"I adhere," he says, "to the hypothesis of Copernicus, because it seems to me the simplest and clearest. There is no vacuum anywhere in space.... The heavens are full of a universal liquid substance. This is an opinion now commonly received among astronomers, because they cannot see how the phenomena can be explained without it. The substance of the heavens has the common property of all liquids, that its minutest particles are easily moved in any direction, and when it happens that they all move in one way, they necessarily carry with them all the bodies they surround, and which are not prevented from moving by any external cause. The matter of the heaven in which the planets are turns round continually like a vortex, which has the Sun for its centre. The parts that are nearest the Sun move faster than those that are at a greater distance; and all the planets, including the earth, remain always suspended in the same place in the matter of the heaven. And just as in the turns of rivers, when the water turns back on itself and twists round in circles, if any twig or light body floats on it, we see it carry them round, and make them move with it, and even among these twigs we may see some turning on their own centre, and those that are nearest to the middle of the vortex moving quicker than those on the outside; so we may easily imagine it to be with the planets, and this is all that is necessary to explain the phenomena. The matter that is round Saturn takes about thirty years to run its circle; that which surrounds Jupiter carries it and its satellites round in twelve years, and so on.... The satellites are carried round their primaries by smaller vortices.... The earth is not sustained by columns, nor suspended in the air by ropes, but it is environed on all sides by a very liquid heaven. It is at rest, and has no propulsion or motion, since we do not perceive any in it. This does not prevent it being carried round by its heaven, and following its motion without moving itself, just as a vessel which is not moved by winds or oars, and is not retained by anchors, remains in repose in the middle of the sea, although the flood of the great mass of water carries it insensibly with it. Like the earth, the planets remain at rest in the region of heaven where each one is found. Copernicus made no difficulty in allowing that the earth moves. Tycho, to whom this opinion seemed absurd and unworthy of common sense, wished to correct him, but the earth has far more motion in his hypothesis than in that of Copernicus." [Illustration: FIG. 22.--DESCARTES' THEORY OF VORTICES.]