Mathematical Theories of the Earth. — Woodvmrd. 281 
is not improbable that tlieso two seemingly divergent theories are 
really supplementary. 
Closely related to the questions of secular contraction and the 
mechanics of crust movements are those vexed questions of earth- 
quakes, volcanism, the liquidity or solidity of the interior, and the 
rigidity of the earth's mass as a whole, — all (juestions of the greatest 
interest but still lingering on the battlefields of scientific opinion. 
Many of the "thrice slain" combatants in these contests would fain 
risk being slain again ; and whether our foundation be liquid or solid, 
or to speak more precisely, whether the earth may not be at once 
highly plastic under the action of long continued forces and highly 
rigid under the action of periodic forces of short period, it is pretty 
certain that some years must elapse before the arguments will be con- 
vincing to all concerned. The difhculties appear to be due princi- 
pally to our profound ignorance of the properties of matter subject to 
the joint action of great pressure and great heat. The conditions 
which exist a few miles beneath the surface of the earth are quite 
beyond the reach of laboratory tests as hitherto developed, but it is 
not clear how our knowledge is to be improved without resort to 
experiments of a scale in some degree comparable with the facts to be 
explained. In the mean time, therefore, we may exjDect to go on 
theorizing, adding to the long list of dead theories which mark the 
progress of scientific thought, with the hope of attaining the truth not 
so much by direct discovery as by the laborious process of eliminating 
error. 
When we take a more comprehensive view of the problems pre- 
sented by the earth, and look for light on their solution in theories of 
cosmogony, the difficulties which beset us are no less numerous and 
formidable than those encountered along special lines of attack. 
Much progress has recently been made, however, in the elaboration 
of such theories. Roche, Darwin, and others have done much to re- 
move the nebulosity of Laplace's nebular hypothesis. Poincare and 
Darwin have gone far towards bridging the gaps whic-h have long ren- 
dered the theory of rotating fluid masses incomplete. Poincare has 
in fact shown us how a homogeneous rotating mass might, through 
loss of heat and consequent contraction, pass from the spheroidal form 
to the Jacobian ellipsoidal form, and thence, by reason of its increas- 
ing speed of rotation, separate into two une(iual masses. Darwin, 
starting with a swarm of meteorites and gravitation as a basis, has 
reached many interesting and instructive results in the endeavor to 
trace out the laws of evolution of a planetary system. But notwith- 
standing the splendid researches of these and other investigators in 
this field, it must be said that the real case of the solar system, of the 
earth and moon, still defies analysis; and that the mechanics of the 
segregation of a planet from the sun or of a satellite from a planet, if 
such an event has ever happened, or of the mechanics of the evolution 
of a solar system from a swarm of meteorites, are still far from being 
clearly made out. 
