of
800,000 to 1. A corresponding expansion accompanied on each occasion its
retirement from the sphere of observation. Similar changes of volume,
though rarely to the same astounding extent, have been perceived in
other comets. They still remain unexplained; but it can scarcely be
doubted that they are due to the action of the same energetic internal
forces which reveal themselves in so many splendid and surprising
cometary phenomena.

Another question of singular interest was raised by Encke's acute
inquiries into the movements and disturbances of the first known "comet
of short period." He found from the first that its revolutions were
subject to some influence besides that of gravity. After every possible
allowance had been made for the pulls, now backward, now forward,
exerted upon it by the several planets, there was still a surplus of
acceleration left unaccounted for. Each return to perihelion took place
about two and a half hours sooner than received theories warranted.
Here, then, was a "residual phenomenon" of the utmost promise for the
disclosure of novel truths. Encke (in accordance with the opinion of
Olbers) explained it as due to the presence in space of some such
"subtle matter" as was long ago invoked by Euler[247] to be the agent of
eventual destruction for the fair scheme of planetary creation. The
apparent anomaly of accounting for an accelerative effect by a retarding
cause disappears when it is considered that any check to the motion of
bodies revolving round a centre of attraction causes them to draw closer
to it, thus shortening their periods and quickening their circulation.
If space were filled with a resisting medium capable of impeding, even
in the most infinitesimal degree, the swift course of the planets, their
orbits should necessarily be, not ellipses, but very close elliptical
spirals along which they would slowly, but inevitably, descend into the
burning lap of the sun. The circumstance that no such tendency can be
traced in their revolutions by no means sets the question at rest. For
it might well be that an effect totally imperceptible until after the
lapse of countless ages, as regards the solid orbs of our system, might
be obvious in the movements of bodies like comets of small mass and
great bulk; just as a feather or a gauze veil at once yields its motion
to the resistance of the air, while a cannon-ball cuts its way through
with comparatively slight loss of velocity.

It will