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[TRANSLATED FROM THE LATIN.]
OF THE EBB AND FLOW OF THE SEA.
THE investigation of the causes of the ebb and | which return at regular periods of the year. That flow of the sea, attempted by the ancients and in consequence of these and similar causes, they then neglected, resumed by the moderns, but rather frittered away than vigorously agitated in a variety of opinions, is generally, with a hasty anticipation, directed to the moon, because of certain correspondences between that motion, and the motion of that orb. But to a careful inquirer certain traces of the truth are apparent, which may lead to surer conclusions. Wherefore, to proceed without confusion, we must first distinguish the motions of the sea, which, though thoughtlessly enough multiplied by some, are in reality found to be only five; of these one alone is eccentric, the rest regular. We may mention first the wandering and various motions of what are called currents: the second is the great sixhours motion of the sea, by which the waters alternately advance to the shore, and retire twice a day, not with exact precision, but with a variation, constituting monthly periods. The third is the monthly motion itself, which is nothing but a cycle of the diurnal motion periodically recurring: the fourth is the half-monthly motion, formed by the increase of the tides at new and full moon, more than at half-moon: the fifth is the motion, once in six months, by which, at the equinoxes, the tides are increased in a more marked and signal manner.
It is the second, the great six-hours or diurnal motion, which we propose for the present as the principal subject and aim of our discourse, treating of the others only incidentally and so far as they contribute to the explanation of that motion.
First, then, as relates to the motion of currents, there is no doubt that to form it the waters are either confined by narrow passages, or liberated by open spaces, or hasten as with relaxed rein, down declivities, or rush against and ascend elevations, or glide along a smooth, level bottom, or are ruffied by furrows and irregularities in the channel, or fall into other currents, or mix with them and become subject to the same influences, or are affected by the annual or trade winds,
vary their states of flow and eddy, both as relates
are adverse considerably restrains them. Waiving and supply its place. If there were a fresh then the motion of the currents, we proceed to the four regular motions; that in the six hours, in the month, in the half month, and in six months, of which the sexhorary motion alone seems to produce and develope the ordinary tide, the monthly to determine that motion and define its renewal; the half-monthly and half-yearly to increase and strengthen it. For the ebb and flow, which cover and quit again a certain extent of shore, both vary at various hours, and according to the momentum and quantity of the water; whence these three other motions are rendered more perceptible.
We must, therefore, contemplate, singly and specifically, as we purposed, the motion of ebb and flow. And, first, it is necessary to grant that this motion, the subject of inquiry, is one of these two: either the motion of an elevation and depression, or the motion of a progression of the waters. The motion of elevation and depression we understand to be such, as is found in boiling water, mounting and subsiding alternately in a caldron: the motion of progression to be such as is observed in water carried in a basin, which quitting the one side, is projected to the opposite. Now, that the motion we treat of is not of the former sort, is in the first place suggested by this fact, that in different parts of the world the tides vary according to the times, so that in certain places there are floods and accumulations of the mass of waters, in others at the same hours ebb and diminutions. Now, the waters, if they did not travel from place to place, but rose ebullient from the bottom, ought to rise everywhere at once, and to subside together. For we see those two other motions, the monthly and half monthly, in full movement and operation at the same periods throughout the globe. For the waves increase at the equinoxes in all parts, not in certain places under the equator, or in others under the tropics: and the same is true of the half-monthly motion. For, everywhere over the world, the waters are elevated at new moon and full moon, nowhere at half-moon. The waters, therefore, are manifestly raised, and again depressed in these two motions, and like the heavenly bodies have their apogees and perigees. But in the ebb and flow of the sea, which we now discuss, the contrary takes place, an unequivocal sign of progressive motion. Besides, ere we set down the flow of the sea as an elevation of the waters, we ought to consider a little more carefully how that elevation can take place. For the swelling must either be produced by an augmentation of the mass of waters, or from an extension or rarefaction of fluid in that mass, or from simple elevation of the mass or body. The third supposition we must dismiss entirely. For if the water united in the same body were lifted up, a vacuum would necessarily be left between the earth and the under face of the water, there being no body ready to succeed
