64 Mr. Mauuet on the Dynamics of Earthquakes. 
have been told by officers who were present that the ground rocked sensibly 
for miles away, and the wave was felt at a distance before the sound of the 
explosion was heard. 
We are now in a position to return and explain why a body displaced, as the 
stones of the Calabrian obelisks, is not replaced, in an earthquake. 
The centre of gravity of the stone, and therefore the stone itself, in accord- 
ance with the principles we have laid down, describes an ellipse in space, and, 
from the specific elasticity of almost all rocky solids, forming the earth’s crust, 
this will be an ellipse of great eccentricity; the horizontal major axis may be, 
perhaps, a few feet; the minor vertical axis is but a few inches, or the fraction 
of an inch, because the form of the waye is very low and flat, its amplitude 
great, and its velocity immense. Now the horizontal velocity of the centre of 
gravity will be greatest at the top and bottom of the ellipse; but when approach- 
ing the top, the inertia of the mass assists in holding it fast to its bed; while on 
the back stroke, or when the centre of gravity is returning to the bottom of the 
ellipse, the inertia of the mass acts in freeing it from the friction or adhesion of 
its bed, and the mass readily becomes shifted upon it. Thus the conjoint effect 
of the upward, downward, and horizontal motion of the mass, combined with its 
inertia, is such that it is not moved from its bed by the forward motion of the 
wave, but is left behind, as it were, by its backward motion, that is, it is finally left, 
having been moved in the direction of translation of the wave. It is possible that 
some movement may take place occasionally by the forward stroke, but it can 
rarely occur that the conditions of equilibrium of the body after displacement, and 
the force, direction, &c., of the subsequent back stroke, are such as just to undo 
what the forward stroke or shock had done in way of disturbance.* 
But it is stated that occasionally bodies are thrown down in opposite direc- 
tions, for instance, walls standing north and south are prostrated both to the east 
and to the west. This and many other accidental anomalies, of course, will occur, 
but do not affect the truth of the general view. Thus a mass of masonry may be 
so conditioned as to height, weight, and connexion of parts, as to offer more 
resistance to the forward than to the back stroke, or vice versa, or may be shat- 
tered by the one and thrown down by the other. 
* The singular inversion of the pavements of towns by the Calabrian earthquake is also thus 
explained.—See Lyell, vol. i. p. 488, 2nd edit. 
