Alexis Perrey on Earthquakes. 9 
Breaks in the rocks, as the caverns referred to, must modify 
their propagation, vary their direction and weaken their intensity, 
and may extinguish them; and this may account for the simulta- 
@ neous shaking of two regions while an intermediate locality is 
undisturbed—a phenomenon of so frequent occurrence in certain 
parts of America that the people speak of it under the expres- 
sion of the earth being bridged within, or suspended. 
Boussingault recognized, as the principal cause of the earth- 
uakes of the Andes, the continual and progressive sliding of 
the dislocated rocks of which they consist; and he considered 
the phenomenon as incessant in South America, an earthquake 
taking place, in his view, somewhere in the Andes at every 
instant of time. 
ese views are not at variance with my own. Any slidings 
due to gravity will be caused, or favored, by the daily vibrations 
whose effects and causes have been considered. 
Calculation demonstrates the existence of two kinds of waves 
moving with different velocities around a centre of vibration; 
Tadmit readily, with Mr. Wertheim, the coéxistence of these 
two kinds of waves. If then there are several successive sets 
of vibrations at a given point, each will propagate the double 
system of waves. It will be the same, also, if there are simulta- 
heous disturbances at a number of neighboring points. ) 
waves of greatest velocity of one set will overtake and pass by 
those of least velocity in the preceding set, and at an interval of 
distance depending on the interval of time between the successive 
ns. 
vibratio 
disturbance than the passage of two successive waves. In this 
case, the surface of the earth under vibration, if epg homoge- 
neous, should present concentric zones in whic the disturbance 
will be alternately more and less great. I would say, however, 
that I do not believe that such an alternation of effects from earth- 
tures on the surface to be upset or damaged should have an 
identity of construction and of position with reference to the 
points of compass which cannot be looked for. ee 
: At some future time, I propose to consider, from this point of 
View, the occurrence of the first shock more or less light which 
Precedes often the great shakings, and of the harmless vibrations 
which separate the disastrous shocks; and also the short interval 
Am. Jour. Sct.—Szconp Sens, Vou. XXXVI, No. 109.—Jax., 1864. 
2 
