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 figures of the celestial bodies.  In the fourth book he considers the oscillations of the ocean and the atmosphere, arising from the disturbing action of the celestial bodies.  The fifth book is devoted to the investigation of the movements of the celestial bodies around their centres of gravity.  In this book the author gives a solution of the great problems of the precession of the equinoxes and the libration of the moon, and determines the conditions upon which the stability of Saturn’s ring depends.  The sixth book is devoted to the theory of the planetary movements; the seventh, to the lunar theory; the eighth, to the theory of the satellites of Jupiter, Saturn, and Uranus; and the ninth, to the theory of comets.  In the tenth book the author investigates various subjects relating to the system of the universe.  Among these may be mentioned the theory of astronomical refractions; the determination of heights by the barometer;