so mysterious are all the physical phenomena of the sea that it is, perhaps, hardly possible to say of any particular one that it is more wonderful than the rest. and yet one is sorely tempted thus to distinguish when meditating upon the movements of the almost inconceivable mass of water which goes to make up that major portion of the external superficies of our planet which we call “the sea.” in spite of all the labours of investigators, notwithstanding all the care and patience which science has bestowed upon oceanography, it is nevertheless true that, except in a few broad instances, the direction, the rate, and the dependability of ocean currents still remain a profound mystery. nor should this excite any wonder. if we remember how great is the influence over the sea possessed by the winds, how slight an alteration in the specific gravity of water is sufficient to disturb its equilibrium and cause masses hundreds of square miles in area to exchange levels with the surrounding ocean, we shall at once admit that, except in those few instances hinted at which may be referred to constant causes, ocean currents must of necessity be still among the phenomena whose operations cannot be reckoned upon with any certainty, but must be watched for[320] and guarded against with the most jealous care by those who do business in great waters.

perhaps one of the commonest of the many errors made in speaking of marine things is that of confounding current with tide. now tide, though a variable feature of the circulation of the waters near land, is fairly dependable. that is to say, the navigator may calculate by means of the moon’s age and the latitude of the place not only the time of high water, but knowing the mean height at full and change of the moon, he may and does ascertain to what height the water will rise, or how low it will fall at a certain place on a given date. true, a heavy gale of wind blowing steadily in or against the same direction of the ebbing or flowing tide will accelerate or retard, raise or depress, that tide at the time; but these aberrations, though most unpleasant oftentimes to riparian householders, are rarely of much hindrance or danger to navigation. this cannot be said of the currents of the sea. the tides have their limits assigned to them both inland and off-shore, although in the latter case it is almost impossible to tell exactly where their influence becomes merged in the vaster sway of the ocean currents, with all their unforeseen developments. the limits of tidal waters in rivers, on the other hand, being well under observation at all times, may be and are determined with the greatest exactitude.

with regard to the few instances of dependability among ocean currents, the first place will undoubtedly by common consent be given to the[321] gulf stream. owing its existence primarily to the revolution of the earth upon its axis, its outflow through the tortuous channel connecting the gulf of florida with the north atlantic is more constant and steady in direction than any ebbing or flowing tide in the world, inasmuch as its “set” is invariably upon one course. its rate is not so uniform, varying somewhat with the season, but in the narrowest part of the channel remaining fairly constant at about four knots an hour. yet sail but a few score leagues into the florida gulf whence this great river in the sea takes its apparent rise, and its influence disappears! the mariner may seek there in vain for that swift, silent flow which in the straits of florida sweeps him north-eastward irresistibly in the teeth of the strongest gale. what has happened? does the mighty stream drain westward into that great land-locked sea by hundreds of channels from the equatorial regions, but far below the surface, and, obeying some all-compelling impulse, rise to the light upon reaching the bahama banks, pouring out its beneficent flood as it comes at the rate of a hundred miles per day? it sweeps into the broad atlantic, and immediately spreads out into a breadth to which the amazon is but a brooklet, losing its velocity meanwhile, until, having skirted the north american coast as far as the grand banks, it rolls in sublime grandeur eastward towards these “fortunate isles.” as it does so the mystery attendant upon it deepens. its balmy presence cannot be mistaken, for the air on either side of it may be piercing in its keenness,[322] while immediately above it there is summer. a gale blowing at right angles to its course will raise that terrible combination of waves which gives alike to the “western ocean” and the “pitch of the cape” their evil reputation as the most dangerous in the world; and yet who among navigators has ever been able to determine what, if any, rate of speed it has in mid-atlantic? look through hundreds of log-books kept on board ships that are, perhaps, more carefully navigated than any others, the north atlantic liners, and you shall not find a trace of the gulf stream “set” mentioned. in order to make this clear, it should be said that in all properly navigated ships the course steered and the speed made are carefully noted throughout the twenty-four hours; and this course, with distance run, calculated from the position accurately fixed by observation of the celestial bodies at the previous noon, gives the ship’s position by “dead reckoning.” the ship’s position being also found by the celestial bodies at the same time, the difference between the latter and the “dead reckoning” position should give the “set” and direction of the current for the twenty-four hours. and in vessels so carefully steered, and whose speed is so accurately known, as the great liners are, such current data are as trustworthy as any nautical data can be. but according to the records kept by these able navigators, there is no current setting eastward across the north atlantic. perhaps the explanation is that it is so very sluggish as to be unnoticeable, for those dreadful monuments of misfortune[323] to themselves and others, the derelict ships, have been known to drift completely backwards and forwards across the atlantic, finding not only a current to carry them eastward, but its counter-current to carry them back again.

