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EBB AND FLOW OF THE SEA.
THE consideration of the causes of the ebb and flow of the sea, attempted by the ancients and afterwards dropped, taken up again by the moderns and yet by variety of opinions rather unsettled than discussed, is commonly by a light conjecture referred to the moon, by reason of some correspondence, between the motion of the tides and that of the moon. But yet if we look more closely we shall find some vestiges of truth which may lead to greater certainty. Therefore that there may be no confusion, we must first distinguish the motions of the sea, which, though some have very inconsiderately multiplied them, are in reality only five in number; whereof one is a kind of anomalous motion, the others constant. Let the first motion be set down as that wandering and various motion of the currents (as they call them). The second as that great motion of the ocean every six hours, by which the waters alternately approach and retire from the shore twice a day; not exactly, but with such a difference as makes the period of revolution a month. The third as the monthly motion itself, being no other than the restoration of the daily motion (before mentioned) to the same times. The fourth as the half-monthly motion, whereby the tides are increased more at the new and full
moons, than at the quarters. The fifth as the halfyearly motion, whereby the tides receive a great and remarkable increase at the equinoxes. Now it is of the second, or great diurnal motion of the ocean I intend principally to discourse at present; only touching on the others in passing, and as far as they tend to explain this motion. First therefore with respect to the motion of the currents, there is no doubt but that accordingly as the waters are either confined by straits, or released by open spaces ; either run and as it were pour down declivities, or encounter and run up acclivities ; either glide smoothly over a level, or are disturbed by the furrows and inequalities of the bottom ; either fall in with other currents with which they mingle and are carried along, or are agitated by the winds, especially the anniversary or periodical, which return at certain seasons of the year; there is doubt, I
that from these and similar causes waters vary their forces and eddies as well in the direction and course as in the velocity or measure of the motion, and that thence these currents are formed. In seas therefore the depth of the channel, the intervention of submarine rocks, chasms, the windings of shores, promontories, gulfs, straits, scattered islands, and the like, produce many effects, and drive the courses and streams of the waters to all points of the compass, to east and west, as well as to north and south, according to the positions and relative configurations of these obstructions, open spaces and declivities. Let therefore this particular and as it were fortuitous motion of the waters be set aside, that it may
not confuse us in the inquiry which we are pursuing. For it is not fair to deny the truth of what I shall presently propound with regard to the natural
and universal motions of the ocean, on the ground that this motion of the currents is at variance with my positions. For currents are mere compressions of water, or liberations from compression ; and are (as I have said) particular and respective to the positions of water and land, or even to the pressure of the wind. And this should be the more remembered and observed, because this general motion of the ocean, whereof I am now treating, is so mild and gentle, that it is entirely subdued and overpowered by the force of the currents, and yields to the impulse and direction of their violence. Now that this is so, is principally shown by the fact, that the simple motion of the ebb and flow of the sea is not felt in the middle of the sea, especially in vast and extensive seas, but only near the shores. Therefore no wonder if (being inferior in strength) it is hidden and as it were destroyed by the currents ; except that this very motion, when it is with the stream of the currents, somewhat assists and increases their force ; whereas when it is against the currents it slightly checks it. Dismissing then the motion of the currents, I go on to the four constant motions, the sixhourly, the monthly, the half-monthly, and the halfyearly; whereof the first alone seems to move and stir the flow of the sea, the second only to determine and restore that motion, and the two last to increase and strengthen it. For the ebb and flow of the sea, which floods the shores to a certain distance and then retires again, varies both at different hours and in the force and quantity of water, whereby the other three motions become visible. This motion therefore of ebb and flow must (as we propose) be distinctly and properly considered. And first it must absolutely be