Mr. Ma tet on the Dynamics of Earthquakes. 65 
But another solution is possible for such cases of prostration in opposite direc- 
tions. When the earth wave, as we may call the shock, comes from seaward, 
and travels through a mass of stratified rock, or of diluvial matter, or other 
material of low elasticity, towards an inland range of mountains, consisting of 
masses of hard, crystalline, and highly elastic rocks, part of the wave will pass 
on, and a slight earthquake will be felt throughout the mountain range; but 
a portion of the wave will be reflected, and will return back, and pass a second 
time through the country first agitated, but now in an opposite direction; and 
thus walls, or other such masses, which remained standing from the primary 
earth-wave, may be prostrated by the secondary or reflected wave, and in the 
contrary direction to those first thrown down. 
I proceed now to explain, in accordance with my theory, the circumstances 
of the great sea wave and of the aérial sound wave, attending most great earth- 
quakes. 
Michell rightly attributes the origin of the great sea wave to the primary 
disturbance of the ocean water, directly over the centre of disturbance, propagated 
in every direction, like the circles upon a pond when a pebble is dropped into it ; 
but he does not give any satisfactory account of why it is that this wave succeeds 
the earth-wave or shock by a long interval; indeed the theory of wave-motion 
was then in too backward a state to have enabled him to do so. Recent 
authors have not been even as near the truth as he was when he attributed the 
different rates at which the great sea wave moves towards several distant sur- 
rounding shores, to its true cause, namely, the variable depth of the water. 
Thus Darwin says :* “In almost every severe earthquake the neighbouring 
waters of the sea are said to have been greatly agitated. The disturbance seems 
generally, as in the case of Conception, to have been of two kinds, first, at the 
instant of the shock the water swells high up on the beach, with a gentle motion, 
and then as quickly retires; secondly, some time afterwards the whole body of 
the sea retires from the coast, and then returns in waves of overwhelming force. 
The first movement seems to be an immediate consequence of the earthquake 
affecting differently a fluid and a solid, so that their respective levels are slightly 
deranged; but the second case is a far more important phenomenon.” 
* Voyage of a Naturalist, p. 309. 
VOL. XXI. K 
