Underwater current

“ At these points the large volume of returning water has an easily seen retarding effect upon the incoming waves, thus adding to the condition causing the rip current. The waves on one or both sides of the rip, having greater energy and not being retarded by the concentration of backwash, advance faster and farther up the beach. From here, they move along the beach as feeder currents. At some point of low resistance, the water flows seaward through the surf, forming the neck of the rip current. Outside the breaker line the current widens and slackens, forming the head. ”

“ Now we may judge how exceedingly small a current this would produce when we consider that there is a rise of about two feet in the open ocean at one place and a fall of the same amount at another every six hours, caused by the tides, and yet the flowing of the water from the one place to the other place to produce this rise at one place, and fall at the other, it is well known does not produce any sensible currents in the open sea. ”

“ It is to these inequalities at the bottom of the sea, we must attribute the origin of currents, for if the bottom was equal and level, there would be no other current than the general motion from east to west, and a few others which might be caused by the action of the winds; but a certain proof that most currents are produced by the flux and reflux, and directed by the inequalities at the bottom of the sea, is, that they regularly follow the tides, and change their direction at each ebb and flow. ”

“ No man knows all the influences that affect the currents of the ocean. You may find the current in one place the same forty times in succession; on the forty-first trip it may be entirely changed. Sometimes a big storm that has ended four or five hours before the steamship passes a certain place may have given the surface current a strong set in one direction. There is no means of telling when these influences may have been at work save by taking the ship's position frequently. ”

“ The currents are so very slow in their movements, compared with the undulations of the tide wave, that it cannot be ascertained as yet, what are the magnitudes of such elements as inertia and friction, and how they are to be corrected for, so as to predict the time and velocity of the current from observations of the vertical rise and fall. ”

“ The velocity of all tidal currents would also be greater than at present, and as the power of a current of water for transporting solid material held in suspension increases rapidly with the velocity, so we may infer that the efficiency of tidal currents as a vehicle for the transport of comminuted rocks would be greatly increased. It is thus obvious that tides with a rise and fall double in vertical height of those which we know at present would add a large increase to their efficiency as geological agents. ”

“ Suppose this under current from the Mediterranean to extend one hundred and sixty fathoms down, so as to chafe the barrier across the Straits. Upon the bottom of this current, then, there is a pressure of more than fifty atmospheres. Have we not here a source of power that would be capable of drawing up, by almost an insensibly slow motion, water from almost any depth? At any rate, it appears that the effect of currents by traction, or friction, or whatever force, does extend far below the level of their beds in shallow places. ”

“ There are currents in the sea where it is 3000 fathoms deep, and some of them—as the Gulf Stream—run with a velocity of four miles an hour and even more. Every square foot of the earth's crust at the bottom of a four-knot current 3000 fathoms deep would have no less than 506,880—in round numbers, half a million—of such columns of water daily dragging, and rubbing, and scouring, and chafing over it, under a continuous pressure of 648 tons. ”

“ The results also indicate that when the current is at its maximum the surface flow is faster than at any depth below it, but when at its minimum the velocity at a depth of 15 fathoms or even down to 65 fathoms is greater than at the surface, and that there is at times a current running south along the bottom in all parts of the stream except on the extreme eastern side. ”

“ This is much less than the opposing force arising from the great specific gravity of the water in the northern part of the Atlantic on account of its lower temperature, as we have shown by calculations. The same reasoning may be applied to any other part of the ocean; for, if the salt of the ocean has any influence in producing currents, it must be to produce an undercurrent from the torrid zone, where evaporation is supposed to be in excess, toward the poles, and consequently, a counter-current at the surface from the poles toward the equator. ”

“ Disturbed equilibrium is the great cause of oceanic currents. Heat and cold are the chief agents in creating this disturbance.
It is obvious that when a portion of water in any vessel sinks, another portion must of necessity flow into the space which it has left, and if the cause which induced the sinking continue, so the flow to fill up will continue, and thus a current will be established.
Heat at the equator warms the sea-water, and makes it light; cold at the poles chills it, and makes it heavy. Hot water, being light, rises; cold water, being heavy, sinks. ”

“ This current receives volume and velocity from another cause, which is thus explained: "Immediately under the sun, or where the beams of that luminary are direct, a vacuum is produced, into which the circumambient air rushes; and as this vacuity is carried westward along the equator, upwards of 1,035 miles hourly, an atmospheric current follows, which, acting on the ocean waters, impels them westward, and adds force and mass to the tropic current. In the Atlantic Ocean, from the peculiar structure of its shores, a very remarkable phenomenon—the Gulf Stream—is produced. ”

“ The tides at the two ends of a strait are seldom in phase or equal in range, and the current, called hydraulic current, is generated largely by the continuously changing difference in height of water at the two ends. The speed of a hydraulic current varies nearly as the square root of the difference in height. The speed reaches a maximum more quickly and remains at strength for a longer period than shown in Figure 914b, and the period of weak current near the time of slack is considerably shortened. ”

“ The polar origin of the cold deep water found in low latitudes has long been considered probable, and has given rise to a theory of a general oceanic circulation in a vertical and horizontal direction, produced by differences of temperature and density. Recent theoretical investigations, however, seem to indicate that these causes alone are incapable of producing currents, and, to-day, the theory that the winds are mainly responsible for all current movements very largely predominates. ”

