theory has been supplanted by a compromise theory rather than
completely overthrown, and our knowledge of the condition of the
telluric depths is still far from definite. If so much uncertainty
attends these fundamental questions as to the earth's past and present,
it is not strange that open problems as to her future are still
more numerous. We have seen how, according to Professor Darwin's
computations, the moon threatens to come back to earth with destructive
force some day. Yet Professor Darwin himself urges that there are
elements of fallibility in the data involved that rob the computation of
all certainty. Much the same thing is true of perhaps all the estimates
that have been made as to the earth's ultimate fate. Thus it has been
suggested that, even should the sun's heat not forsake us, our day will
become month-long, and then year-long; that all the water of the globe
must ultimately filter into its depths, and all the air fly off into
space, leaving our earth as dry and as devoid of atmosphere as the moon;
and, finally, that ether-friction, if it exist, or, in default of that,
meteoric friction, must ultimately bring the earth back to the sun. But
in all these prognostications there are possible compensating factors
that vitiate the estimates and leave the exact results in doubt. The
last word of the cosmic science of our generation is a prophecy of
evil--if annihilation be an evil. But it is left for the science of
another generation to point out more clearly the exact terms in which
the prophecy is most likely to be fulfilled.

PHYSICAL PROBLEMS

In regard to all these cosmic and telluric problems, it will be seen,
there is always the same appeal to one central rule of action--the law
of gravitation. When we turn from macrocosm to microcosm it would
appear as if new forces of interaction were introduced in the powers of
cohesion and of chemical action of molecules and atoms. But Lord Kelvin
has argued that it is possible to form such a conception of the forms
and space relations of the ultimate particles of matter that their
mutual attractions may be explained by invoking that same law of
gravitation which holds the stars and planets in their course. What,
then, is this all-compassing power of gravitation which occupies so
central a position in the scheme of mechanical things?

The simple answer is that no man knows. The wisest physicist of
to-day will assure you that he knows absolutely nothing of the