The Ocean—its Size, Colour, Pressure, and Saltness—Tides—Waves—their
Height and Force—Currents—their Effect on Voyages—Temperature—The
Stratum of Constant Temperature—Line of Maximum Temperature—North and
South Polar Ice—Inland Seas.
THE ocean, which fills a deep cavity in the globe, and covers
three-fourths of its surface, is so unequally distributed that there is
three times more land in the northern than in the southern hemisphere.
The torrid zone is chiefly occupied by sea, and only one twenty-seventh
part of the land on one side of the earth has land opposite to it on the
other. The form assumed by this immense mass of water is that of a
spheroid, flattened at the poles; and as its mean level is nearly the
same, for anything we know to the contrary, it serves as a base to which
all heights of land are referred.
The bed of the ocean, like that of the land, of which it is the
continuation, is diversified by plains and mountains, table-lands and
valleys, sometimes barren, sometimes covered with marine vegetation, and
teeming with life. Now it sinks into depths which the sounding-line has
never fathomed, now it appears in chains of islands, or rises near to
the surface in hidden reefs and shoals, perilous to the mariner. Springs
of fresh water rise from the bottom, volcanos eject their lavas and
scoriæ, and earthquakes trouble the deep waters.
The ocean is continually receiving the spoils of the land, and from that
cause would constantly be decreasing in depth, and, as the quantity of
water is always the same, its superficial extent would increase. There
are, however, counteracting causes to check this tendency: the secular
elevation of the land over extensive tracts in many parts of the world
is one of the most important. Volcanos, coral islands, and barrier-reefs
show that great changes of level are constantly taking place in the bed
of the ocean itself—that symmetrical bands of subsidence and elevation
extend alternately over an area equal to a hemisphere, from which it may
be concluded that the balance is always maintained between the sea and
land, although the distribution may vary in the lapse of time.
The Pacific, or Great Ocean, exceeds in superficies all the dry land on
the globe. It has an area of 50 millions of square miles; including the
Indian Ocean, its area is nearly 70 millions; and its breadth from Peru
to the coast of Africa is 16,000 miles. Its length is less than the
Atlantic, as it only communicates with the Arctic Ocean by Behring’s
Straits, whereas the Atlantic, as far as we know, stretches from pole to
pole.
The continent of Australia occupies a comparatively small portion of the
Pacific, while innumerable islands stud its surface many degrees on
either side of the equator, of which a great number are volcanic,
showing that its bed has been, and indeed actually is, the theatre of
violent igneous eruptions. So great is its depth, that a line five miles
long has not reached the bottom in many places; yet as the whole mass of
the ocean counts for little in the total amount of terrestrial
gravitation, its mean depth is but a small fraction of the radius of the
globe.
The bed of the Atlantic is a long deep valley, with few mountains, or at
least but few that raise their summits as islands above its surface. Its
greatest breadth, including the Gulf of Mexico, is 5000 miles, and its
superficial extent is about 25 millions of square miles. This sea is
exceedingly deep: in 27° 26ʹ S. latitude and 17° 29ʹ W. longitude Sir
James Ross found the depth to be 14,550 feet; about 450 miles west from
the Cape of Good Hope it was 16,062 feet, or 332 feet more than the
height of Mont Blanc; and 900 miles west from St. Helena a line of
27,600 feet did not reach the bottom, a depth which is equal to the
height of some of the most elevated peaks of the Himalaya; but there is
reason to believe that many parts of the ocean are still deeper. A great
part of the German Ocean is only 93 feet deep, though on the Norwegian
side, where the coast is bold, the depth is 190 fathoms.
Immense sandbanks often project from the land, which rise from great
depths to within a few fathoms of the surface. Of these, the Aghullas
Banks, off the Cape of Good Hope, are amongst the most remarkable; those
of Newfoundland are still greater in extent: they consist of a double
sandbank, which is supposed to reach to the north of Scotland. The
Dogger Bank, in the North Sea, and many others, are well known.
According to Mr. Stevenson, one-fifth of the German Ocean is occupied by
sandbanks, whose average height is 78 feet, an area equal to about
one-third of Great Britain. Currents are sometimes deflected from their
course by sandbanks whose tops do not come within 50 or even 100 feet of
the surface. Some on the coast of Norway are surrounded by such deep
water that they must be submarine table-lands. All are the resort of
fish.
The pressure at the great depths is enormous. In the Arctic Ocean, where
the specific gravity of the water is lessened, on account of the greater
proportion of fresh water produced by the melting of the ice, the
pressure at the depth of a mile and a quarter is 2809 pounds on a square
inch of surface; this was confirmed by Captain Scoresby, who says, in
his “Arctic Voyages,” that the wood of a boat suddenly dragged to a
great depth by a whale was found, when drawn up, so saturated with water
forced into its pores, that it sank in water like a stone for a year
afterwards. Even sea-water is reduced in bulk from 20 to 19 solid inches
at the depth of 20 miles. The compression that a whale can endure is
wonderful. Many species of fish are capable of sustaining great
pressure, as well as sudden changes of pressure. Divers in the
pearl-fisheries exert great muscular strength, but man cannot bear the
increased pressure at great depths, because his lungs are full of air,
nor can he endure the diminution of it at great altitudes above the
earth.
