The tides at Tonquin are the most remarkable in the world. In this part there is but one tide and one ebb every twenty-four hours; whereas in other places there are two. Besides twice in each month there is no tide at all, when the moon is near the equinoctial, the water being for some time quite stagnant. These, with other peculiar appearances attending the same phenomena, were considered by many as inscrutable; but Sir Isaac Newton adjudged them to arise from the concurrence of two tides, one from the South Sea, and the other from the Indian Ocean. Of each of these tides there come successively two every day; two at one time greater, and two at another that are less. The time between the arrival of the two greater is considered by him as high tide; the time between the two lesser as ebb. In short, with this clue that great mathematician solved every appearance, and so established his theory as to silence every opposer. This fluctuation of the sea from the tides, observes the same author, produces another and more constant rotation of its waters from the east to the west, in this respect following the course of the moon.
This may be considered as one great and general current of the waters of the sea; and although it be not every where distinguishable, it is nevertheless every where existent, except when opposed by some particular current or eddy produced by partial and local causes. This tendency of the sea towards the west is plainly perceivable in all the great straits of the ocean; as for instance, in those of Magellan, in South America, where the tide running in from the east nearly twenty feet high, and continues flowing six hours, whereas the ebb continues but two hours, and the current is directed to the west. This proves that the flux is not equal to the reflux, and that from both results a motion of the sea westward, which is more powerful during the time of the flux than the reflux. This motion westward has been sensibly observed by navigators in their passage back from India to Madagascar, and so on to Africa. In the great Pacific, also, it is very perceivable; but the places where it is most obvious are, as it was said, in those straits which join one ocean to another. In the straits between the Maldivia Islands, in the gulph of Mexico, between Cuba and Jucatan. In the straits in the gulph of Paria, the motion is so violent, that it has received the appellation of the Dragon’s Mouth. Northward, in the sea of Canada, in Waigat’s straits, in the straits of Java, and in short, where the ocean on one part pours into the ocean on the other. In this manner is the sea carried with an unceasing circulation round the globe, and at the same time that its waters are pushed backward and forward with the tide; they have thus a progressive current to the west, which, though less observable, is not the less real. [19]
CHAPTER II.
THE ORIGIN OF CURRENTS THEIR VARIATION, EFFECTS, AND VELOCITY CONSIDERED.
Another impulse communicated to the waters of the ocean arises from its currents. These are caused by the winds blowing for many months in one direction, which produce on an expansive ocean movements of considerable magnitude: this may be easily conceived when we observe the effects produced on our own seas by the temporary action of the same cause.
A strong south-west or north-west wind invariably raises the tides to an unusual height along the east coast of England and the Channel. Smeaton ascertained by experiment that in a canal four miles in length, the water was kept up four inches higher at one end than at the other, merely by the action of wind along the canal; and Rennell informs us that a large piece of water, ten miles broad, and generally only three feet deep, has by a strong wind had its waters driven to one side, and sustained so as to become six feet deep, while the windward side was laid dry. He also observes, “As water, when pent up so that it cannot escape, acquires a higher level, so, in a place where it can escape, the same operation produces a current, and this current will extend to a greater or less distance according to the force by which it is produced.”
Currents flowing alternately in opposite directions are also occasioned by the rise and fall of the tides. The effect of this cause is, as before observed, in estuaries and channels between islands.
Evaporation by solar heat is another cause of oceanic currents, of which the great current setting through the Straits of Gibraltar into the Mediterranean, is a remarkable example. A stream of colder water always flows from the Black Sea into the Mediterranean. It must happen in many other parts of the world that large quantities of water, raised from one tract of the ocean by solar heat, are carried to some other, where the vapour is condensed, and falls in the shape of rain, and this, in flowing back again to restore equilibrium, will cause sensible currents. There is still another way in which heat and cold must occasion great movements in the ocean; a cause to which, perhaps, currents are principally due. It is now ascertained that there is in sea water no point, as in fresh water, at which an increase of cold causes the fluid to begin again to expand. In the ocean, therefore, whenever the temperature of the surface is lowered, condensation takes place, and the superficial water having its specific gravity increased, falls to the bottom, upon which lighter water rises immediately, and occupies its place. When this circulation of ascending and descending currents has gone on for a certain time in high latitudes; the inferior parts of the sea are made to consist of colder or heavier fluid than the corresponding depths of the ocean between the tropics. If there be a free communication, if no chain of submarine mountains divide the polar from the equatorial basins, a horizontal movement will arise by the flowing of colder water from the poles to the equator, and there will then be a reflux of warmer superficial water from the equator to the poles. A well-known experiment has been adduced to elucidate this mode of action in explanation of the “trade winds.” If a long trough, divided in the middle by a sluice or partition, have one end filled with water, and the other with quick silver, both fluids will remain quiet so long as they are divided, but when the sluice is drawn up, the heavier fluid will rush along the bottom of the trough, while the lighter, from being displaced, will rise, and flowing in an opposite direction, spread itself at the top. The expansion and contraction of sea water by heat and cold, have in a similar manner, a tendency to set under currents in motion from the poles to the equator, and to cause counter currents at the surface, which are impelled contrary to that of prevailing-trade winds. The geographical and other circumstances being very complicated, we cannot expect to trace separately the movements due to each cause, but must be prepared for many anomalies, especially as the bed of the ocean must often modify and interfere with the course of the inferior currents, as much as the position and form of continents and islands alter the direction of those on the surface. Thus, on sounding at great depths in the Mediterranean, Captains Berard and D’Urville have found that the cold does not increase in a high ratio, as in the tropical regions of the ocean, the thermometer remaining fixed at about 55° F. between the depths of 1000 and 6000 feet; and Captain Smith has shown in his survey, that the deepest part in the Straits of Gibraltar is only 1320 feet, so that a submarine barrier exists there, which must prevent the influx of any under current of the ocean cooled by the polar ice.
The rotation of the earth on its axis is another cause which can only come into play when the waters have been already set in motion by some one or all of the forces above described, and when the direction of the current so raised happens to be from south to north, or from north to south, the principle on which this operates has been long recognized in the case of trade winds; thus, when a current flows from the Cape of Good Hope towards the Gulph of Guinea, it consists of a mass of water, which, on doubling the Cape, in latitude 35°, has a rotatory velocity of about 800 miles an hour; but when it reaches the line, it arrives at a parallel where the surface of the earth is whirled round at the rate of 1000 miles an hour, or about 200 miles faster. If this great mass of water was transferred suddenly from the higher to the lower latitude, the deficiency of its rotatory motion, relatively to the land and water with which it would come into juxta position, would be such as to cause an apparent motion of the most rapid kind (of no less than 200 miles an hour) from east to west. [23]
In the case of such a sudden transfer, the eastern coast of America being carried round in an opposite direction, might strike against a large body of water with tremendous violence, and a considerable part of the continent might be submerged. This disturbance does not occur, because the water of the stream, as it advances gradually into new zones of the sea, acquires by friction an accelerated velocity. Yet as this motion is not imparted instantaneously, the fluid is unable to keep up with the full speed of the new surface over which it is successively brought; and Herschel, in his Treatise on Astronomy, observes, when speaking of the trade winds, it lags or hangs back in a direction opposite to the earth’s rotation, that is from east to west; [24a] and thus a current which would have run simply towards the north but for the rotation, may acquire a relative direction towards the west, or become a south-easterly current. [24b]