The flux and reflux of the sea is evidently so closely connected with the movements and changes of the moon, that the intimate relations between both could not possibly escape the penetrating sagacity of the Greeks. Thus we read in Plutarch, that Pytheas of Marseilles, the great traveller who sailed to the north as far as the Ultima Thule, and lived in the times of Alexander the Great, ascribed to the moon an influence over the tides. Aristotle expressed the same opinion, and Cæsar says positively (Commentaries, _De Bel. Gal._ book iv. 29,) that the full-moon causes the tides of the ocean to swell to their utmost height. Strabo distinguishes a three-fold periodicity of the tides according to the daily, monthly, and annual position of the moon, and Pliny expresses himself still more to the point, by saying that the waters move as if obeying the thirsty orb which causes them to follow its course. This vague notion of obedience or servitude was first raised by Kepler to the clear and well defined idea of an attractive power. According to this great and self-taught genius, all bodies strive to unite in proportion to their masses. "The earth and moon would mutually approach and meet together at a point, so much nearer to the earth as her mass is superior to that of the moon, if their motion did not prevent it. The moon attracts the ocean, and thus tides arise in the larger seas. If the earth ceased to attract the waters, they would rise and flow up to the moon." The general notion of a mutual attraction, however, did no more than point out the way for the solution of the problem, and it was reserved to our great Newton to accomplish the prophecy of his great predecessor, "that the discovery of the true laws of gravitation would be accomplished in a future generation, when it should please the Almighty Creator of nature to reveal her mysteries to man." Newton was the first who proved that the tide-generating power of a celestial body arises from the difference of the attraction it exerts on the centre and the surface of the earth. Thus it was at once made clear how the water not only rises on the surface facing the moon, but also on the opposite side of the earth, as in the latter case the moon acts more strongly on the mass of the earth than on the waters which cover the hemisphere most distant from her. The evident consequence is that the earth _sinks_ (so to say), on the surface turned from the moon, whereby a deepening of the waters, or, in other words, a rising of the tide, is occasioned. It now also became clear how the moon, whose attractive power upon the earth is 160 times smaller than that of the sun, is yet able to occasion a stronger tide, since, from her proximity to the earth, she attracts the surface more forcibly than the centre with the thirtieth part of her power, while the distant sun occasions a difference of attraction on these two points equal only to one twelve-thousandth part of her attractive force.