Alexis Perrey on Earthquakes. 3 
and have the character of true earthquakes. Such is the first or 
principal cause of the phenomenon. 
The two opposite protuberances of the central nucleus together 
constitute, in their movement of rotation, what we call the great 
or primary earthquake or seismic wave. The greater the lunar 
influence, the greater will be the protuberances and the higher 
the seismic wave. 
should result, which, in the case supposed, would also cause, 
when its crests pass under the points of least resistance, the same 
two seismic waves should ad to, or diminish, one another, or 
coalesce in one wave, as with oceanic tidal waves. They will 
therefore manifest themselves at the surface only by their differ- 
ential or their resultant effects; and their union will form the 
great luni-solar wave. Its effect will therefore be the greatest 
possible at the syzygies; and hence the ruptures of the earth, 
consequent thereupon, should be most frequent at these two 
epochs in the lunar period: 
Let us now take note of the diurnal motion of the earth. We 
now have two new seismic waves; a lunar, the crests of which 
It is easy to conceive that in their simultaneous progress, these 
1 ; 
Points of view as the luni-solar wave depending solely on the 
motion of the moon in its orbit. 
n their progress, these different waves are similar, or, at least, 
h 
Sure on the earth’s crust, which, supposing it homogeneous, will 
experience at these points maxima and minima in c ange o 
form, and consequently in frequency of fractures; and there- 
ee maxima and minima in vibrations of the crust, or earth- 
quakes 
leal expressions of the physical laws of the phenomenon) will 
®nter necessarily the distances of the sun and moon from the 
farth. But the action being in the inverse ratio of the squares 
of the distances, the effect should be, under this point of view, 
Sreater at the perigee than at the apogee. In accordance with 
this, I have found, that, relatively to the lunar motion, earth- 
quakes are more frequent at the perigee than at the apogee; and 
Into these periodical functions of the seismic waves (or analyt- ¢ 
