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}
The most extensive and best determined system of currents is that which
has its source in the Indian Ocean, under the influence of the trade
winds; and which, after doubling the Cape of Good Hope, inclines to the
northward, along the western coast of Africa; then crosses the Atlantic
near the Equator, and is lost in the Caribbean Sea; yet seems to be again
revived in the current which issues from the Gulph of Mexico, by the
Straits of Bahama, and flows rapidly in a north-easterly direction, by
the bank of Newfoundland, towards the Azores.
Rennell informs us, that the Lagullas current, so called from the cape
and bank of that name, is formed by the junction of two streams flowing
from the Indian Ocean, the one from the channel of Mozambique, down the
south-east coast of Africa, the other from the ocean at large.—The
collective stream is from ninety to one hundred miles in breadth, and
runs at the rate of from two and a half to more than four miles per hour.
It is at length turned westward by the Lagullas bank, which rises from a
sea of great depth, to within one hundred fathoms of the surface. It
must therefore be inferred, says Rennell, that the current here is more
than one hundred fathoms deep, otherwise the main body of it would pass
across the bank, instead of being deflected eastward, so as to flow round
the Cape of Good Hope. From this Cape it flows northward, along the
western coast of Africa, taking the name of the South Atlantic current.
It then enters the Bight or Bay of Benin, and is turned westward, partly
by the form of the coast there, and partly, perhaps, by the Guinea
current, which runs from the north into the same great bay. From the
centre of this bay proceeds the Equatorial current, holding a westerly
direction towards the Atlantic, which it traverses from the coast of
Guinea to that of Brazil, flowing afterwards by the shores of Guiana to
the West Indies. The breadth of this current varies from one hundred and
sixty to four hundred and fifty geographical miles, and its velocity is
from twenty five to seventy nine miles per day, the mean rate being about
thirty miles. The length of its whole course is about four thousand
miles. As it skirts the coast of Guiana, it is increased by the influx
of the waters of the Amazon and Orinoco, and by their junction acquires
accelerated velocity. After passing the island of Trinadad, it expands,
and is almost lost in the Caribbean Sea; but there appears to be a
general movement of that sea towards the Mexican Gulph, which discharges
the most powerful of all currents through the Straits of Florida, where
the waters run in the northern part with a velocity of five miles an
hour, having a breadth of from thirty five to fifty miles. {25}
The temperature of the Gulph of Mexico is 86° in summer, or 6° higher
than that of the ocean in the same parallel (25° N. lat.) and a large
proportion of this warmth is retained, even where the stream reaches the
43° N. lat. After issuing from the Straits of Florida, the current runs
in a northerly direction to Cape Hatteras, in North Carolina, about 35°
N. lat. where it is more than seventy miles broad, and still moves at the
same rate of seventy five miles per day. In about 40° N. lat. it is
turned more towards the Atlantic by the extensive banks of Nantucket and
St. George, which are from two hundred to three hundred feet beneath the
surface of the sea; a clear proof that the current exceeds that depth.
On arriving near the Azores, the stream widens and overflows, as it were
forming a large expanse of warm water in the centre of the north
Atlantic, over a space of two or three hundred miles from north to south,
and having a temperature of from 8° to 10° Fahr. above the surrounding
ocean. The whole area covered by the gulph water is estimated by Rennell
at two thousand miles in length, and at a mean, three hundred and fifty
miles in breadth, an area more extensive than that of the Mediterranean.
The warm water has been sometimes known to reach the Bay of Biscay, still
retaining five degrees of temperature above that of the adjoining ocean;
and a branch of the gulf current drifts fruits, plants, and wood, the
produce of America and the West Indies, to the shores of Ireland and the
Hebrides. {26}
From the above statements, observes Mr. Lyell, we may understand why
Rennell has characterised some of the principal currents as oceanic
rivers, which he describes as being from fifty to two hundred and fifty
miles in breadth, and having a rapidity exceeding that of the largest
navigable rivers of the continent, and so deep as to be sometimes
obstructed and occasionally turned aside by banks, the tops of which do
not rise within forty, fifty, or even one hundred fathoms of the surface
of the sea.
