relation to that sphere as a part of a thing stands to the whole. These spheres have not a common centre; the centres of some of them are identical with the centre of the Universe, while those of the rest are different from it. Some of the spheres have a motion independent of that of the whole Universe, constantly revolving from East to West, while other spheres move from West to East. The stars contained in those spheres are part of their respective orbits; they are fixed in them, and have no motion of their own, but participating in the motion of the sphere of which they are a part, they appear themselves to move. The entire substance of this revolving fifth element is unlike the substance of those bodies which consist of the other four elements, and are enclosed by the fifth element. The number of these spheres encompassing the Universe cannot possibly be less than eighteen; it may even be larger; but this is a matter for further investigation. It also remains an open question whether there are spheres which, without moving round the centre of the Universe, have nevertheless a circular motion. Within that sphere which is nearest to us, a substance is contained which is different from the substance of the fifth element; it first received four primary forms, and then became in these four forms, four kinds of matter: earth, water, air, fire. Each of the four elements occupies a certain position of its own assigned to it by nature; it is not found in another place, so long as no other but its own natural force acts upon it; it is a dead body; it has no life, no perception, no spontaneous motion, and remains at rest in its natural place. When moved from its place by some external force, it returns towards its natural place as soon as that force ceases to operate. For the elements have the property of moving back to their place in a straight line, but they have no properties which would cause them to remain where they are, or to move otherwise than in a straight line. The rectilinear motions of these four elements when returning to their original place are of two kinds, either centrifugal, viz., the motion of the air and the fire; or centripetal, viz., the motion of the earth, and the water; and when the elements have reached their original place, they remain at rest.