OF THE FLUX AND REFLUX.
Water has but one natural motion; like other fluids it always descends from the higher into the lower places, unless obstructed by some intervening obstacle. When it reaches the lowest place it remains there calm and motionless, at least without some foreign causes which agitates and disturbs it. All the waters of the ocean are collected in the lowest parts of the surface of the earth, of course the motions of the sea must proceed from external causes, the principal of which is the flux and reflux, which is alternatively made in a contrary direction, and from which results a general and continual motion in the sea from east to west. These two motions have a constant and regular relation with the motions of the moon. When the moon is new, or at the full, this motion from east to west is more sensible, as well as that of the tides, which upon most shores ebb and flow every six hours and a half: that it is always high tide whenever the moon is at the meridian, whether above or below the horizon of the place; and low tide when the moon rises or sets. The motion of the sea from east to west is constant and invariable, because the ocean in its flux moves from east to west, and impels towards the west a great quantity of water, and the reflux seems to be made in a contrary direction, by reason of the small quantity of water then driven towards the west; the flux, therefore, must rather be regarded as a swelling, and the reflux as a subsiding of the water, which instead of its disturbing the motion from east to west, produces and continually restores it, although in fact it is stronger during the rise, and weaker during the fall, from the above reason.
The principal circumstances of this motion are, 1. That it is more sensible when the moon is new, or at the full, than in the quadratures: in spring and in autumn it is also more violent than at any other time of the year; and it is weaker in the solstices, which, is occasioned by the combination of the attraction of the moon and sun. 2. The wind often alters the direction and quantity of this motion, particularly that which constantly blows from the same quarter. It is the same with respect to large rivers which convey their waters into the sea and produce a current there, often extending several leagues, which is strongest when the direction of the wind agrees with the general motion. Of this we have an example in the Pacific Ocean, where the motion from east to west is constant and very perceptible. 3. We must remark that when one part of a fluid moves, the whole mass receives the motion; now in the motion of the tides a great part of the ocean moves in a very sensible manner, and consequently the ocean is agitated by this motion throughout its whole extent.
Perfectly to comprehend this we must attend to the nature of the power which produces the tides. We have observed that the moon acts upon the earth by a power called attraction by some, and by others gravity: this force penetrates through the globe, is exactly proportioned to the quantity of matter, and decreases as the square of the distance increases. Let us next examine what must happen to the waters when the moon is at the meridian of any one place.—The surface of the waters being immediately under the moon is then nearer that planet than any other part of the globe; hence this part of the sea must be elevated towards the moon, by forming an eminence, the summit of which must be opposite to the moon's centre; for the formation of this eminence the waters at the bottom, as well as at the surface, contribute their share, in proportion to the proximity they are in of the moon, which acts upon them in the inverse ratio of the squares of their distances: thus the surface of that part of the sea is first raised; the surface of the neighbouring parts will be likewise elevated, but to a less height, and the water at the bottom of all these parts will be raised by the same cause; so that all this part of the sea growing higher and forming an eminence, it is necessary that the water of the remote parts, and on which this force of attraction does not act, proceeds with precipitation to replace the waters which are thus elevated and drawn towards the moon. This is what produces the flux, or high tide, which is more or less sensible on different coasts, and which agitates the sea not only at its surface but even to the greatest depths. The reflux, or ebb, happens afterwards by the natural inclination of the water, for when the moon no longer uses its power, the water which was raised by this foreign power retakes its level, and returns to the shores and places it had been forced to quit. When the moon passes to the antipode, or opposite meridian, the same effect ensues, though from a different cause, In the first case the waters rise because they are nearer the planet than any other parts of the globe; and in the second it is from the contrary reason, they rise because she is the most remote from them; and this it is easily perceived must produce the same effect, for the waters of this part being less attracted than those of the opposite hemisphere, they will naturally recede and form an eminence, the summit of which will answer to the point of the least action that is directly opposite to the moon's station, or where she was thirteen hours before. When the moon arrives at the horizon the tide is ebb, the sea is then in its natural state, and the water in a direct equilibrium; but when she is at the opposite meridian this equilibrium can no longer exist, since the waters of the part opposite to the moon being at the greatest distance possible from her, they are less attracted than the remaining part of the globe, and hence their relative weight, which always retains them in an equilibrium, impels them towards the opposite point to the moon. Thus in the two cases, when the moon is at the meridian of a place, or at the opposite meridian, the water must be raised nearly to the same height, and consequently fall and rise, when the moon is at the horizon either at her rising or setting. Thus a motion, such as we have just mentioned, necessarily disturbs the whole mass of the sea, and agitates it throughout its whole extent and depth; and if this motion appears insensible in the open seas, it is nevertheless no less real; but as the winds cannot ruffle the bottom in an equal degree with the surface, the motion of the tides is necessarily more regular there, although directed alternately in the same manner as at the top.
From this alternative motion of flux and reflux there results, as already observed, a continual motion of the sea from east to west, because the moon, which produces the tides, proceeds from east to west, and successively acting in the same direction, the water follows her course. This motion is most considerable in all sraits; for example, at the straits of Magellan the water rises nearly 20 feet, and continues so for six hours, whereas the reflux lasts only two[C], and the water runs towards the west. This evidently proves that the reflux is not equal to the flux, and that from both there results a motion towards the west, much stronger in the time of the flux than in that of the reflux. This is the reason that in open seas, remote from land, the tides are only felt by the general motion of the waters from east to west.
[C] See Narborough's Voyage.
The tides are stronger in the torrid zone between the tropics than in the rest of the ocean: they are also more sensible in places which extend from east to west, in long and narrow gulphs, and on the coasts where there and isles and promontories. The greatest known flux is at one of the mouths of the river Indus, where the water rises thirty feet. It rises also very remarkably near Malays, in the straits of Sund, in the Red Sea, in Nelson's Bay, at the mouth of the river St. Lawrence, on the coasts of China, Japan, Banama, in the Gulph of Bengal, &c.
The motion of the sea from east to west is more sensible in particular places. Mariners have observed it in sailing from India to Madagascar and Africa; it is also very perceptible in the Pacific Sea, and between the Malaccas and Brazil: but this motion is most violent in the Straits; for example, the waters are carried with such great force in that direction through the Straits of Magellan that it is felt to a great distance in the Atlantic; and it is supposed that this caused Magellan to conjecture there was a strait by which the two seas had a communication. In the Manilla straits, and in all the channels which divide the Maldivian islands, the sea flows from east to west, as well as in the Gulph of Mexico, between Cuba and Jucatan. In the gulph of Paria this motion is so violent that the strait is called the Dragon's Mouth. In the Canadian and Tartarian Seas it flows also with violence, as well as in the Strait of Waigat, through which it conveys enormous masses of ice into the northern seas of Europe. The Pacific Ocean flows from east to west, through the Straits of Java; the sea of Japan flows towards China, the Indian Ocean flows towards the west, through the Straits of Java and other Indian islands; we cannot, therefore, doubt that the sea has a constant and general motion from east to west, and it is certain the Atlantic flows towards America, and that the Pacific Sea goes from it, as is evident at Cape Current between Lima and Panama.