The ordinary velocity of the principal currents of the ocean is from one
to three miles per hour; but when the boundary lands converge, large
bodies of water are driven gradually into a narrow space, and then,
wanting lateral room, are compelled to raise their level. Whenever this
occurs, their velocity is much increased. The current which runs through
the Race of Alderney, between the island of that name and the main land,
has a velocity of about eight English miles an hour. The late Captain
Hewett found that in the Pentland Firth the stream, in ordinary spring
tides, runs ten miles and a half an hour, and about thirteen miles during
violent storms. The greatest velocity of the tidal current through the
“Shoots or New Passage,” in the Bristol Channel, is fourteen miles an
hour; and Captain King observed, in his recent survey of the Straits of
Magellan, that the tide ran at the same rate through the “First Narrows,”
and about eight geographical miles an hour in other parts of those
straits.
The course of currents on the British shores is ascertained to be as
winding as that of ordinary rivers. Sometimes they run between banks of
sand, which consist of matter thrown down at certain points where the
velocity of the stream has been retarded, but it very frequently occurs,
that as in a river one bank is made of low alluvial gravel, while the
other is composed of some hardy and lofty rocks constantly undermined, so
the current in its bends strikes here and there upon a coast which then
forms one bank, whilst a shoal under water forms the other. If the coast
be formed of solid materials, it yields slowly; so also if it be of great
height, for in that case a large quantity of matter must be removed
before the sea can penetrate to any distance.
Currents depend, like tides, on no temporary or accidental circumstances,
but on the laws which preside over the motions of the heavenly bodies.
The height to which tides rise, and the violence and velocity of the
currents, depend in a great measure on the actual configuration of the
land, the contour of a long line of continental or insular coast, the
depth and breadth of channels, the peculiar form at the bottom of the
seas—in a word, on a combination of circumstances which are made to vary
continually by many igneous and aqueous causes, and among the rest, by
the tides and currents themselves. Although these agents of decay and
reproduction are local in reference to periods of short duration, such as
those which history embraces, they are nevertheless universal, if we
extend our views to a sufficient lapse of ages. {28}
Currents, observes Goldsmith, act their part in a smaller sphere, being
generally greatest where the motions of the sea are least, namely,
nearest the shores, and with the tides, produce the most rapid changes;
their motion agitates the substances of which their bed is composed, and
at the bottom of the sea, the greatest wonders are performed, for while
the sea has been known to recede from some lands, so it has been found to
encroach upon others, and probably these depredations on one part of the
shore may account for the dereliction of another, for the current which
rested upon some certain bank, having got an egress in some other place,
it no longer presses upon its former bed, but pours all its stream into
the new entrance, so that every inundation of the sea may be attended
with some correspondent dereliction of another shore, where the sea meets
no obstacles, it spreads with a gentle intumescence, till all the power
is destroyed by wanting depth to aid the motion, but when the progress is
checked in the midst by the prominence of rocks or the abrupt elevation
of land, it dashes with all its force its depth against the obstacle, and
forms, by its repeated violence, that abruptness of the shore which
confines its impetuosity. Where the sea is extremely deep, and very much
vexed with tempests, it is no small obstacle that can confine its rage;
and for this reason, we see the boldest shores projected against the
deepest waters, all less impediments having long before been surmounted
and washed away. In places where the force of the sea is less violent,
or its tides less rapid, the shores are generally seen to descend with a
more gradual declivity. Upon these shores the sea seldom beats with any
great violence, as a large wave has not depth sufficient to float it
onwards, so that here only are to be seen gentle surges making towards
the land, and lessening as they approach. As the sea, in the former
description, is generally seen to present prospects of tumult and uproar,
here it more usually exhibits a repose and tranquil beauty. Its waters
which, when surveyed from the precipice, afforded a muddy greenish hue,
arising from their depth and position to the eye, {29} when regarded from
a shelving shore, were the colour of the sky, and seem rising to meet it.
The deafening noise of the deep sea is here converted into gentle
murmurs; instead of the waters dashing against the face of the rock, it
advances and recedes, still going forward but with just force enough to
push its weeds and shells, by insensible approaches, to the shore.