er it is
sufficient to achieve all that has been observed by geologists is a
matter of opinion.

Once more, the whole diversity of the seasons depends on the tilt
of the earth's axis, the 23 deg. by which it is inclined to a
perpendicular to the orbital plane; and this obliquity or tilt is
subject to slow fluctuations. Hence there will come eras when all
causes combine to produce a maximum extremity of seasons in the
northern hemisphere, and other eras when it is the southern
hemisphere which is subject to extremes.

But a grander problem still awaited solution--nothing less than the fate
of the whole solar system. Here are a number of bodies of various sizes
circulating at various rates round one central body, all attracted by
it, and all attracting each other, the whole abandoned to the free play
of the force of gravitation: what will be the end of it all? Will they
ultimately approach and fall into the sun, or will they recede further
and further from him, into the cold of space? There is a third possible
alternative: may they not alternately approach and recede from him, so
as on the whole to maintain a fair approximation to their present
distances, without great and violent extremes of temperature either way?

If any one planet of the system were to fall into the sun, more
especially if it were a big one like Jupiter or Saturn, the heat
produced would be so terrific that life on this earth would be
destroyed, even at its present distance; so that we are personally
interested in the behaviour of the other planets as well as in the
behaviour of our own.

The result of the portentously difficult and profoundly interesting
investigation, here sketched in barest outline, is that the solar system
is stable: that is to say, that if disturbed a little it will oscillate
and return to its old state; whereas if it were unstable the slightest
disturbance would tend to accumulate, and would sooner or later bring
about a catastrophe. A hanging pendulum is stable, and oscillates about
a mean position; its motion is periodic. A top-heavy load balanced on a
point is unstable. All the changes of the solar system are periodic,
_i.e._ they repeat themselves at regular intervals, and they never
exceed a certain moderate amount.

The period is something enormous. They will not have gone through all
their changes until a period of 2,000,000 years has elapsed. This is
the period of the pl