There is no magical power in the center of our earth, as some have supposed from the fact that all bodies seek that point. Indeed, that is the one spot where there is no attraction, and where all substances would weigh nothing. The path described by a plummet, or any falling body, is simply a resultant motion produced by opposite particles of the earth making it pass half way between their lines of attraction. This will ordinarily be toward the center of the earth. As attraction of gravity is in proportion to mass, a body suspended near a mountain will be deflected toward it.
This has been shown by an experiment performed by Dr. Maskelyne,[7] near Mt. Schehallien, in Wales. Upon suspending a light body on opposite sides of this mountain, he observed that it swerved from the perpendicular toward the mountain. The amount of this variation measured the attraction of the mountain, as compared with the attraction of the earth. As the geological structure of this eminence was known, it was not difficult to compute its mass, and a comparison was made between it and the earth. From this calculation the entire weight of the earth was obtained, proving its specific gravity to be five times that of an equal bulk of water. Dr. Cavendish[8] afterward arrived at precisely the same result by experimenting with a pendulum.
A is a rotating machine; a is a skein of thread; á is the skein rotated; b is a chain; c is an onion; d is an apple; e is a glass fish aquarium, one tenth full of water, and rotated. A stick, hoop, shingle, or any such body suspended by a cord, when rapidly rotated will rise and revolve around its shortest axis.
Terrestrial gravity is constantly affecting the motion of bodies. Motion is the act of changing place, and always indicates the presence of some force; force or energy being that which tends to produce motion or rest. Motion in curved lines is produced by two or more forces acting upon a body, one of which must be constant. Example: A cannon ball is acted upon by the sudden explosion of the powder, the resistance of air, and the constant downward attraction of gravity. Nature seems to delight in curved motion; the waves, the flight of birds, the running brooks, the clouds, even the waving trees and grasses, all furnish illustrations of this. A little reflection upon any such instances will show that they are usually produced by the united action of an instantaneous and constant force.
The center of gravity is that point around which the opposite particles of a body balance each other. This point does not necessarily coincide with center of figure or center of motion, the former of which is a point equally distant from opposite parts of a regular body, while the latter is a point in a substance around which it revolves.
Ex.—A lead pencil poised on the finger. This experiment can be varied in many ways, showing the nature of stable and unstable equilibrium.
If the sun and all the planets could be strung on a rod passing through their centers, with the planets to the east, the center of gravity of the solar system would be somewhere in the sun, east of its center. As the planets assume various positions with reference to the sun, it must follow that the center of gravity in our system must vary accordingly.
The same is true of objects on the earth. The center of gravity may be elevated or depressed, moved to the right or left. We instinctively adjust our bodies so that a perpendicular let fall from the center of gravity will constantly fall within the base. The most surprising exhibition of this power of automatic adjustment was seen in Blondin, in his performances on the tight rope.