all these suppositions were disproved, the very argument of Tycho Brahe for the stability of the earth turned the other way, and proved as incontestably that it moved. In the Copernican system, however, these questions are of no consequence; if the sun be at rest, this mass makes no difference; if the earth moves like the planets, their relative size does not alter anything; and if stars are immovable they may be at any distance and of any magnitude. The objections of Tycho Brahe to the earth's motion were: First, that it was too heavy--we know now, however, that some other planets are heavier--and that the sun, which he would make move instead, is 340,000 times as heavy. Secondly, that if the earth moved, all loose things would be carried from east to west; but we have experience of many loose things being kept by friction on moving bodies, and can conceive how, all things may be kept by the attraction of the earth under the influence of its own motion. Thirdly, that he could not imagine that the earth was turned upside down every day, and that for twelve hours our heads are downwards. But the existence of the antipodes overcomes this objection, and shows that there is no up and down in the universe, but each man calls that _down_ which is nearer to the centre of the earth than himself. A variation on Tycho Brahe's system was attempted by one Longomontanus, who had lived with him for ten years. It consisted in admitting the diurnal rotation, but not the annual revolution, of the earth; but it made no progress, and was soon forgotten. More remarkable than this was the attempt by Descartes in the same direction, namely, to hold the principles of Copernicus, and yet to teach the immobility of the earth. His idea of immobility was however very different from that of Tycho Brahe, or of any one else, and would only be called so by those who were bound to believe it at all costs. His Theory of Vortices, as it is called, will be best given in his own words as contained in his _Les Principes de la Philosophie_, third part, chap. xxvi., entitled, "That the earth is at rest in its heaven, which does not prevent its being carried along with it, and that it is the same with all the planets."