Page:Popular Science Monthly Volume 64.djvu/324

320 of heavy bodies towards the earth's center other men are dropping balls into holes bored entirely through the globe and these balls are falling to the earth's midmost point—

as Virgil explains to Dante in the thirty-fourth canto of the Inferno. The cosmogony of Dante in the Divina Commedia was accepted for centuries by roman Catholics, as Milton's in the Paradise Lost has been adopted by protestants. For Dante the globe of the earth was the center of the world. It was surrounded by nine transparent spheres moved by angels. There was a crystal sphere for the moon, and others for Mercury, Venus, the sun, Mars, Jupiter, Saturn and the fixed stars, and beyond them the Primum Mobile—nine in all. Beyond the outer sphere was the Empyrean—here God sate. Below the earth is hell and here its god—Lucifer—reigned over bad angels. All the discord in the world came from them, even its storms, hail and lightning. The spheres of Eudoxus served as a base to Dante's system, which was adapted, with a poet's license, to a poet's use.

In the De Cœlo, Aristotle lays down certain fundamental principles: The things of which the world is made are all solid bodies, and all have, therefore, three dimensions. The simple elements of nature must also have simple motions. So, indeed, fire and air have their natural motions upwards, water and earth, downwards, both in straight lines. But besides these motions there is also a circular motion, not natural to these elements, although it is a much more complete motion than the rectilinear. For the circle is, in itself, a complete line, which a straight line is not: There must, therefore, be certain things to which complete circular motion is natural: It follows that there must be a certain sort of bodies very different from the four elementary bodies, bodies that are more godlike, that must therefore stand above them: This finer essence was later named by the commentators 'Quinta Essentia'—our quintessence. The heavenly bodies are formed of this; they are spheres endowed with life and activity.

The question of the revolution of the earth in an orbit round the sun is discussed by Aristotle, and he rejects the idea for the reason that such a motion would necessarily produce a corresponding alteration in the place of each and every fixed star. The objection was perfectly valid. If the stars were only a little farther from us than Saturn, as Aristotle believed, a motion of the earth in an orbit would cause each star to move in an apparent parallactic orbit, a miniature copy of that of the earth. No such alteration of place was observable. Hence, said he, the earth did not move. Even the nearest stars are,