1.78 
MR. J. H. JEAXS OX THE MBRATIOXS AXD 
§ 33. The most reliable evidence as to the actual values of /jl is to be obtained 
from the pheuomeuca of earthquake propagation."^ From the “time curves” given in 
the British Association Report presented at the 1902 meeting, there seems to be 
little doubt that the so-called “large-waves” are propagated merely through a thin 
crust on the earth’s surface, while the “ preliminaiy tremor” is propagated in a 
sensibly straight line tlirough the earth itself The average velocity of propagation 
is found to be about 97 kiloms. per second, and this is independent of the length of 
the path. The inference is that (X-f 2 p ,)/|0 is nearly constant throughout the earth’s 
interior, and that its value is about (97 X 10’)- or 9 A X lO^b If we suppose the 
mean value of p to he 5‘5, tins gives for the mean value ol X + 2/x, 
\ + 2 ijl = 517 X Bbb 
Now, the critical mean value of X -j- 2p, which corresponds to tlie first point of 
l)ifurcation has already l)een seen to be about 4 X 10^^. It would, therefore, appear 
tliat the error introduced in the classical solution for the displacements and stresses is 
appreciable, although not great—it is j.'robably comparable with tlie error to which 
attenti(->n lias already been attracted by CTiREE.f 
Figuke" uf the Eaktii. 
TheoiX'ticaJ Conclusions. 
§ 34. From the evidence of the last section it will be .seen that there is an over- 
Avliehuing probability that the ^■aIues of the elastic constants ot the earth are such 
that a state of spherical symmetry would be one of stable equilibrium. 
Whether or not .the earth is at present in a state of .spherical .symmetry is a 
difierent (jue.stion; various indications and, in particular, the inequality in the 
distril)ution of land l)etween the two liemi.spheres of the globe .suggest that it is 
not so. 
Now, even if the material of the eaith is at the present moment ot .sutficient 
strength to maintain a spherical configuration in sj)ite of the gravitational tendency 
to instability, it does not seem proba.ble that it has alvaiys been so. Looking back¬ 
wards in time we mirst come to a stage in wliicli the material ot the earth was pTa.stic, 
and, further back .still, fiuid. At this time the value of X Avould be much smaller 
than its pre.sent value, and, as already pointed out in § 29, would probably be about 
etpud to the critical value for the })lanet at that period of its exi.stence. There woidd, 
therefore, seem to be a .sufficient reason for con.sidering the possibility that the earth, 
at the moment at Avhich consolidation set in, was not in a .state of spherical symmetry. 
Let us examine some of the consequences of tliis conjecture. 
* Ri-ofessor 51 ilne has kindly assisted me in this question, 
t Loc. dt. ante. 
