Alexis Perrey on Earthquakes. 5 
But are these fractures, as has been said, the only cause of the 
Sounds which so often precede, accompany, or follow, earth- 
quakes? It is difficult to believe it. We acknowledge that we 
are not ready to explain the sounds that so often precede earth- 
quakes. In the case of earthquake shocks which are continued 
for a length of time, these sounds are often repeated: And how 
does the sound-vibration differ from the dynamical vibration 
which immediately follows it? Moreover, in such earthquake- 
shocks, continued for a length of time, both aérial and subterra- 
nean detonations are frequently repeated without any sensible 
movement of the ground. Many instances of this kind occurred 
in the valley of Visp in 1855 and 1856.” The sounds are, in 
fact, one of the most obscure elements connected with earth- 
uakes 
_ But to proceed, the ruptures which take place at certain points 
in the crust shake the neighboring parts, which, in their turn, 
.under the action of successive earthquake waves, lead to other 
like fractures. Such catastrophes may again and again follow. 
e thus account for the shocks which are repeated for a greater 
or less time after every great earthquake. d 
he fractures opened at any point will become prolonged in 
the direction of the line of least resistance. Hence comes the 
is seen that the waves that are propagated laterally arrive later, 
Telatively to the passage of the moon over the meridian, at the 
this kind are the shocks on the Mississippi in 1811; those of 
Maurienne in 1838; those of Scotland in 1842 and 1843. 
* The detonations in the valley of Visp continued to occur at intervals even till 
May, 1861. The later months of the year do not appear to have been marked by 
any repetition of the phenomena of 1855.—(Letters and Journal of M. Tscheinen, 
“urate at Greechen.)—Note added August 26th, 1862. 
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