but who among us with the slightest smattering of physiography is there that is not assured that but for the genial warmth of this mighty silent sea-river our islands would revert to their condition at the glacial epoch; who is there but feels a shiver of dread pass over his scalp when he contemplates the possibility of any diversion of its life-giving waters from our shores? the bare suggestion of such a calamity is most terrifying.

as steady and reliable in its operations is the great equatorial current which, sweeping along the line from east to westward, is doubtless the fountain and origin of the gulf stream, although its operations among that ring of islands guarding the entrance to the mexican gulf are involved in such obscurity that none may trace them out. and going farther south, we find the agulhas current, beloved of homeward-bound sailing-ships round the cape of good hope, pursuing its even, resistless course around the southern horn of africa changelessly throughout the years. how its stubborn flow frets the stormy southern sea! no wonder that the early navigators doubling the cape outward-bound, and fearing to go south, believed that some unthinkable demon held sway over those wild waves. the passage of cape horn from east to west holds the bad eminence to-day among seafarers of being the most[324] difficult in the world, but what the outward passage around the cape of storms must have been before men learned that it was possible to avoid the stream of the agulhas current by going a few degrees south we of these later days can only imagine. what becomes of the agulhas current when once it has poured its volume of indian ocean waters into the atlantic? does it sink below the surface some hundreds of fathoms, and silently, smoothly, glide south to the confines of the antarctic ice barrier, or does it wander northward into warmer regions? in any case, it fulfils the one grand function of all currents, whether of air or water—the avoidance of stagnation, the circulation of health among the nations of the earth.

coming northward in the pacific, let us note the counterpart of the gulf stream, the kuro siwo, or black river of japan, with the multitudinous isles of the east indian archipelago for its caribbean sea, and nippon for its british isles. it is, however, but a poor competitor in benevolence with our own gulf stream, as all those who know their japan in winter can testify. others there are that might be noted and classified if this aimed at being a scientific article, but these will suffice. these are surely wide fields enough for the imagination to rove in, wonderful depths of energy in plenty wherein the reverent and thoughtful mind may find all-sufficient food for its workings. remembering that the known is but the fringe of the unknown, and that the secrets of the ocean are so well kept that man’s hand shall never fully tear aside the veil,[325] we may patiently ponder and wonder. that great sea of the ancients beyond whose portals, according to their wisdom, lay cimmerian darkness—what keeps its almost tideless waters sweet? unseen currents enter and leave by the pillars of hercules at differing levels, and could we but penetrate those dim regions we should doubtless find the ingress and egress of that incalculable mass of water proceeding continually, the one above the other, renewing from the exhaustless stores of the atlantic the staleness of the great midland lake, itself apparently remaining in unchanging level.

but when all these great well-known movements of the ocean have been considered, there still remain an infinite number of minor divagations influenced by who knows what hidden causes. the submarine upheavals of central heat, when from out of her glowing entrails the old earth casts incandescent stores of lava, raising the superincumbent mass of water for many square miles almost to boiling-point—who can estimate the effect that these throes have upon the trend of great areas of ocean? the almost infernal energy of those gyrating meteors of the tropics as they rage across the seas—how can any mind, however acute, assess the drag upon the whole body of surface water that is manifested thereby? to say nothing of the displacement caused by the less violent but far more frequent stress laid upon the much-enduring sea by extra-tropical gales, whereby the baffled mariner’s calculations are all overset, and his ship that should be careering safely in the[326] wide offing is suddenly dashed in ruins upon the iron-bound shore!

great efforts have been made to lay down for the benefit of seafarers a comprehensive scheme of ocean currents all over the watery surface of the globe, but in the great majority of cases the guidance is delusive, the advice untrustworthy, through no fault of the compilers. they have done their best, but mean results can never help particular needs. and so the wary mariner, as far as may be, trusts to the old-fashioned three “l’s,”—lead, log, and look-out; knowing full well how little reliance is to be placed in the majority of cases upon any advice soever concerning the mystery of ocean currents.