of Cambridge University, into the question of the
probable birth of our moon. He comes to the conclusion that at least
fifty-four millions of years ago the earth and moon formed one body,
which had a diameter of a little over 8000 miles. This body rotated on
an axis in about five hours, namely, about five times as fast as it does
at present. The rapidity of the rotation caused such a tremendous strain
that the mass was in a condition of, what is called, unstable
equilibrium; very little more, in fact, being required to rend it
asunder. The gravitational pull of the sun, which, as we have already
seen, is in part the cause of our ordinary tides, supplied this extra
strain, and a portion of the mass consequently broke off, which receded
gradually from the rest and became what we now know as the moon. Sir
George Darwin holds that the gravitational action of the sun will in
time succeed in also disturbing the present apparent harmony of the
earth-moon system, and will eventually bring the moon back towards the
earth, so that after the lapse of great ages they will re-unite once
again.

In support of this theory of the terrestrial origin of the moon,
Professor W.H. Pickering has put forward a bold hypothesis that our
satellite had its origin in the great basin of the Pacific. This ocean
is roughly circular, and contains no large land masses, except the
Australian Continent. He supposes that, prior to the moon's birth, our
globe was already covered with a slight crust. In the tearing away of
that portion which was afterwards destined to become the moon the
remaining area of the crust was rent in twain by the shock; and thus
were formed the two great continental masses of the Old and New Worlds.
These masses floated apart across the fiery ocean, and at last settled
in the positions which they now occupy. In this way Professor Pickering
explains the remarkable parallelism which exists between the opposite
shores of the Atlantic. The fact of this parallelism had, however, been
noticed before; as, for example, by the late Rev. S.J. Johnson, in his
book _Eclipses, Past and Future_, where we find the following passage:--

"If we look at our maps we shall see the parts of one Continent that jut
out agree with the indented portions of another. The prominent coast of
Africa would fit in the opposite opening between North and South
America, and so in numerous other instances. A general rending asunder
of the World would seem to have