quantity of water added, it must be by flowing and eruption from the earth. If there were dilatation only, this must take place either by solution into greater rarity, or by a tendency to approach another body, which, as it were, evokes the waters, attracts them, and lifts them to greater elevation. And, doubtless, that state of the waters, whether considered as ebullition, or rarefaction, or harmony with some one or other of the heavenly bodies, cannot seem incredible, that is, to a moderate extent, and on the supposition of the lapse of considerable time, in which such swellings and accretions may gather and accumulate. Therefore the difference observable between the ordinary, and the half-monthly tide, or the most copious of all, the half-yearly one, in which the addition to the mass of waters is not equal to the difference between ordinary ebb and flow, and has besides a large interval of time insensibly to form, may, on the hypothesis of elevation and depression, be consistently explained. But that so great a mass of water should burst forth as to explain that difference which is found between the ebb and flow, and that this should take place with such extreme rapidity, namely, twice a day, as if the earth, according to the fantastic notion of Apollonius, performed respiration, and breathed waters every six hours, and then again inhaled them, is very hard to believe. And let no man be misled by the unimportant fact that in some places wells are said to have a simultaneous motion with the ebb and flow of the sea, whence one might conjecture, that waters enclosed in the entrails of the earth boil up in like manner, in which case that swelling of the waters cannot be attributed to a progressive motion. For the answer is an easy one, that the flow of the sea by its encroachment may perforate and gorge many hollow and loose places of the earth, turn the course of subterraneous waters, or cause a reverberation of the enclosed air, which by a continued series of impulsions may raise the water in this sort of wells. Accordingly, this does not take place in all wells, nor even in many, which ought to be the case if the entire mass of waters had a property of periodically boiling up, and a harmony with the tide. But, on the contrary, this rarely happens, so as to be regarded almost as a miracle, because, in fact, such apertures and spiracles as reach from wells to the sea, without circuity or impediment, are very rarely found; nor is it unimportant to mention, what some relate, that in deep pits situated not far from the sea, the air becomes thick and suffocating at the time of ebb, from which it may seem manifest, not that the waters boil up, (for none are seen to do so,) but that the air is reverberated. No doubt, there is another objection, not despicable, but of great weight, every way deserving of an answer, one which had been the subject of careful observation,
ately follow from this, and we would have men note the observation, that those things which agree in their periods and curriculum of time, or even in their mode of relation, are of a nature subjected the one to the other, and stand respectively as cause and effect. Thus we do not go so far as to affirm, that the motions of the sun ought to be set down as the causes of the inferior motions which are analogous to them; or that the sun and moon (as is commonly said) have dominion over these motions of the sea, although such notions are easily insinuated into our minds from veneration of the heavenly bodies; but in that very half-monthly motion, if it be rightly noted, it were a new and surprising kind of subjection to influence, that the tides at new and at full moon should be affected in the same manner, when the moon is affected in contrary ways; and many other things might be instanced, destroying similar fancies of this sort of dominant influence, and leading to this inference, that those corres
and that not incidentally, but a thing especially bodies. Notwithstanding, it will not immedi and of purpose inquired into and discovered, namely, that the water at the opposite shores of Europe and of Florida ebb at the same hours from both shores, and do not quit the shore of Europe when they roll to the shore of Florida, like water (as we have said before) agitated in a basin, but are manifestly raised and depressed at either shore at once. But a clear solution of this objection will be seen in the observations which shall presently be made about the path and progression of the ocean; the substance, however, is this; that the waters, setting out in their course from the Indian ocean, and obstructed by the remora of the continents of the old and new world, are impelled along the Atlantic from south to north; so that it is no wonder if they are driven against either shore equally at the same time, as waters are wont to be, which are propelled from the sea into estuaries and up the channels of rivers, evidently showing that the motion of the sea is progressive as respects the rivers, and yet that it at once inundates both shores. Notwith-pondences arise from the catholic affections of standing, according to our custom we freely confess, and would have men observe and remember, that if it is found in experience that the tide advances at the same time on the coast of China and Peru, as on that of Europe and Florida, this our opinion, that ebb and flow is a progressive motion of the sea, must be repudiated.
