in planes inclined at small angles towards each
other, substitute different conditions and the stability of the universe
will again be put in jeopardy, and according to all probability there
will result a frightful chaos.[31]

Although the invariability of the mean distances of the planetary
orbits has been more completely demonstrated since the appearance of the
memoir above referred to, that is to say by pushing the analytical
approximations to a greater extent, it will, notwithstanding, always
constitute one of the admirable discoveries of the author of the
_Mecanique Celeste_. Dates, in the case of such subjects, are no luxury
of erudition. The memoir in which Laplace communicated his results on
the invariability of the mean motions or mean distances, is dated
1773.[32] It was in 1784 only, that he established the stability of the
other elements of the system from the smallness of the planetary masses,
the inconsiderable eccentricity of the orbits, and the revolution of the
planets in one common direction around the sun.

The discovery of which I have just given an account to the reader
excluded at least from the solar system the idea of the Newtonian
attraction being a cause of disorder. But might not other forces, by
combining with attraction, produce gradually increasing perturbations as
Newton and Euler dreaded? Facts of a positive nature seemed to justify
these fears.

A comparison of ancient with modern observations revealed the existence
of a continual acceleration of the mean motions of the moon and the
planet Jupiter, and an equally striking diminution of the mean motion
of Saturn. These variations led to conclusions of the most singular
nature.

In accordance with the presumed cause of these perturbations, to say
that the velocity of a body increased from century to century was
equivalent to asserting that the body continually approached the centre
of motion; on the other hand, when the velocity diminished, the body
must be receding from the centre.

Thus, by a strange arrangement of nature, our planetary system seemed
destined to lose Saturn, its most mysterious ornament,--to see the
planet accompanied by its ring and seven satellites, plunge gradually
into unknown regions, whither the eye armed with the most powerful
telescopes has never penetrated. Jupiter, on the other hand, the planet
compared with which the earth is so insignificant, appeared to be moving
in the opposite direction, so as to be