4 Alexis Perrey on Earthquakes. 
relatively to the earth’s orbital motion, they are more frequent 
at the winter solstice ea! at the summer, that is, at the peri- 
helion than at the aphelio 
these waves are, siaiale cally, not single waves, but are 
groups of successive undulations, like the tidal in the ocean. 
ence there must be a oe, of pressures and tensions in 
the passage of a seismic wave over a given point. Hence, also, 
a possible, and probable, succession in the vibrations of the crust. 
Hence, also, an undulatory character in the earthquake shocks, 
with alternations of intensity during their passage. 
Thus e have regarded the crust as eee interiorly an 
ellipsoidal Hariice, and the central nucleus as li 
Let us now suppose the nucleus the same, but het inner surface 
of the crust as having irregularities like the outer,—that 1 
mountain elevations projecting inward, and immersed in the fluid 
mass, and valleys whose depressions are excavated in the crust. 
Such an internal orographic cdo ould modify the progress 
of the seismic waves. A wa Se would rise and increase its velo- 
city and, consequently, its sitive force, between two mountains 
or elevations that obstruct its passage; it would spread and lose 
velocity over a plain or in a valley where it could expand and 
develop itself; and would beat against the declivities or pro- 
jections encountered. Hence a new kind of ¢ compression, and, 
therefore, of molecular vibrations, whscls should propagate them- 
selves to the earth’s surfac ce, and appear as earthquakes. Hence 
also, beyond question, some partial displacements in the walls of 
the vaulted crust, and ruptures causing vibrations more or less 
intense. Hence, ‘also, fssures in the vault, of greater or less ex- 
tent, and more or less.a 
An introduction of the incandescent liquid from the earth- 
quake-wave into these fissures could hardly take place without 
shocks or vibrations more or less apparent. But it is a question 
whether such vibrations would reach the earth’s surface. This 
would depend on their intensity; and also on the thickness and 
ra peel of the crust, which would necessarily have an important 
ence. 
These displacements and ruptures could not take place with- 
: ote aciulelecs to a an or less ata ne pe ee 
condition and nature of the region. 
