ON THE

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 half-yearly 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

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 say, 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. 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 six-hourly, the monthly, the half-monthly, and the half-yearly; 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 granted, that this motion concerning which we are inquiring be one of these two,—either a motion of rising and falling of the waters, or a motion of progression. Now by motion of rising and falling I mean such motion as is found in boiling water, which rises up in the boiler and then sinks again; whereas by progressive motion I mean such as is found in water carried in a basin, which runs from one side up against the other. But that this motion is not of the first kind appears principally from this, that in the different parts of the world tides vary in point of time; so that in some places there is a flow and increase, when elsewhere there is an ebb and decrease. Now, if waters did not move from place to place but boiled up from the bottom, they ought to rise and fall everywhere at once. For we see that those two other motions, the half-monthly and the halfyearly, act and operate over the whole world at the same time. For the flow of the tide is increased everywhere at the equinox, not in some places at the equinox and in others at the tropics; and so it is with the half-monthly motion. For the tide is highest at the new moon everywhere, and at the quarter nowhere. In these two motions therefore the waters really seem plainly to rise and fall, and to have, as it were, their apogees and perigees like the celestial bodies. Now, in the ebb and flow of the sea, of which I am speaking, it is quite the contrary; which is the surest sign of motion in progression. Besides, if the flow of the tide be set down as a rising, we must observe somewhat more carefully how this rising is caused. For the swelling must be caused either by an increase in the quantity of water, or by an extension or rarefaction of the water in the same quantity, or by a simple lifting up in the same quantity and the same body. But this third cause is to be absolutely rejected. For if the water be lifted up as it is, there

must of necessity be a vacuum between the ground and the bottom of the water, since there is no body to take its place. And if there is a fresh body of water, it must emanate and spring from the earth. But if it be only an extension, that will be caused either by a solution into a rarer body, or by a desire of approaching some other body, which, as it were, summons out and attracts the water and raises it up. And certainly this, whether it be ebullition or rarefaction, or agreement of the waters with some one of the higher bodies, does not appear incredible, if it be in a moderate quantity, and a tolerable length of time likewise be allowed for the swelling or increase of the water to collect and rise. Therefore the excess of water observable between the ordinary tide and the half-monthly which is fuller, or even the half-yearly which is fullest of all, seeing that it is not greater than the difference between the flow and ebb, and has likewise a long enough interval to make this increase gradually,—is nothing contrary to reason. But that so great a mass of water should burst forth, as to account for the difference between the ebb and flow; and that this should be done so quickly, namely, twice a day; as if the earth, according to that foolish conceit of Apollonius, were taking respiration, and breathing out water every six hours and then taking it in again; is a very great difficulty. And let no one be influenced by the trifling experiment, that some wells in some places are said to have a correspondence with the ebb and flow of the sea; whence one might suspect that the waters inclosed in the cavities of the earth boil up in a like manner; in which case the swelling could not be well referred to the progressive motion of the waters. For the answer is easy, that the coming in of the tide may close up and fill many hollows and loose places of the earth, turn the subterraneous waters, and beat back the inclosed air; which in a continued succession may raise up the waters of such wells by simple protrusion. Therefore this does not happen in all wells, nor indeed in many; which should be the case if it were the nature of the universal mass of waters to rise and fall by turns, and to correspond with the tide of the sea. But on the contrary, it is so extraordinary as almost to be regarded as a miracle; because (no doubt) such openings and passages extending from wells to the sea are very seldom found without some stoppage or impediment.

And it is not

out of the way to mention what some say, that in deep mines near the sea the air becomes so thick on the flow of the tide as to threaten suffocation; from which it would appear not that the waters boil up (there being none seen), but that the air is driven back. But indeed there is another experiment which is not to be despised, but is of great weight, and by all means deserves an answer; namely, that it has been found by careful observation (not accidentally noticed, but purposely inquired and discovered) that the tide ebbs on the opposite coasts of Europe and Florida at the same time, and that it does not leave the coast of Europe when it moves to that of Florida, like water stirred in a basin (which I spoke of before), but that it plainly rises and falls on both coasts at the same time. But the solution of this objection will clearly appear in the observations I shall make presently on the course and progression of the ocean. Now the sum of the matter is this, that the waters which set out from the Indian Ocean, being obstructed by the opposition of the old and new worlds, are driven through the Atlantic from south to north; so no wonder that they approach equally at the same time to both shores, as waters use to do which are driven by the sea into the mouths and channels of rivers, wherein it is most evident that the motion of the sea is progressive with respect to the river, and yet overflows the opposite shores both at the same time. This however I candidly admit, as my manner is, and I would have men attend and remember it; if on experience it be found that it is high water on the coasts of Peru and China at the same time as on the above-mentioned coasts of Europe and Florida, my opinion that the ebb and flow of the sea is a progressive motion must be given up. For if it be high water at the same time on the opposite shores both of the Southern ocean and the Atlantic, there are no other shores left in the world where there can be at the same time a corresponding ebb. But on the result of an appeal to experience (to which I have submitted the cause) in this matter, I feel tolerably secure. For I am plainly of opinion that, if we knew how the case stands all over the world, we should find that the arrangement is fair enough, and that there is at any given hour an ebb in some parts of the globe equal to the flow in others. Wherefore, from what has been said, let this motion of ebb and flow be set down as a pro