ust have been, by Kepler's
Law, more than two and less than two and a half hours. Now it happens
that the most rapid rate of rotation of a fluid mass of the earth's
average density, consistent with spheroidal equilibrium, is two hours
and twenty minutes. Quicken the movement but by one second and the globe
must fly asunder. Hence the inference that the earth actually _did_ fly
asunder through over-fast spinning, the ensuing disruption representing
the birth-throes of the moon. It is likely that the event was hastened
or helped by solar tidal disturbance.

To recapitulate. Analysis tracks backward the two bodies until it leaves
them in very close contiguity, one rotating and the other revolving in
approximately the same time, and that time certainly not far different
from, and quite possibly identical with, the critical period of
instability for the terrestrial spheroid. "Is this," Professor Darwin
asks, "a mere coincidence, or does it not rather point to the break-up
of the primeval planet into two masses in consequence of a too rapid
rotation?"[1175]

We are tempted, but are not allowed to give an unqualified assent. Mr.
James Nolan of Victoria has made it clear that the moon could not have
subsisted as a continuous mass under the powerful disruptive strain
which would have acted upon it when revolving almost in contact with the
present surface of the earth; and Professor Darwin, admitting the
objection, concedes to our satellite, in its initial stage, the
alternative form of a flock of meteorites.[1176] But such a congregation
must have been quickly dispersed, by tidal action, into a meteoric ring.
The same investigator subsequently fixed 6,500 miles from centre to
centre as the minimum distance at which the moon could have revolved in
its entirety; and he concluded it "necessary to suppose that, after the
birth of a satellite, if it takes place at all in this way, a series of
changes occur which are quite unknown."[1177] The evidence, however, for
the efficiency of tidal friction in bringing about the actual
configuration of the lunar-terrestrial system is not invalidated by this
failure to penetrate its natal mystery. Under its influence the
principal elements of that system fall into interdependent mutual
relations. It connects, casually and quantitatively, the periods of the
moon's revolution and of the earth's rotation, the obliquity of the
ecliptic, the inclination and eccentricity of the lunar orbit. All this