204. EARTHQUAKES. 
not interfere with the phenomena of precession and nu- 
tation. But Sir William Thomson showed that this 
theory ‘‘breaks down when tested by a simple calcula- 
tion of the amount of tangential force required to give to 
any globular portion of the interior mass the precession- 
al and nutational motions which, with other physical 
astronomers, he attributes to the earth as a whole.”’ 
Sir William Thomson has re-examined this problem 
and gives as his opinion (1 quote from Geikie) that the 
solar semi-annual and lunar fortnightly nutations abso- 
lutely disprove the existence of a thin rigid shell full of 
liquid. A thin crust requires a rigidity which is not 
possessed by any known substance, and a crust less 
than two thousand or twenty-five hundred miles in 
thickness could not bear the strain of the tide-producing 
force of the sun and moon, and maintain the earth’s 
configuration. He concludes that the mass of the earth 
‘is on the whole more rigid certainly than a continuous, 
solid globe of glass of the same diameter.” 
This view, which Thomson has again reviewed and 
modified, is strengthened by the mathematical investi- 
cations of Mr. George H. Darwin on the bodily tides of 
viscous and semi-elastic spheroids and the character of 
ocean tides on a yielding nucleus, whereby he shows 
that ‘‘no very considerable portion of the interior of the 
earth can even distantly approach the fluid condition.” 
Seeking to reconcile the geological facts ‘‘ which seem 
to demand the existence of a mobile mass of intensely 
hot matter’? beneath the earth’s surface, with the re- 
quirements of physics, Mr. Fisher suggested the ex- 
istence of a fluid or viscous material lying between the 
external crust—which is solid by cooling, and an in- 
tensely hot but solid nucleus which is rigid by 
pressure. 
What the entire truth is concerning these very im- 
portant questions, is a problem for future solution ; 
ss 
