PTOLEMAIC SYSTEM. A few words of general explanation constitute all that can be given under this head, and we are not now speaking with particular reference to Ptolemy, but of the astro nomical part of that system which, founded on early metaphysical and physical doctrines, adopted by Plato and Aristotle, reinforced by mathematical hypotheses drawn from Hipparchus and Ptolemy, received by the Mohammedans and by them imparted to the Christians of the middle ages, was the doctrine universally established in Europe till the 17th century. As a whole it combines the physics of the Aristotelian school, the geometry of Euclid and his successors, the sexagesimal arithmetic of the Greeks, and the astronomy of Hipper chus and Ptolemy, with some slight additions from later names. The geometry remains, the arithmetic has been supplanted by the decimal system of the Hindus ; the physics and astronomy stood and fell together • and as under the words Ptolemaic System the astronomy is meant, we only here notice the physical notions so far as they are connected with it.
The early separation of perceptible matter into the four elements of earth, water, air, and fire, with observation of the relative places they appear to assume, led to the formation of an elementary system. Earth (and solids generally) sink in water, while air rises in water, and flame in air. Hence the notion that the mass of the earth is the central body of the universe ; above is a region of water, through which rises that portion of earth on which men and animals live. Above this is a region of air,'and above this again a region of fire. Nothing is at rest until it arrives at its proper or natural place,and all the motions of a part separated from its whole are rectilinear ; fire rises, and bodies fall, in straight lines. Gravity and levity are only the efforts of bodies separated from their natural places to return to them.
Above the earth and the elementary zones which encompass it, are other successive zones, called heavens. Each heaven contains an immense crystal spherical surface, to which one of the heavenly luminaries is attached, or would be attached, if it moved uniformly in a circle, as it would then do if the crystal sphere were made to revolve uniformly. But the varied motions of the heavenly bodies
made it necessary that smaller orbs should be placed with their centres upon the larger ones, as hereafter noticed, and that the planets should move with the smaller ones. It is hardly to be believed (at least so many think) that Ptolemy and the mathematicians received these orbs, in the physical sense, or as anything but hypotheses for representing the actual motions of the planets ; it is certain however that the actual solid orbs continued to be received till a late period, Copernicus uses _ .
_ .
whiek has exposed him to some stupielon of receiving them : even In the final scholium of the Princlpia Newton thinks it worth while once more to overturn them, as Tycho Bralio had done before him, by showing that if they existed, the comets could not move as they were known to move.
The first leaven is that of the moon ; the second that of Mercury ; the third that of Venus; the fourth that of the Sun; the fifth that of Mars; the sixth that of Jupiter; the seventh that of Saturn ; the eighth that of all the fixed stars. The heavens of Aristotle end here; later theorists add two more, a ninth, to make the precession of the equinoxes, and a tenth, or primum mobile, to make the diurnal revolution. All beyond this is the empyreal heaven.
The office of the primum mobile is to revolve from east to west in twenty-four hours, carrying with it (but how, we do not know) the whole of the subordinate heavens, and making all the phenomena of day and night. All the heavenly motions are to be circular and uniform ; this doctrine of the Platonic school is the keystone of the whole system. The poles of the primum mobile are those of the equator ; but the ninth heaven moves slowly round the poles of the ecliptic, carrying the whole system forward in longitude, so as to give the phenomena arising from the precession of the equinoxes. The heavens of the other heavenly bodies move round with the mean motions of the bodies depending upon them; and this completes the general view of the system.