Mr. Matter on the Dynamics of Earthquakes. 69 
through the solid earth. These are so far exactly the phenomena observed : the 
shock is felt, and the rolling sounds are heard at the same instant, or as nearly so 
as can be told. It may, however, occasionally happen that the rolling sounds 
shall precede considerably the actual shock, because the amount and peculiar cha- 
racter of the disturbance at the centre of impulse may be such as, by several par- 
tial disturbances, to set in motion waves of sound through the earth or sea, before 
any sufficient impulse has been given to propagate a sensible shock; and further, 
as the velocity of the sound wave through the earth will probably be from 7000 
to 10,000 feet per second, or even more, while the velocity of the sound wave 
through the sea will be about 4700 (both in round numbers), so it will generally 
occur that the sound will be heard accompanying the shock, as transmitted by 
the former medium, and still the same sound be heard some time after the shock, 
thus transmitted more slowly through the latter medium, viz., the sea. 
Occasionally waves of sound may be wholly wanting, owing to no fractures 
taking place in the earth’s crust; the elastic earth wave, or shock, in such cases, 
being due merely to compression produced by sudden flexure. 
So also, if the centre of impulse lie deep in hard, elastic primary rock, ex- 
tending beneath a country consisting superficially of soft rock, or of diluvial 
matter of very low elasticity, the sound wave will reach the ear at a very distant 
point of the surface, by passing horizontally through the elastic rock below, and 
then vertically through the small distance to the surface occupied by the softer 
and less elastic materials, thus reaching the ear by a quicker channel than if it 
passed first vertically up from the deep seat of disturbance, and then passed hori- 
zontally through the superficial deposits ; but the latter course is that which the 
great earth wave or shock must take to be felt horizontally at the surface ; hence, 
the sound must be heard, in such circumstances, before the shock ; and while the 
sound, occasionally accompanied by a slight vertical shock, will appear to come up 
directly from under the feet of the hearers, the principal shock will be felt late- 
rally as coming from a distance—a fact actually recorded by Dolomieu in his 
Dissertation on the Calabrian Earthquake of 1783, where, on the granite moun- 
tains, the sound was heard before the shock was felt; but in the great diluvial 
plain the shock was felt before the sound was heard. 
In similar conditions, the great earth wave itself may, when transmitted 
trom profound depths, be diverted by difference as to elasticity of formation, from 
