CHAPTER VIII.

§ 370-409.-CURRENTS OF THE SEA.

§ 370. WE here set out with the postulate that the sea, as well Obedient to order. as the air, has its system of circulation, and that this system, whatever it be, and wherever its channels lie, whether in the waters at or below the surface, is in obedience to physical laws. The sea, by the circulation of its waters, doubtless has its offices to perform in the terrestrial economy; and when we see the currents in the ocean running hither and thither, we feel that they were not put in motion without a cause. On the contrary, we know they move in obedience to some law of Nature, be it recorded down in the depths below, never so far beyond the reach of human ken; and being a law of Nature, we know who gave it, and that neither chance nor accident had any thing to do with its enactment. Nature grants us all that this postulate demands, repeating it to us in many forms of expression; she utters it in the blade of green grass which she causes to grow in climates and soils made kind and genial by warmth and moisture that some current of the sea or air has conveyed far away from under a tropical sun. She murmurs it out in the cooling current of the north; the whales of the sea tell of it (§ 158); and all its inhabitants proclaim it.

of the sea.

371. The fauna and the flora of the sea are as much the creaThe fauna and flora tures of climate (§ 164), and are as dependent for their well-being upon temperature as are the fauna and the flora of the dry land. Were it not so, we should find the fish and the algae, the marine insect and the coral, distributed equally and alike in all parts of the ocean. The arctic whale would delight in the torrid zone, and the habitat of the pearl oyster would be also under the iceberg, or in the frigid waters of polar seas.

372. Nevertheless, though the constituents of sea water be the Those of southern same in kind, we must not infer that they are the same in degree for all parts of the ocean, for there

unlike those of north

ern seas.

The capacity of wa

is a peculiarity, perhaps of temperature, perhaps of transparency, which marks the inhabitants of trans-equatorial seas. MM. Peron and Le Sueur, who have turned their attention to the subject, assert that out of many thousand cases they did not find a single one in which the inhabitants of trans-equatorial were not distinguishable from those of their species in cis-equatorial seas. 373. Water, while its capacities for heat are scarcely exceeded by those of any other substance, is one of the most ter to convey heat. complete of non-conductors. Heat does not permeate water as it does iron, for instance, or other good conductors. Heat the top of an iron plate, and the bottom becomes warm; but heat the top of a sheet of water, as in a pool or basin, and that at the bottom remains cool. The heat passes through iron by conduction, but to get through water it requires to be conveyed by a motion, which in fluids we call currents. Therefore the study of the climates of the sea involves a knowledge of its currents, both cold and warm. They are the channels through which the waters circulate, and by means of which the harmonies of old ocean are preserved.

to be considered in

pairs.

374. Hence, in studying the system of oceanic circulation, we Currents of the sea set out with the very simple assumption, viz., that from whatever part of the ocean a current is found to run, to the same part a current of equal volume is bound to return; for upon this principle is based the whole system of currents and counter-currents of the air as well as of the water. Hence the advantage of considering them as the anatomist does the nerves of the human system-in pairs. Currents of water, like currents of air, meeting from various directions, create gyrations, which in some parts of the sea, as on the coast of Norway, assume the appearance of whirlpools, as though the water were drawn into a chasm below. The celebrated Maelstrom is caused by such a conflict of tidal or other streams. The late Admiral Beechey, R.N.,* gave diagrams illustrative of many "rotatory streams in the English Channel, a number of which occur between the outer extremities of the channel tide and the stream of the oceanic or parent wave." "They are clearly to be accounted for," says he, "by the streams acting obliquely upon each other." * See an interesting paper by him on Tidal Streams of the North Sea and English Channel, pp. 703; Phil. Transactions, Part ii., 1851.

%

375. It is not necessary to associate with oceanic currents the Marine currents do idea that they must, of necessity, as on land, run

not, like those on

land, run of neces- from a higher to a lower level. So far from this

sity from higher to

lower levels. being the case, some currents of the sea actually run up hill, while others run on a level. The Gulf Stream is of the first class (§ 83).

376. The currents which run from the Atlantic into the MedThe Red Sea current. iterranean, and from the Indian Ocean into the Red Sea, are the reverse of this. Here the bottom of the current is probably a water-level, and the top an inclined plane, running down hill. Take the Red Sea current as an illustration. That sea lies, for the most part, within a rainless and riverless district. It may be compared to a long and narrow trough. Being in a rainless district, the evaporation from it is immense; none of the water thus taken up is returned to it either by rivers or rains. It is about one thousand miles long; it lies nearly north and south, and extends from latitude 13° to the parallel of 30° north. From May to October, the water in the upper part of this sea is said to be two feet lower than it is near the mouth.* This change or difference of level is ascribed to the effect of the wind, which, prevailing from the north at that season, is supposed to blow the water out. But from May to October is also the hot season; is is the season when evaporation is going on most rapidly; and when we consider how dry and how hot the winds are which blow upon this sea at this season of the year; that it is a narrow sea; that they blow across it, and are not saturated, we may suppose the daily evaporation to be immense. The evaporation from this sea and the Persian Gulf is probably greater than it is from any other arms of the ocean. We know that the waste from canals by evap oration, in the summer time, is an element which the engineer, when taking the capacity of his feeders into calculation, has to consider. With him it is an important element; how much more so must the waste by evaporation from this sea be when we consider the physical conditions under which it is placed. Its feeder, the Arabian Sea, is a thousand miles from its head; its shores are burning sands; the evaporation is ceaseless; it is a natural evaporating dish (§ 525) on a grand scale; none of the vapors which the scorching winds that blow over it carry away are returned to * Johnston's Physical Atlas.

