tensive glaciation; while the opposite hemisphere,
with a short, mild winter, and long, cool summer, enjoyed an approach to
perennial spring. These conditions were exactly reversed at the end of
10,500 years, through the shifting of the perihelion combined with the
precession of the equinoxes, the frozen hemisphere blooming into a
luxuriant garden as its seasons came round to occur at the opposite
sites of the terrestrial orbit, and the vernal hemisphere subsiding
simultaneously into ice-bound rigour.[901] Thus a plausible explanation
was offered of the anomalous alternations of glacial and semi-tropical
periods, attested, on incontrovertible geological evidence, as having
succeeded each other in times past over what are now temperate regions.
They succeeded each other, it is true, with much less frequency and
regularity than the theory demanded; but the discrepancy was overlooked
or smoothed away. The most recent glacial epoch was placed by Dr. Croll
about 200,000 years ago, when the eccentricity of the earth's orbit was
3.4 times as great as it is now. At present a faint representation of
such a state of things is afforded by the southern hemisphere. One
condition of glaciation in the coincidence of winter with the maximum of
remoteness from the sun, is present; the other--a high eccentricity--is
deficient. Yet the ring of ice-bound territory hemming in the southern
pole is well known to be far more extensive than the corresponding
region in the north.

The verification of this ingenious hypothesis depends upon a variety of
intricate meteorological conditions, some of which have been adversely
interpreted by competent authorities.[902] What is still more serious,
its acceptance seems precluded by time-relations of a simple kind. Dr.
Wright[903] has established with some approach to certainty that glacial
conditions ceased in Canada and the United States about ten or twelve
thousand years ago. The erosive action of the Falls of Niagara qualifies
them to serve as a clepsydra, or water-clock on a grand scale; and their
chronological indications have been amply corroborated elsewhere and
otherwise on the same continent. The astronomical Ice Age, however,
should have been enormously more antique. No reconciliation of the facts
with the theory appears possible.

The first attempt at an experimental estimate of the "mean density" of
the earth was Maskelyne's observation in 1774 of the deflection of a
plumb-line through the