l forces as we are familiar with--that did not, in any
respect, change the interpretation, for there could have been no abrupt
diminution in the intensity of those forces, which, if they had lessened
in power, must have passed through a long, a gradual decline. [Sidenote:
These necessarily imply long time.] In that very decline there thus
spontaneously came forth evidences of a long lapse of time. The whole
course of Nature satisfies us how gradual and deliberate are her
proceedings; that there is no abrupt boundary between the past and the
present, but that the one insensibly shades off into the other, the
present springing gently and imperceptibly out of the past. If volcanic
phenomena and all kinds of igneous manifestations--if dislocations,
injections, the intrusion of melted material into strata were at one
time more frequent, more violent--if, in the old times, mundane forces
possessed an energy which they have now lost, their present diminished
and deteriorated condition, coupled with the fact that for thousands of
years, throughout the range of history, they have been invariably such
as we find them now, should be to us a proof how long, how very long ago
those old times must have been.

[Sidenote: Support from astronomical facts.] Thus, therefore, was
perceived the necessity of co-ordinating the scale of time with the
scale of space, and such views of the physical history of the earth were
extended to celestial bodies which were considered as having passed
through a similar course. In one, at least, this assertion was no mere
matter of speculation, but of actual observation. The broken surface of
the moon, its volcanic cones and craters, its mountains, with their
lava-clad sides and ejected blocks glistening in the sun, proved a
succession of events like those of the earth, and demonstrated that
there is a planetary as well as a terrestrial geology, and that in our
satellite there is evidence of a primitive high temperature, of a
gradual decline, and, therefore, of a long process of time. Perhaps
also, considering the rate of heat-exchange in Venus by reason of her
proximity to the sun, the pale light which it is said has been observed
on her non-illuminated part is the declining trace of her own intrinsic
temperature, her heat lasting until now.

[Sidenote: Astronomical facts imply slow secular changes.] If
astronomers sought in systematic causes an explanation of these facts
if, for instance, they were dispo