Mr. Matter on the Dynamics of Earthquakes. 8 
constraining motion of the wave of fluid upon which it rests,—then it follows that 
the speed of transit of the wave of fluid below, limits that of the undulating crust 
above—that the velocity of the earth wave or shock cannot be greater than that 
of the fluid wave below. Now the latter must follow the laws of a tidal wave—ot 
the great sea wave of which we have already spoken ; its velocity, like that of the 
tidal wave of our seas, will be a function of its length, and of the depth of the fluid, 
diminished, in this case, by certain considerations as to the density and degree of 
viscidity of the.liquid ; and although it would be at present impossible, for want 
of data, to calculate the exact velocity with which this subterraneous lava wave 
could move, it may be certainly affirmed that its velocity would be immeasurably 
short of the observed or theoretic velocity of the great earth wave, or true shock, 
in earthquakes, 
Even if we suppose, as indeed would probably be the fact, that, the original 
impulse once given, there would be mutual constraint between the wave motions 
of the fluid below and those of the elastic solid crust above, so that the crust, if taken 
alone, would vibrate like a sonorous plate, while the fluid wave below it alone 
would follow the law of motion of a tidal wave, but that by their mutual reaction, 
a wave passes through both, having a common time of transit, or period of undu- 
lation, slower than that due to the vibrations of the solid elastic crush, but quicker 
than that due to the undulation of the fluid nucleus; still, the velocity of transit 
of the earth wave at the surface would be far too slow to answer the observed 
phenomena of earthquake shocks. 
The rate of ascertained progress of the great Lisbon shock, the only one, 
unfortunately, that has been observed with any pretension to accuracy, is stated 
by Michell at twenty miles per minute, or 1750 feet per second*—a speed more 
than ten times that of the attendant great sea wave, and at least twenty times as 
great as it is possible to admit the velocity of propagation of a similar wave in 
imperfectly fluid lava; and yet this velocity is probably underrated, or, if 
not, the elasticity of the earth’s crust must be greatly impaired by its increased 
* Phil. Trans. vol. li. part 2, p. 572.—Humboldt (Cosmos), gives six to seven geographical 
miles per minute as the wave period of horizontal transit, without stating how this result has been 
obtained; yet he admits the vertical wave transit to have a period of vastly greater velocity, as, for 
instance, in what he calls the ‘‘ upward explosions” at Riobambe, which projected the bodies of the 
inhabitants nearly 100 feet. It would be an important fact if future observation should prove a 
different wave period for the same formation in the vertical and horizontal directions. 
