t must have been originally of slight
eccentricity, but that tidal friction is capable not only of extending,
but also of elongating it. The accelerating force is vastly greater at
periastron (when the two bodies are nearest each other) than at apastron
(when their distance is greatest). At periastron the disturbing force
will, therefore, increase the apastron distance by an enormous amount,
while at apastron it increases the periastron distance by a very small
amount. Thus, while the ellipse is being gradually expanded, the orbit
grows more and more eccentric, until the axial rotations have been
sufficiently reduced by the transfer of axial to orbital moment of
momentum.

And now we must draw this chapter to a close, though there are many
other subjects that might be included. The theory of tidal evolution is,
indeed, one of quite exceptional interest. The earlier mathematicians
expended their labour on the determination of the dynamics of a system
which consisted of rigid bodies. We are indebted to contemporary
mathematicians for opening up celestial mechanics upon the more real
supposition that the bodies are not rigid; in other words, that they are
subject to tides. The mathematical difficulties are enormously enhanced,
but the problem is more true to nature, and has already led to some of
the most remarkable astronomical discoveries made in modern times.

* * * * *

Our Story of the Heavens has now been told. We commenced this work with
some account of the mechanical and optical aids to astronomy; we have
ended it with a brief description of an intellectual method of research
which reveals some of the celestial phenomena that occurred ages before
the human race existed. We have spoken of those objects which are
comparatively near to us, and then, step by step, we have advanced to
the distant nebulae and clusters which seem to lie on the confines of the
visible universe. Yet how little can we see with even our greatest
telescopes, when compared with the whole extent of infinite space! No
matter how vast may be the depth which our instruments have sounded,
there is yet a beyond of infinite extent. Imagine a mighty globe
described in space, a globe of such stupendous dimensions that it shall
include the sun and his system, all the stars and nebulae, and even all
the objects which our finite capacities can imagine. Yet, what ratio
must the volume of this great globe bear to the whole extent