For if the flow of the sea takes place at the same time at the opposite shores, as well of the Pacific or Southern Ocean as of the Atlantic Ocean, there are not in the universe any shores remaining, at which a corresponding ebb, at the same time, might afford a satisfactory solution of the objection. But we propose with confidence of a trial of this by experiment, to whose test we submit our cause: for we are clearly of opinion, that were the general result of a trial of this fact through the world known to us, this compact of nature would be found effected on sufficiently reciprocal conditions, namely, that at any given hour as much reflux took place in some parts of the world as flow in others. Therefore, from what we have stated, this motion of ebb and flow may be affirmed progressive.
matter, from the primary concatenation of causes, and connexion of things; not as if such were governed the one by the other, but both flowed from the same sources and from joint causes. Notwithstanding this, however, it remains true, as we have said, that nature delights in harmony, and scarcely admits of any thing isolated or solitary. We must therefore look, in treating of the sexhorary ebb and flow of the sea, with what other motions it is found to agree and harmonize. And first we must inquire with respect to the moon, in what manner that motion blends relations or natures with the moon. But this we do not see prevail except in the monthly repairing of the moon, for the periodical course of six hours has no affinity with the monthly course; nor again are the tides found to follow any affections of the moon. For, whether the moon be crescent or waning, whether she be under the earth or above the earth, whether her elevation above the horizon be higher or lower, whether her position be in the zenith or elsewhere, in none of these relations do the ebb and flow of the tide correspond with her.
Therefore, leaving the moon, let us inquire Now follows the inquiry, from what cause and concerning other correspondences; and from all what combination of things this motion of ebb the motions of the heavenly bodies, it is certain and flow arises and is presented to view. For all that the diurnal motion is the shortest, and is the great movements (if these be regular and per- accomplished in the least period of time, that is, petual) are not isolated, or (to use here an expres- in the space of twenty-four hours. It is therefore sion of the astronomers) ferine, but have some- in harmony with this, that the motion of which thing in nature with which they move harmoni- we inquire, which is yet three times shorter than ously. Therefore those motions, as well as the the diurnal one, should be referred immediately half-monthly one of increase as the monthly of to that motion which is the shortest of the reparation, appear to accord with the motion of the heavenly ones. But this notion has no great moon; and again the half-monthly, or equinoctial, weight with us in this matter. Another hypowith the motion of the sun; also the elevations thesis has more influence with us, that this motion and depressions of the water, with the approxi- is so distributed, that, though the motion of the mation and revolution in the orbits of the heavenly waters is slower by innumerable degrees, still it
15 referable to a common measure. For the space of six hours is a quarter of the diurnal motion, which space (as we said) is found in that motion of the sea, with a difference coinciding with the measure of the moon's motion. Whereupon this belief sinks deep into our mind, and looks as it were an oracular truth, that this motion is of the same kind with the diurnal motion. With this, therefore, as a basis, we shall proceed to a thorough inquiry: and we think that the whole subject is exhausted in three points of investigation.
