enotes that accelerated motion with which they all, as well as the earth, advance from the perigee to the apogee of their orbits. This acceleration is most readily observed by comparing the successive diurnal motions of the planet in its orbit. When the actual diurnal motion exceeds the mean diurnal motion, the planet is accelerated; and, on the other hand, when it falls short of it, it is retarded, as takes place between the apogee and perigee.

Acceleration of the Moon is a remarkable increase which has been discovered in the moon's motion in her orbit, which has been going on increasing from age to age by a gradation so imperceptible, that it was only discovered or suspected by Dr. Halley, on comparing the ancient eclipses observed at Babylon and others with those of his own time. The quantity of this acceleration was afterwards determined by Mr. Dunthorne from more accurate data regarding the longitudes of Alexandria and Babylon, and from the most authentic eclipse of which any good account remains, observed at Babylon in the year 721 before Christ. The beginning of this eclipse, as observed at that time, was about an hour and three quarters sooner than he found it would have been by computation; and hence he found the mean acceleration, or what has since been termed the moon's secular equation, about 10" of a degree each century. According to Laplace, it amounts to \( 11\cdot135'' \). This remarkable fact had long excited the attention of astronomers; as, along with several others of the same kind among the heavenly bodies, it seemed to betray imperfection; exhibiting inequalities which were continually increasing, instead of correcting themselves or being somehow compensated by that admirable design which prevailed in every other part of the system. At last, however, it was discovered, by the application of a refined analysis, that these inequalities were not perpetual; that they actually terminate in the lapse of ages, and again return in the opposite direction, thus preserving entire the harmony of the celestial motions. This fine discovery, which observation alone could never have disclosed, we owe to the genius of Laplace. See Astronomy in this work; also Phil. Trans. No. 204, 218, and vol. xlvii. 1749, 1750, 1777; Mem. de l'Acad. Par. 1757, 1763, 1786; Accélération de l'Acad. Berlin, 1778, 1782; Connaissances des Temps, 1779, 1782, 1790; Newton's Principia, second edition; Say's Astronomy; Vince's Astronomy; Astronomie, par Lalande, &c.

Acceleration of Bodies on inclined Planes. The same general law obtains here as in bodies falling perpendicularly; the effect of the plane is to make the motion slower; but the inclination being everywhere equal, the retardation arising therefrom will proceed equally in all parts, at the beginning and the ending of the motion.