planets to act as
secondary rulers of their destinies. By their influence they were, in
all likelihood, originally fixed in their present tracks; and by their
influence, exerted in an opposite sense, they may, in some cases, be
eventually ejected from them. Careers so varied, as can easily be
imagined, are apt to prove instructive, and astronomers have not been
backward in extracting from them the lessons they are fitted to convey.
Encke's comet, above all, has served as an index to much curious
information, and it may be hoped that its function in that respect is by
no means at an end. The great extent of the solar system traversed by
its eccentric path makes it peculiarly useful for the determination of
the planetary masses. At perihelion it penetrates within the orbit of
Mercury; it considerably transcends at aphelion the farthest excursion
of Pallas. Its vicinity to the former planet in August, 1835, offered
the first convenient opportunity of placing that body in the
astronomical balance. Its weight or mass had previously been assumed,
not ascertained; and the comparatively slight deviation from its regular
course impressed upon the comet by its attractive power showed that it
had been assumed nearly twice too great.[244] That fundamental datum of
planetary astronomy--the mass of Jupiter--was corrected by similar
means; and it was reassuring to find the correction in satisfactory
accord with that already introduced from observations of the asteroidal
movements.

The fact that comets contract in approaching the sun had been noticed by
Hevelius; Pingre admitted it with hesitating perplexity;[245] the
example of Encke's comet rendered it conspicuous and undeniable. On the
28th of October, 1828, the diameter of the nebulous matter composing
this body was estimated at 312,000 miles. It was then about one and a
half times further from the sun than the earth is at the time of the
equinox. On the 24th of December following, its distance being reduced
by nearly two-thirds, it was found to be only 14,000 miles across.[246]
That is to say, it had shrunk during those two months of approach to
1/11000th part of its original volume! Yet it had still seventeen days'
journey to make before reaching perihelion. The same curious
circumstance was even more markedly apparent at its return in 1838. Its
bulk, or the actual space occupied by it, appeared to be reduced, as it
drew near the hearth of our system, in the enormous proportion