Such is the celebrated theory of vortices. The comparison of the rotation of the earth and planets and their revolution round the sun to the turning of small portions of a rapid stream, may contain an idea yet destined to be developed to account for these motions; but as used by Descartes it is a mere playing upon words admirably adapted to secure the concurrence of all parties; those who believed in the motion of the earth seeing that it did not interfere with their ideas in the least, and those who believed in its stability being gratified to find some way by which they might still cling to that belief and yet adopt the new ideas. This was its purpose, and that purpose it well served; but as a philosophical speculation it was worthless. When former astronomers declared that any planet moved, whether it were the earth or any other, they had no idea of attraction, but supposed the planet fixed to a sphere; this sphere moving and carrying the planet with it was what they meant by the planet moving: the theory of vortices merely substituted a liquid for a solid sphere, with this disadvantage, that if the planet were fixed to a solid moving sphere, it _must_ move; if only placed in a liquid one, that liquid might pass it if it did not have motion of its own. [Illustration: FIG. 23.--VORTICES OF THE STARS.] A variation on Descartes' system of vortices was proposed in the eighteenth century, which supposed that the sun, instead of being fixed in the centre of the system, itself circulated round another centre, carrying Mercury with it. This motion of the sun was intented to explain the changes of magnitude of its disc as seen from the earth, and the diurnal and annual variations in its motion, without discarding its circular path. [Illustration: FIG. 24.--VARIATION OF DESCARTES' THEORY.] We have thus noticed all the chief astronomical systems that have at any time been entertained by astronomers. They one and all have given way before the universally acknowledged truth about which there is no longer any dispute. Systems are not now matters of opinion or theory. We speak of facts as certain as any that can be ascertained in any branch of knowledge. We have much to learn, but what we have settled as the basis of our knowledge will never more be altered as far as we can see.