56 Mr. Matter on the Dynamics of Earthquakes. 
cut stone, but may be increased to any amount by the intervention of cement. 
The stone, however, is possessed of weight, and therefore of inertia, that is to 
say, being at rest, its whole mass cannot be instantly brought into motion by the 
plane on which it rests; and if the amount of adhesion between the stone and its 
bed be less than the inertia due to any given velocity of horizontal movement of 
the bed, the bed will move more or less from under the stone, or the stone will 
appear to move in a contrary direction to that of the motion of its bed. 
Now the inertia of the stone, which is here the resisting force, may be con- 
sidered to act at its centre of gravity. The impelling force is the grasp of the 
stone which its bed holds of it by friction or adhesion, and this may also be re- 
ferred to some one point in the surfaces of contact, which we may call the centre 
of adherence. 
If, then, a stone, or other solid, rest upon a plane which is suddenly moved 
with sufficient velocity to produce motion in the incumbent body, three several 
conditions of motion of the body may occur, according to the respective positions 
of its centre of gravity and of the centre of adherence. 
Ist. The centre of gravity of the body may be at such a height above the 
base that it shall upset by its own inertia. This is the case with houses, towers, 
walls, &c. &c., when they fall by earthquakes, accompanied, however, by disloca- 
tion of their parts. 
2nd. The centre of adherence may be ina point of the base plumb under 
the centre of gravity of the body, or in a vertical plane passing through its 
centre of gravity, and in the direction of motion of the base. 
In this case the stone will appear to move in the opposite direction to that in 
which the base has moved, that is to say, the body may have acquired more or 
less the direction of motion of the base, according as the motion of the latter has 
been of longer or shorter continuance, or less or more rapid; but in so far as the 
movement in opposite directions has taken place, the base in reality has slipped 
from under the body. 
3rd. The centre of adherence may neither be plumb under the centre of gra- 
vity of the body, nor in the plane of motion passing through its centre of gravity, 
but in some point of the base outside the line of its intersection by the plane; in 
which case the effect of the rectilmear motion in the plane of the base will be to 
twist the body round upon its bed, or to move it laterally, and twist it at the 
