BOOK III.] OF NATURAL PHILOSOPHY. 417 other side of the equator; L the place of the moon three Lours before; H the place of the earth directly under it ; h the opposite place ; K, k the places at 90 degrees distance ; CH, Ch, the greatest heights of the sea from the centre of the earth; and CK, Ck, its least heights: and if with the axes H//, K/.*, an ellipsis is described, and by the revolution of that ellipsis about its longer axis H/i a spheroid HPKhpk is formed, this sphe roid will nearly represent the figure of the sea; and CF, C/, CD, Cd, will represent the heights of the sea in the places F/, Dd. But far ther ; in the said revolution of the ellipsis any point N describes the circle NM cutting the parallels F/, Dd, in any places RT, and the equator AE in S : CN will represent the height of the sea in all those places R, S, T, situated in this circle. Wherefore, in the diurnal revolution of any place F, the greatest flood will be in F, at the third hour after the appulse of the moon to the meridian above the horizon ; and afterwards the great est ebb in Q,, at the third hour after the setting of the moon ; and then the greatest flood in/, at the third hour after the appulse of the moon to the meridian under the horizon ; and, lastly, the greatest ebb in Q,, at the third hour after the rising of the moon ; and the latter flood in / will be less than the preceding flood in F. For the whole sea is divided into two hemispherical floods, one in the hemisphere KH/J on the north side, the other in the opposite hemisphere Khk, which we may therefore call the northern and the southern floods. These floods, being always opposite the one to the other, come by turns to the meridians of all places, after an interval of 12 lunar hours. And seeing the northern countries partake more of the northern flood, and the southern countries more of the southern flood, thence arise tides, alternately greater and less in all places without the equator, in which the luminaries rise and set. But the greatest tide will happen when the moon declines towards the vertex of the place, about the third hour after the appulse of the moon to the meridian above the hori zon ; and when the moon changes its declination to the other side of the equator, that which was the greater tide will be changed into a lesser. And the greatest difference of the floods will fall out about the times of the solstices ; especially if the ascending node of the moon is about the Hrst of Aries. So it is found by experience that the morning tides in winter exceed those of the evening, and the evening tides in summer ex ceed those of the morning ; at Plymouth by the height of one foot, but at Bristol by the height of 15 inches, according to the observations of Colepress and Sturmy. But the motions which we have been describing suffer some alteration from that force of reciprocation, which the waters, being once moved, retain a little while by their vis insita. Whence it comes to pass that the tides may continue for some time, though the actions of the luminaries should 27