The first is, whether that diurnal motion is confined within the regions of heaven, or descends, and penetrates to the lower parts? The second is, whether the seas move regularly from east to west, as the heaven does? The third, whence and how that six hours' motion of the tides takes place which coincides with a quarter of the diurnal motion, with a difference falling in with the measure of the moon's motion. Now, as relates to the first inquiry, we think that the motion of rotation, or of turning from east to west, is not properly a motion merely of the heavenly bodies, but manifestly of the universe, and a primary motion in all the great fluids, found to prevail from the highest part of heaven to the lowest part of the waters, in direction the same in all, in impulse, that is, in rapidity and slowness, widely different; in such wise, however, that in an order not in the least confused, the rapidity is diminished in proportion as the bodies approach the globe of the earth. Now this, it seems, may be taken as a probable reason for supposing that that motion is not limited to the heavens, because it prevails and is in force through so great a depth of heaven as lies between the starry heaven and the moon, (a space much more extensive than that between the moon and the earth,) with a regular diminution; so that it is probable that nature does not at any point abruptly break off a harmonious motion of this kind, diffused through such vast spheres and gradually lessening. And that this is so in the heavenly bodies is evinced by two inconsistencies, which follow from the opposite hypothesis. For, since the planets visibly perform a diurnal motion, unless we are to suppose that motion natural and self-moved in all the planets, we must unavoidably have recourse for an explanation either to the supposition of the primum mobile, which is evidently opposed to nature; or to the rotation of the earth, which is a notion extravagant enough, if we look to the methods of nature. Therefore, the motion exists in the heavenly bodies. And, quitting heaven, that motion is most distinctly visible in the inferior comets; which, though lower than the orb of the moon, evidently move from east to west. For, though they have their solitary and eccentric motions, yet in performing them they for a time have a common movement, and are borne along with the motion of the ether,
and with the same conversion: but in the tropics they are not generally so confined, nor move in the regular course, but sometimes straggle towards the poles, yet, nevertheless, pursue their rotatory motion from east to west. And thus this motion, though it suffers great diminution, since the nearer it descends towards earth the conversion is performed in smaller circles, and more slowly, still remains powerful, so as to traverse great distances in a short time. For these comets are carried round the whole circumference, both of the earth and the lower atmosphere, in the space of twenty-four hours, with an excess of one or two hours more. But after, by a continued descent, it has reached these regions upon which the earth acts, this motion, not only by the com│munication of the earth's nature and influence, which represses and lowers circular motion, but also by a substantial immission of the particles of its matter, by means of vapours and gross exhalations, becomes infinitely relaxed, and almost falls off, yet it is not therefore wholly annihilated or ceases, but remains feeble and verging to imperceptible. For mariners now begin to confess that between the tropics, where, in the open sea, the motion of the air is best perceived; and where the air itself, as well as heaven, revolves in a larger circle, and therefore more rapidly, that a perennial and gentle breeze blows from east to west, insomuch that those who wish to use the south-west wind often seek and avail themselves of it outside the tropics. Consequently, this motion is not extinguished, but becomes languid and obscure, so as to be scarcely perceptible outside the tropics. Yet, even outside the tropics, in our own part of the globe, Europe, at sea, in serene and peaceful weather, there is observed a certain wind, which is of the same species; we may even conjecture that what we experience here in Europe, where the east wind is sharp and dry, and, on the contrary, the south-west winds are cherishing and humid, does not depend merely on the circumstance that the one blows from a continent, the other from the ocean, but on this, that the breath of the east wind, since it is in the same train with the proper motion of the air, accelerates and heightens that motion, and therefore disperses and rarefies the air, but that of the west wind, which is in the contrary direction to the motion of the air, makes it rebound upon itself, and become inspissated. Nor ought this to be neglected, which is admitted into the number of common observations, that the clouds which are in motion in the upper part of the air generally move from east to west; while the winds about the earth's surface generally blow at the same time the contrary way. And if they do not this always, the reason is this, that there are sometimes opposite winds, some acting on the high, others on the lowest exhalations. Now, those blowing on high, if they be adverse, confound the real motion of the
air. It is sufficiently clear, then, that the motion ing, and reverberated by the interposition of lands, is not confined within the limits of heaven.
Then follows in order the second inquisition: whether the waters move regularly from east to west. Now, when we speak of waters, we mean those accumulations or masses of waters which are such large portions of nature as to have a relation of harmony to the fabric and system of the universe. And we are fully of opinion that the same motion is natural to, and inherent in, the body of waters, but is slower than in the air; though, on account of the grossness of the body, it is more palpable and manifest. Of this we shall content ourselves with three selected from many experimental proofs, but these weighty and marked ones, which prove that this is so.