“ Tidal currents are caused by gravitational interactions between the sun, moon, and earth and are a part of the same general movement of the sea that is manifested in the vertical rise and fall, called TIDE. Tidal currents are periodic with a net velocity of zero over the tidal cycle. Nontidal currents include the permanent currents in the general circulatory systems of the sea as well as temporary currents arising from more pronounced meteorological variability. The SET of a current is the direction toward which it flows; the DRIFT is its speed. ”

“ All hands were busy on the new rigging. The captain had got up a spare spar and Old Tom Thornton and Stronson, went to work on that. The captain was determined to get up a new mizzen topmast and bend on new sails. Every square inch of canvas spread to the favoring breeze would aid us in the race home.
We had gotten now into the greatest ocean [181] current in the world—the Gulf Stream. Ocean currents are mysterious phenomena. The source of energy required to set and keep the vast masses of water in motion has been productive of endless discussion. ”

“ James Croll, the Scottish geologist, in his Climate and Time, and latterly the old theory that ocean currents are due to the trade-winds has again come into favor. Indeed, very recently a model has been constructed, with the aid of which it is said to have been demonstrated that prevailing winds in the direction of the actual trade-winds would produce such a current as the Gulf Stream. ”

“ And besides this, we have to consider that no part of the Gulf Stream flows strictly before the trade-winds. Where the current flows most rapidly, namely, in the Narrows of Bemini, it sets against the wind, and for hundreds of miles after it enters the Atlantic ‘it runs,’ says Maury, ‘right in the “wind’s eye.128”‘ It must be remembered that a current of air directed with considerable force against the surface of still water has not the power of generating a current which can force its way far through the resisting fluid. ”

“ Also of a low barometer.—But, be that as it may, the air which these vapour-bearing winds—vapour-bearing because they blow over such an immense tract of ocean—pour into this stopping-place has to ascend and flow off as an upper current, to make room for that which is continually flowing in below. In ascending it expands and grows cool, and, as it grows cool, condensation of its vapour commences; with this, vast quantities of latent heat, which converted the water out at sea into vapour for these winds, are set free in the upper air. ”

“ The innermost warm band is the one which exhibits the highest temperature and greatest rate of flow, its velocity being greatest where it is pressed on laterally by the Arctic Current, so that a rate of 4 miles per hour is occasionally observed. Capt. Nares estimates the depth of the Stream in this part of its course at about 100 fathoms, and its rate of flow in the line of most rapid movement at 3 miles per hour. The outermost band, on the other hand, graduates insensibly, both as to temperature and rate of movement, into the general sur face-water of the Atlantic. ”

“ This is the essential cause for the difference in strength between the currents observed in salt and fresh waters, for, of all the current-producing causes which act in fresh waters, only the one resulting from variations of temperature can sensibly affect the specific gravity, while the specific gravity of sea water, besides being much more affected by variation of temperature, is still further influenced by the fresh water which rains upon the surface of the ocean. ”

“ Since the incoming wave affects water which is at the same time under the influence of the undertow, it gives to that current a pulsating character, for the wave-motion sometimes supports and sometimes opposes the undertow, and thus endows it with a higher transporting power than belongs to its mean velocity. ”

“ Then the whole velocity at any point will be inversely proportional to the radius of the point, and the fluid will describe stream lines having a constant inclination to the radius drawn to the axis of the current. That is, the stream lines will be logarithmic spirals. When water is delivered from the circumference of a centrifugal pump or turbine into a chamber, it forms a free vortex of this kind. The water flows spirally outwards, its velocity diminishing and its pressure increasing according to the law stated above, and the head along each spiral stream line is constant. ”

“ During the flow in each direction, the speed varies from zero at the time of slack water to a maximum, called strength of flood or ebb, about midway between the slacks. Reversing currents can be indicated graphically, as in Figure 914b, by arrows that represent the speed of the current at each hour. The flood is usually depicted above the slack waterline and the ebb below it. The tidal current curve formed by the ends of the arrows has the same characteristic sine form as the tide curve. ”

“ No one can doubt that this agent is one of adequate power. Indeed, the winds, conceived by Franklin to be the primary cause of the Atlantic currents, are in reality due to the merest fraction of the energy inherent in the sun’s heat.
We have other evidence that the indraught is from below in the comparative coldness of the equatorial current. The Gulf Stream is warm by comparison with the surrounding waters, but the equatorial current is cooler than the tropical seas. ”

“ Normally current waters do not mix readily with the water mass through which they pass. 21 Because of the slowness of vertical circulation in the ocean, radionuclides deposited on the surface of the ocean may take a thousand years to reach the bottom. But the vertical transport sometimes is much more rapid: When the wind piles too much water against a coastline, the resultant downwelling (sinking) may move radionuclides suddenly into the deeper ocean. ”

“ Tidal scour may go on at any depth, provided the current is confined to a narrow channel, so as to obtain the requisite velocity. If in addition there is a to-and-fro motion, such as that caused by the Atlantic swell at depths of at least 50 fathoms, the actual current required to remove even coarse sand need only be very gentle. The oscillation in one direction may not reach the critical velocity ”

“ Where they strike the borders of land, the wind-waves, therefore, generate two other movements, the undertow and the littoral current. Any particle of water near shore may be affected by any two or by all three of these movements at the same moment. The effect of littoral current and undertow is to give a particle of water on which both are working a direction between the two, as be. The effect of other combinations can be readily inferred. These various combinations are of consequence in the transportation of débris.
WORK OF THE WAVES. Erosion. ”