tion, it
will again begin to diminish, and thus it will perpetually oscillate
between certain fixed limits. Laplace found that the extent to which the
tropical year is liable to vary from this cause, amounts to thirty-eight
seconds. If it were not for the effect produced upon the inclination of
the equator to the ecliptic by the mean disturbing action of the sun and
moon upon the terrestrial spheroid, the extent of variation would amount
to 162 seconds.

_Motion of the perihelion of the terrestrial orbit._--The major axis of
the orbit of each planet is in a state of continual movement from the
disturbing action of the other planets. In some cases, it makes the
complete tour of the heavens; in others, it merely oscillates around a
mean position. In the case of the earth's orbit, the perihelion is
slowly advancing in the same direction as that in which all the planets
are revolving around the sun. The alteration of its position with
respect to the stars amounts to about 11" in a year, but since the
equinox is regressing in the opposite direction at the rate of 50" in a
year, the whole annual variation of the longitude of the terrestrial
perihelion amounts to 61". Laplace has considered two remarkable epochs
in connection with this fact; viz: the epoch at which the major axis of
the earth's orbit coincided with the line of the equinoxes, and the
epoch at which it stood perpendicular to that line. By calculation, he
found the former of these epochs to be referable to the year 4107,
B.C., and the latter to the year 1245, A.D. He accordingly suggested
that the latter should be used as a universal epoch for the regulation
of chronological occurrences.

(B.)

The _Mecanique Celeste_.--This stupendous monument of intellectual
research consists, as stated by the author, of five quarto volumes. The
subject-matter is divided into sixteen books, and each book again is
subdivided into several chapters. Vol. I. contains the first and second
books of the work; Vol. II. contains the third, fourth, and fifth books;
Vol. III. contains the sixth and seventh books; Vol. IV. contains the
eighth, ninth, and tenth books; and, finally, Vol. V. contains the
remaining six books. In the first book the author treats of the general
laws of equilibrium and motion. In the second book he treats of the law
of gravitation, and the movements of the centres of gravity of the
celestial bodies. In the third book he investigates the subject of the
f