The first is, that there is found a manifest motion and flow of waters from the Indian Ocean, even to the Atlantic, and that more swift and strong towards the Straits of Magellan, when an outlet is opened to them westwards; and a great current also on the other side of the world from the Northern Ocean to the British Sea. And these currents of waters manifestly roll from east to west; in which fact we must note in the first place, that in those two places alone the seas find thoroughfares, and can describe in flowing a complete circle: whereas, on the contrary, at the central regions of the globe, by the two ramparts of the old and new world, they are thrown off and driven (as it were into the estuaries of rivers) into the basins of the Atlantic and Pacific, the two oceans extending between the south and north, and open to the motion of a current from east to west. So that the true course of the waters is inost safely inferred from the extremities of the globe, as we have stated, where they meet with no impediment, but sweep round in full circuit. And the first experiment is thus, the second is the following.
Let us suppose that the tide takes place at the mouth of the Straits of Gibraltar at any given hour: it is certain that the tide sets in at Cape St. Vincent later in the day than at the mouth of the Straits at Cape Finisterre later than at Cape St. Vincent, at King's Island later than at Cape Finisterre, at the Island Heek later than at King's Island, at the entrance of the English channel later than at Heek,—at the shore of Normandy later than at the entrance of the channel. Thus far in regular order: but at Graveling, as if by an entire inversion of the order, and that with a great leap, as it were, at the same hour, with a velocity like that which it has at the mouth of the Straits of Gibraltar. This second observation we apply to, and compare with the first. For we think, as has already been said, that in the Indian and northern oceans the true currents of the waters, that is, from the east to the west, are open and unimpeded, but in the channels of the Atlantic and Southern Oceans imprisoned and cross
which extend both ways longitudinally from south to north; and nowhere but toward their extremities afford a free canal to the waters. But that strong direction of the waters, which is caused by the Indian Ocean towards the north, and in the opposite direction from the North Sea towards the South, differ infinitely in the extent of sea, affected on account of the different force and quantity of waters. But that this should take place is unavoidable. For the two great islands of the old and new world have the same figures, and are so stretched out as to broaden to the north, and taper to the south. The seas, therefore, on the contrary, towards the south occupy a vast space, but to the north a small one, at the back of Asia, Africa, and America; consequently, that great mass of waters which is discharged from the Indian Ocean, and is refracted into the Atlantic, is capable of forcing or propelling the course of the waters in a continued movement nearly to the British Sea, which is a part of the line described northwards. But that much smaller portion of the waters which issues from the north sea, and which has also a free passage westwards at the back of America, is not strong enough to turn the course of the waters southwards, except towards that point which we mentioned, namely, about the British Sea. Now, in these opposite currents, there must be some goal where they meet and contend, and where within short space the order of advance is suddenly changed, as we have said occurs about Graveling-the focus of the currents from the Indian and Northern Oceans, and that a certain ocean stream is formed by opposite currents on the coast of Holland has been noted by numbers, not only from the inversion of the hour of the tide, which we have stated, but also from the peculiar visible effect. Now, if this is so, we return to the position, that it must needs be, that in proportion as the parts and shores of the Atlantic extend southwards and approach the Indian Sea, in the same proportion the tide is prior, and early in the order of approach, and in proportion as you go northwards, (as far as their common goal,) where they are forced back by the antagonist stream of the Northern Ocean, they are backward and late. Now, that this is the case, the observation of the progression from the Straits of Gibraltar to the British Sea manifestly proves. Wherefore we think that the tide about the shores of Africa is at an earlier hour than that of the Straits of Gibraltar, and, in reversed order, the tide about Norway earlier than the tide about Sweden-but this we have not ascertained by experiment or testimony.
A third experiment is the following: The seas confined by land on one side, which we call bays, if they stretch out with any inclination from east to west, which is in the same line of impetus with the true motion of the waters, have heavy