TERRESTRIAL GRAVITY.
To the ordinary observer, our earth seems, in general, a flat surface, here and there varied by hills and valleys. In reality it is a sphere, with a curvature of eight inches to the mile. The equatorial diameter is twenty-six and five-elevenths miles greater than the polar. The irregularities of the earth’s surface are relatively far less than they seem. Very thin letter paper, spread over a globe sixteen inches in diameter, would by its thickness adequately represent the highest mountain ranges. The greatest ocean depth is about equal to the height of the highest mountain; we see by this that the earth is essentially a smooth, round body.
Its shape is proven in four ways: First, two different navigators may start from the same point, one sailing east and the other west, and reach the same destination. Second, navigators have sailed around the world, Magellan having first performed the task. Third, in moving toward elevated objects, their upper portion first strikes the eye. Fourth, the shadow of the earth, when it falls upon the moon, is round.
The enlargement of the equatorial diameter is supposed to be due to the fact that the earth was once in a plastic state, and the centrifugal force, which is directly proportioned to the rotating speed of a body, caused the matter in the equatorial region to bulge. This action can easily be shown by revolving rapidly a flexible steel hoop, or other mobile substance. All bodies tend to revolve around their shortest axis. A great variety of interesting experiments showing this can easily be performed, some of which are indicated in an accompanying picture.
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.
Stability in structures is usually secured by lowering the center of gravity in one of two ways: either by broadening the base or by making it of heavy materials.
Specific gravity is the weight of a substance as compared with an equal bulk of something taken as a standard; water having been selected as the standard for solids and liquids, and air for gases.
Gravity furnishes more units of measure of various kinds—weight, work, heat, tenacity—than any other force of nature.
It will be remembered that Physics is that branch of science that considers the general properties of matter, and the character of those forces which affect matter without destroying its molecule. It includes many subdivisions. In addition to those already mentioned, we find Molecular Attraction, or the operation of forces that act at insensible distances; Hydrostatics, which treats of liquids at rest; Hydraulics, of liquids in motion; Pneumatics, of gases; Machines, of means for applying force; Acoustics, of the laws of sound; Heat; Light; and Electricity.
As many physical properties have been mentioned in the articles on Air, Water, and Fire, they will not now be considered. Our discussion here applies more especially to those substances which, at ordinary temperatures, are solid.
Ex.—A body buoyed up in water displaces its own weight of the liquid. The glass is nicely graded, and as the water rises in the vessel, the registration at once indicates the amount of water displaced. This proves the truth of the “Law of Archimedes”[9], ascertained while he was investigating the problem of the golden crown.
The most characteristic properties of solid bodies are the following: Hardness, tenacity, malleability, ductility, and crystalline form. Hardness is the resistance which a body offers to being scratched. Tenacity is the resistance offered by a body to a separation of its parts. Malleability is that property of a body which makes it capable of being rolled into sheets. Ductility is capacity for being drawn into wire, and crystalline form is the property which causes it to assume regular shapes.
As will be observed, these peculiarities are closely dependent upon cohesion and adhesion. By the former we understand the force which holds together the similar molecules of a substance; and by the latter, the force which unites the surfaces of different materials. Familiar as we are with these two agencies, their nature is not yet understood. We can easily discover that they are very dependent upon heat, by the application of which most solids pass from the stable form, to one in which, instead of cohesive force between the molecules, there is repulsion; as in the conversion of ice into water, and then into steam.
This movement of molecules is also dependent upon pressure. The most interesting illustration of this is seen in the action of glaciers. It has been ascertained that the melting temperature of ice lowers one two hundred and fiftieth of a degree for every fifteen pounds of pressure to the square inch.
The immense superincumbent mass of ice must, in many places, set free so much latent heat that a portion of the ice melts, so that here and there cells and liquid veins would be opened in the interior of the glacier. But the particles which separate these thin layers of water would almost immediately close up. This is the brilliant demonstration of Prof. Tyndall, who has given the operation the name of “regelation.” It has been thus described: “This phenomenon takes place at every point in the thickness of the glacier. Particles of ice approach one another, and unite across little veins of water, which permeate it in every direction; fresh liquid films are formed under the pressure from above; fresh unions take place between the divided morsels of ice; and, by this continual process of change, the air contained in the mass of that which once was snow, is gradually expelled. Thus it happens that the whole mass ultimately assumes an almost perfect transparency and a beautiful azure color.”