M

it again in the shape of rains. The Red Sea vapors are carried off and precipitated elsewhere. The depression in the level of its head waters in the summer time, therefore, it appears, is owing to the effect of evaporation, as well as to that of the wind blowing the waters back. The evaporation in certain parts of the Indian Ocean is supposed to be (§ 103) from three fourths of an inch to inchi an daily. Whatever it be, it is doubtless greater in the Red Sea. Let us assume it, then, in the summer time to average only half an inch a day. Now, if we suppose the velocity of the current which runs into that sea to average, from mouth to head, twenty miles a day, it would take the water fifty days to reach the head of it. If it lose half an inch from its surface by evapo ration daily, it would, by the time it reaches the Isthmus of Suez, have lost twenty-five inches from its surface. Thus the waters of the Red Sea ought to be lower at the Isthmus of Suez than they are at the Straits of Babelmandeb. Independently of the forcing out by the wind, the waters there ought to be lower from two other causes, viz., evaporation and temperature; for the temperature of that sea is necessarily lower at Suez, in latitude 30°, than it is at Babelmandeb, in latitude 13°. To make it quite clear that the surface of the Red Sea is not a sea level, but is an inclined plane, suppose the channel of the Red Sea to have a perfectly smooth and level floor, with no water in it, and a wave ten feet high to enter the Straits of Babelmandeb, and to flow up the channel, like the present surface current, at the rate of twenty miles a day for fifty days, losing daily, by evaporation, half an inch; it is easy to perceive that, at the end of the fiftieth day, this wave would not be so high by two feet (twenty-five inches) as it was the first day it commenced to flow. The top of that sea, therefore, may be regarded as an inclined plane, made so by evaporation.

rents through straits

explained.

377. But the salt water, which has lost so much of its freshness Upper and under cur- by evaporation, becomes salter, and therefore heavier. The lighter water at the Straits can not balance the heavier water at the Isthmus, and the colder and salter, and therefore heavier water, must either run out as an under current, or it must deposit its surplus salt in the shape of crystals, and thus gradually make the bottom of the Red Sea a salt-bed, or it must abstract all the salt from the ocean to make the Red Sea brine-and we know that neither the one process nor the other is

going on. Hence we infer that there is from the Red Sea an under and outer current, as there is from the Mediterranean through the Straits of Gibraltar, and that the surface waters near Suez are salter than those near the mouth of the Red Sea. And, to show why there should be an outer and under current from each of these two seas, let us suppose the case of a vat of oil and a vat of wine connected by means of a narrow trough-the trough being taken to represent the straits connecting seas the waters of which differ as to specific gravity. Suppose the trough to have a floodgate, which is closed until we are ready for the experiment. Now let the two vats be filled, one with wine the other with oil, up to the same level. The oil is introduced to represent the lighter water as it enters either of these seas from the ocean, and the wine the same water after it has lost some of its freshness by evaporation, and therefore has become salter and heavier. Now suppose the flood-gate to be raised, what would take place? would run in as an upper current, overflowing the wine, and the wine would run out as an under current.

current.

Why, the oil

378. The rivers which discharge their waters into the MediterThe Mediterranean ranean are not sufficient to supply the waste of evaporation, and it is by a process similar to this that the salt which is carried in from the ocean is returned to the ocean again; were it not so, the bed of that sea would be a mass of solid salt. The equilibrium of the seas is a physical necessity. Were it to be lost, the consequences would be as disastrous as would be any other derangement in the forces of attraction. Without doubt, the equilibrium of the sea is preserved by a system of compensation as exquisitely adjusted as are those by which the "music of the spheres" is maintained. It is difficult to form an adequate conception of the immense quantities of solid matter which the current from the Atlantic, holding in solution, carries into the Mediterranean. In his abstract log for March 8th, 1855, Lieutenant William Grenville Temple, of the United States ship Levant, homeward bound, has described the indraught there: "Weather fine; made 11 pt. lee-way. At noon, stood in to Almiria Bay, and anchored off the village of Roguetas. Found a great number of vessels waiting for a chance to get to the westward, and learned from them that at least a thousand sail are weather-bound between this and Gibraltar. Some of them have been so for six weeks, and have even got as far as Malaga, only