that if true a stone dropped from a tower would not fall at its foot, is met by the fact that in a moving vessel a stone dropped from the top of a mast does fall at its foot, although the Aristotelians, not having tried the experiment, maintained that it would fall towards the stern. On the score of simplicity, therefore, a daily rotation of the earth, once admitted as possible, is obviously far more probable than a daily rotation of the whole universe about the earth. The precession of the equinoxes is brought in finally to show how much more complicated and improbable, are the motions necessary to reconcile the facts with the Ptolemaic theory. On the third day the revolution of the earth about the sun is considered, of course as a mere hypothesis. The great difficulty contemplated by Copernicus was that, if the earth really went round the sun, in an orbit nearly two hundred million miles in diameter, the stars ought to show a displacement corresponding to the great change in view-point through the year. The modern reply is that they do, but this could not be proved until nearly two centuries later, though Galileo, speaking as Salviati, grasped the position very clearly. He pointed out that on the one hand the stars must be so far off that the relative displacement of nearer ones is too small to be detected by his instruments, while on the other hand their apparent size, which would on the hypothesis of enormous distance lead to inconceivable dimensions, is an optical illusion; this is shown by their remaining as points when viewed through a telescope, unlike the comparatively near planets which show discs under sufficient magnifying power. Reference to Gilbert's work on the magnet indicates that Galileo had some inkling of the idea that crystallised later in Newton's theory of universal gravitation. The fourth day elaborates Galileo's erroneous theory of the tides, to which previous reference has been made.
