EARTHQUAKES. 5 
tance each particle moves from its original position is called the 
amplitude of the wave. A familiar illustration will perhaps make 
these terms clearer. If a number of billiard balls are placed in 
a line so that they touch one another, and another ball is made 
to roll gently up and strike the first in the line, a wave of elastic 
compression will be transmitted through the line, and the last 
ball will move off. The time between the first ball being struck 
and the movement of the last one will be, when reduced to feet 
per second, the velocity of transit of the wave ; while the velocity 
of the moving ball, also reduced to feet per second, will be the 
velocity of shock. It is evident that the velocity of transit must 
always greatly exceed the velocity of shock. 
Let us now examine the effects of earthquake shocks on 
buildings. It is plain that when the velocity of transit is very 
great, the whole of the foundations of a building will practically 
move at the same time, and if the walls are firmly bound to- 
gether, but little harm will be done. This is always the case 
when an earthquake wave traverses large masses of compact 
~ rock ; but when the wave passes into beds of loose soil or shingle, 
the velocity of transit is very much reduced, and the damage 
done to buildings is proportionately increased. But the velocity 
of transit only determines whether the whole building moves to- 
gether or whether different parts of it move at different times— 
it has nothing to do with the actual movement itself. This de- 
pends upon—(1) the velocity of shock, (2) the direction of the 
shock, and (3) the amplitude of the wave. Of course, the greater 
the velocity of shock, the greater is the destructive power of an 
earthquake, and it can only be guarded against by making build- 
ings low and with light roofs. But the damage done by an earth- 
quake depends very much on the direction of the shock, whether 
it be vertical or nearly horizontal—z.e., its direction in altitude ; 
and from what point of the compass it ccmes—z.e,, its direction in 
azimuth. On the one hand, the nearer the shock approaches to 
the horizontal, the greater will be its overturning power ; while on 
the other hand, the nearer it approaches the horizontal, the greater 
will be the distance of the centre of impulse of the earthquake 
wave, and consequently the less will be the velocity of shock. 
Theoretically it can be shown that the destructive effect of an 
earthquake wave will be greatest when its angle of emergence 
from the earth is about 50deg. The direction of the wave in 
azimuth is a very important consideration, for a wall built in the 
direction of the wave would stand, while one at right angles to 
it would be overthrown. The best position for a house would be 
one in which a corner faced the direction of the wave. 
It is evident that the amplitude of the wave is another very 
important point ; for a small movement, however rapid, might 
not be so destructive as a larger movement with a less velocity. 
Now, in compact rocks, where the whole mass is bound together, 
the amplitude is small, but it is much increased when the ground 
is loose and incoherent ; consequently, earthquakes are almost 
always more severely felt on alluvial plains than on solid rock, 
