As it glides past some island we look from our cabin window and discover that the island seems to be moving by us in an opposite direction. Even so, as our earth sweeps round its axis with a speed of a thousand miles an hour, it seems motionless, while the starry spheres above appear gliding westward.
In spite of the impassioned and reiterated exclamation of Galileo, “It does move!” men would not believe it, until his experiment at the leaning tower of Pisa; and the beautiful demonstration of Foucault[3] from the dome of the Pantheon in Paris, proved beyond question the earth’s rotation. Let us briefly outline these two experiments. If the world has no motion, a heavy weight dropped from the top of a tall shaft would strike exactly at its base. But if the earth were rotating, the top of the tower must move faster than the base. The weight at the top would have the same motion as the top of the shaft, and would keep it, in falling, in accordance with a well known law, and consequently would strike beyond the base. The ball dropped by Galileo did thus strike. Foucault argued: If the earth does not rotate, a pendulum suspended so that a needle fastened to its lowest point would trace a line in sand sprinkled on the surface beneath, would forever move along the same line. But if the earth is rotating, the needle will trace different lines on the sand. If the pendulum was suspended at one of the poles, it would, in twenty-four hours, trace a series of lines like the spokes of a carriage wheel about the pole. Foucault showed that the needle did trace varying lines in the sand, therefore the earth moves. This experiment is repeated annually at Paris, and I have performed it in the Amphitheater at Chautauqua.
Sir Isaac Newton, as all the world knows, discovered the relation and mutual dependence of all matter in the universe. The law of gravitation has been called “Newton’s Darling Child.” It states, in brief, that every body attracts every other directly as the mass, and inversely as the square of the distance.
The sun has three hundred thousand times the amount of matter contained in our earth; its power of attraction is therefore proportional. A man on its surface would be crushed by his own weight, but even Brobdignag,[4] or any other giant, could live comfortably on an asteroid. Indeed, he would weigh so little there that he could leap like the mountain goat.
As an illustration of the second part of the law: one body three times as far away as another from the attracting power, will be held by a force but one ninth as great. A body near the surface of the earth, or 4,000 miles from its center, will fall sixteen feet in a second. The moon is sixty times further from the center of the earth than such a body. Newton found that the moon fell toward the earth, or varied from a straight line, 1.36 of sixteen feet in a second. Now, the square of sixty is 3,600, therefore the moon proves that the force of gravity decreases as the square of the distance increases.
Nothing in nature is more beautiful than the adjustment of forces by which our earth is kept forever revolving in its appropriate orbit. In perihelion,[5] or its nearest approach to the sun, it is 3,000,000 miles nearer than in aphelion, when farthest away from it. Of course it will follow from the law that the attraction of the sun would then be greater. If there were no counteracting influence, there could be but one result—the earth would fly into the sun and be consumed. The very proximity of the earth to the sun, however, increases its speed, and therefore its tendency to fly off on a tangent. Kepler[6] has expressed this truth in one of his laws: “The radius vector (a line drawn from the sun to the earth) passes over equal areas in equal times.” The average speed of the earth in its orbit is 1,100 miles a minute—3,300 times that of the fastest steamship—but it varies throughout its course, and how wonderful that system of breaks by which its motion is regulated! Divine wisdom alone could invent such a plan.
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.