83 
REPORT—1847. 
fraeted wave will become parallel to AB. When the point of incidence is 
beyond P,, the vibrations will not be propagated at all into the upper me¬ 
dium, but will be entirely reflected back into the lower one. 
We may remark that in certain portions of the lower medium the incident 
and reflected waves may coexist. The vibrations of particles in such po^ 
tioiis will of course be roinpouiided of those in both waves. 
^ If the media through which the vibrations are propagated are the 
jncidcnt wave may consist of normal or tangential vibrations. In either 
case there will, genernlly, be two reflected and two refracted sares, witli 
normal and tangential vibrations jespectively. The law of niesion will, 
tor both reflected wavas be the same as that above enunciated; and the 
law ot refraction will also be the same as in fluid media, if in the ratio 
velocity of the refracted wave 
velocity of th^inddelir^ve ’ velociliea. 
§ EariJtquahes. 
the application of the theoretical conclusions above sUtedi 
vibrations, such as are frequently experienced if 
earthquakes, I shall first suppose, as I have already done, that the origtiisl 
disturbance or shock from which the vibrations proceed takes place wilhiD 
comparatively small, as would ho (be case, for instance, if the shock 
nf n “ tieep-seated volcanic cxjilosion, the falling In of the roof 
T flLn 1 T?" rending of the solid rock around it. 
to homogeneous; itviH 
cXiicSd caae7 tlu^ general results in more 
inipoitant point, perhaps, which wc may hope todetenuineffitli 
the denTnf position, and more especially 
ceL a subi/c r 'vddeh earthquakes pro- 
Lnttl.! '"-y >''e cnii tiotermine wiUi suff> 
oartli’s solid enisr' propagation of a wave in the iiilenV of the 
surface to anoth r with which it passes from one point of the 
"vavrand ther/pp^m;/ /horizontal r^heity of the 
This direction uufJ r direction ol propagation at any proposed point, 
coincide S Ciomlrnons stated in the preceding paiigmph, will 
vertically above the ccMrlf’lV .P V’T*^ cartli* 
propagation fVt ^‘^‘-^^iirbance. V\ e maj'supjiose the velocity of 
the two coefficints instance by a prevm 
which 1 shall indiJate velocity by observatwar 
for the purpose. ’ ^ horizontal direction by an iiistriimtat dKigf^ 
a7T''‘ ‘‘r ‘■■’“'i'-" cannbb of Urtornmang ^ * 
servaUot Tr at C, P."""'';’' cruKt at tl,c jJaM of o'" 
is important to h™r , f'!jeclioil of that dirtctioll. But lerr it 
(art. 39) Iwtweeij tliu tw« lU* ^ churnctenstic distinction above indicat^ 
from the same centre of ,,j“S " *''^** proceed simultaneously^ 
one of these waves take vibrations m 
propagation mH f|,«r i directions peryiendicular to the direction of 
vibmtS win giemlwT "'?■ of this diivciou of 
of the wave’s ■ r k? horizontal direction 
