Acceleration, an increase in velocity of a moving body, either constant and uniform or variable. When the velocity receives equal increments in equal times, it is uniform. This is the case with bodies falling in a vacuum, the increase of which is in every second about 32 1/2 feet, but varies with the latitude of the place and the height above the ocean, or the depth under the surface of the earth. As gravitation is the cause of this acceleration, scientists have accepted the letter g for it as a symbol, meaning 31 or 32 or any other number of feet, as the case may be. At the distance of the moon the acceleration is 3,600 times smaller, and g is thus equal to about 1/9 inch; while at the surface of the sun and large planets it is much greater by reason of the greater attraction of large masses. Acceleration is variable when its velocity increases at one instant in a greater or lesser ratio than in another. This is the case with a body falling from a great height toward the earth through the air; the resistance of the latter increasing with the velocity, the ratio of increase must diminish till the accelerating force, that is, gravitation (g), balances the resistance of the air, when! the body will continue to fall with uniform motion, which motion may then become retarded if the body in its downward course enters strata of air of greater density and thus at greater resistance.

The motions of the planets, and especially of the comets, in their orbits around the sun, offer other illustrations of variable acceleration. - Acceleration of the Moon. Halley noticed that the comparison of ancient eclipses with modern shows that the moon moves faster now than formerly, and the solution of the problem as to the cause of this acceleration was first given by Laplace, who showed that the slow diminution of the eccentricity of the earth's orbit must produce an acceleration of the moon's motion and a decrease in the period of its revolution. Adams has recently shown that Laplace overestimated the effect of the change of the earth's orbit on the moon by one half, and that his demonstration therefore only accounts for half of the moon's acceleration. The cause of the other half remains to be found out. De-launy ascribes it to a retardation in the earth's motion of rotation by the influence of the tidal wave raised by the moon, and reacting on the latter. The earth's daily rotation, however, appears to undergo neither acceleration nor retardation; therefore others ascribe it to a resisting medium, filling the interplanetary space and revolving round the sun with the planets, and which thus only can affect their moons, which at every half revolution move in opposite direction to the general motion, and thus having their centrifugal force checked are drawn nearer to the planet, by which their apparent or angular velocity is increased. - Acceleration of the Stars. The so-called stellar acceleration is no acceleration in the true sense, but means only the amount that the apparent daily revolution of the stars gains on that of the sun.

It is easily calculated by considering the fact that the starry heavens make per year one revolution more than the sun; therefore the daily gain must be 1/365 part of 24 hours, which is very near 3 minutes and 56 seconds.