Some minds have difficulty in conceiving this isolation, because they form a false notion of weight.
The astronomers of antiquity, who divined it, knew not how to prevent the Earth from falling. They asked anxiously what the strong bands capable of holding up this block of no inconsiderable weight could be. At first they thought it floated on the waters like an island. Then they postulated solid pillars, or even supposed it might turn on pivots placed at the poles. But on what would all these imaginary supports have rested? All these fanciful foundations of the Earth had to be given up, and it was recognized as a globe, isolated in every part. This illusion of the ancients, which still obtains for a great many citizens of our globule, arises, as we said, from a false conception of weight.
Weight and attraction are one and the same force.
A body can only fall when it is attracted, drawn by a more important body. Now, in whatever direction we may wander upon the globe, our feet are always downward. Down is therefore the center of the Earth.
The terrestrial globe may be regarded as an immense ball of magnet, and its attraction holds us at its surface. We weigh toward the center. We may travel over this surface in all directions; our feet will always be below, whatever the direction of our steps. For us, "below" is the inside of our planet, and "above" is the immensity of the Heavens that extend above our heads, right round the globe.
This once understood, where could the Earth fall to? The question is an absurdity. "Below" being toward the center, it would have to fall out of itself.
Let us then picture the Earth as a vast sphere, detached from all that exists around it, in the infinity of the Heavens. A point diametrically opposed to another is called its antipodes. New Zealand is approximately the antipodes to France. Well, for the inhabitants of New Zealand and of France the top is reciprocally opposed, and the bottom, or the feet, are diametrically in opposition. And yet, for one as for the other, the bottom is the soil they are held to, and the top is space above their heads.
The Earth turns on itself in twenty-four hours. Whatever is above us, e.g., at midday, we call high; twelve hours later, at midnight, we give the same qualification to the part of space that was under our feet at noon. What is in the sky, and over our heads, at a given hour, is under our feet, and yet always in the sky, twelve hours later. Our position, in relation to the space that surrounds us, changes from hour to hour, and "top" and "bottom" vary also, relatively to our position.
Our planet is thus a ball, slightly flattened at the poles (by about 1⁄292). Its diameter, at the equator, is 12,742 kilometers (7,926 miles); from one pole to the other is a little less, owing to the flattening of the polar caps. The difference is some 43 kilometers (about 27 miles).
Its circumference is 40,000 kilometers (24,900 miles). This ball is surrounded by an aerial envelope, the atmosphere, the height of which can not be less than 300 kilometers (186 miles), according to the observations made on certain shooting stars.