposed at: 
any indefinite distance, at which the shock will be, in this respect, a maximum. 
The radius BC will then describe a coseismal circle upon the earth’s sur- 
face, which will be a zone of maximum disturbance. 
Conversely, if we can trace by observation of the shaken country such a 
zone, or ascertain three points in its circle, we can find the centre of the 
circle or the point B, which is plumb over the centre of impulse beneath ; 
andif we have ascertained the angle of emergence that produces the maximum 
effect (and which isa constant), wé*can then calculate the depth of the centre 
of impulse, A, beneath the earth’s surface. 
Fig. 8. 
pP 
A 
Beeieferring to fig. 8, let A be, as before, the centre of impulse; B the 
int upon the earth’s surface (supposed a plane), in the prime vertical pA, 
directly above it. It is required to find a point, C, at which the horizontal 
“overthrowing effects of an impulse in the direction AC, whose intensity 
Varies inversely as the square of the distance, shall be a maximum. 
_ Produce AC to d, and complete the parallelogram of forces, f d being 
parallel to the horizon. 
