sed to examine how far changes in the
obliquity of the ecliptic are connected therewith--it was necessary at
the outset to concede that the scale of time on which the event proceeds
is of prodigious duration, this secular variation observing a slow
process of only 45.7'' in a century; and hence, since the time of
Hipparchus, two thousand years ago, the plane of the ecliptic has
approached that of the equator by only a quarter of a degree. Or if,
again, they looked to a diminishing of the eccentricity of the earth's
orbit, they were compelled to admit the same postulate, and deal with
thousands of centuries. Under whatever aspect, then, the theory was
regarded, if once a former high temperature were admitted, and the fact
coupled therewith that there has been no sensible decline within the
observation of man, whether the explanation was purely geological or
purely astronomical, the motion of heat in the mass of the earth is so
slow, yet the change that has taken place is so great, the variations of
the contemplated relations of the solar system so gradual--under
whatever aspect and in whatever way the fact was dealt with, there arose
the indispensable concession of countless centuries.

To the astronomer such a concession is nothing extraordinary. It is not
because of the time required that he entertains any doubt that the sun
and his system accomplish a revolution round a distant centre of gravity
in nineteen millions of years, or that the year of epsilon Lyrae is
half a million of ours. He looks forward to that distant day when Sirius
will disappear from our skies, and the Southern Cross be visible, and
Vega the polar star. He looks back to the time when gamma Draconis
occupied that conspicuous position, and the builders of the great
pyramid, B.C. 3970, gave to its subterranean passage an inclination of
26 deg. 15', corresponding to the inferior culmination of that star. He
tells us that the Southern Cross began to be invisible in 52 deg. 30' N.,
2900 years before our era, and that it had previously attained an
altitude of more than 10 deg.. When it disappeared from the horizon of
the countries on the Baltic, the pyramid of Cheops had been erected more
than a thousand years.

[Sidenote: Proofs of time from aqueous effects,] We must pass by a
copious mass of evidence furnished by aqueous causes of change operating
on the earth's surface, though these add very weighty proof to the
doctrine of a long period. The filling