The depth to which the sun’s light penetrates the ocean depends upon the
transparency of the water, and cannot be less than twice the depth to
which a person can see from the surface. In parts of the Arctic Ocean
shells are distinctly seen at the depth of 80 fathoms; and among the
West India islands, in 80 fathoms of water, the bed of the sea is as
clear as if seen in air; shells, corals, and sea-weeds of every hue
display the tints of the rainbow.
The purest spring is not more limpid than the water of the ocean; it
absorbs all the prismatic colours, except that of ultramarine, which,
being reflected in every direction, imparts a hue approaching the azure
of the sky. The colour of the sea varies with every gleam of sunshine or
passing cloud, although its true tint is always the same when seen
sheltered from atmospheric influence. The reflection of a boat on the
shady side is often of the clearest blue, while the surface of the water
exposed to the sun is bright as burnished gold. The waters of the ocean
also derive their colour from animalcules of the infusorial kind,
vegetable substances, and minute particles of matter. It is white in the
Gulf of Guinea, black round the Maldives; off California the Vermilion
Sea is so called on account of the red colour of the infusoria it
contains; the same red colour was observed by Magellan near the mouth of
the river Plate. The Persian Gulf is called the Green Sea by eastern
geographers, and there is a trail of green water off the Arabian coast
so distinct that a ship has been seen in green and blue water at the
same time. Rapid transitions take place in the Arctic Sea, from
ultramarine to olive-green, from purity to opacity. These appearances
are not delusive, but constant as to place and colour; the green is
produced by myriads of minute insects, which devour one another and are
a prey to the whale. The colour of clearer shallow water depends upon
that of its bed; over chalk or white-sand it is apple-green, over yellow
sand dark-green, brown or black over dark ground, and grey over mud.
The sea is supposed to have acquired its saline principle when the globe
was in the act of subsiding from a gaseous state. The density of
sea-water depends upon the quantity of saline matter it contains: the
proportion is generally a little above 3 per cent., though it varies in
different places; the ocean contains more salt in the southern than in
the northern hemisphere, the Atlantic more than the Pacific. The
greatest proportion of salt in the Pacific is in the parallels of 22° N.
lat. and 17° S. lat.; near the equator it is less, and in the Polar Seas
it is least, from the melting of the ice. The saltness varies with the
seasons in these regions, and the fresh water, being lightest, is
uppermost. Rain makes the surface of the sea fresher than the interior
parts, and the influx of rivers renders the ocean less salt at their
estuaries; the Atlantic is brackish 300 miles from the mouth of the
Amazons. Deep seas are more saline than those that are shallow, and
inland seas communicating with the ocean are less salt, from the rivers
that flow into them; to this, however, the Mediterranean is an
exception, occasioned by the great evaporation, and the influx of salt
currents from the Atlantic. The water in the Straits of Gibraltar at the
depth of 670 fathoms is four times as salt as that at the surface.
Fresh water freezes at the temperature of 32° of Fahrenheit; the point
of congelation of salt water is much lower. As the specific gravity of
the water of the Greenland Sea is about 1·02664, it does not freeze till
its temperature is reduced to 28-1/2° of Fahrenheit, so that the saline
principle preserves the sea in a liquid state to a much higher latitude
than if it had been fresh, while it is better suited for navigation by
its greater buoyancy. The healthfulness of the sea is ascribed to the
mixing of the water by tides and currents which prevents the
accumulation of putrescent matter.
Besides its saline ingredients, the sea contains bromine and iodine in
very minute quantities, and, no doubt, portions of other substances too
small to be detected by chemical analysis, since it has constantly
received the débris of the land and all its organized matter.
Raised by the moon and modified by the sun, the area of the ocean is
elevated into great tidal waves which keep time with the attractions of
these luminaries at each return to the upper and lower meridian. The
water under the moon is drawn from the earth by her attraction, at the
same time that she draws the earth from the water diametrically opposite
to her, in both cases producing a tide of nearly equal height. The
height to which the tides rise depends upon the relative positions of
the sun and moon, upon their declination and distance from the earth,
but much more upon local circumstances. The spring-tides happen at new
and full moon, consequently, twice in a month, because in both cases the
sun and moon are in the same meridian; for when the moon is new they are
in conjunction, and when she is full they are in opposition, and in each
of these positions their attraction is combined to raise the water to
its greatest height; while, on the contrary, the neap or lowest tides
happen when the moon is in quadrature, or 90° distant from the sun, for
then they counteract each other’s attraction to a certain degree.
The tides ordinarily happen twice in 24 hours, because the rotation of
the globe brings the same point of the ocean twice under the meridian of
the moon; but peculiar local circumstances sometimes affect the tides,
so as to produce only one tide in 24 hours, while, on the other hand,
there have been known three and even four tides in the same space of
time.
As the earth revolves, a succession of tides follow one another, and are
diffused over the Pacific, Indian, and Atlantic Oceans, giving birth to
the tides which wash the shores of the vast continents and islands which
rise above their surfaces; but in what manner those marginal tides
branch off from the parent wave, science has not yet determined: we know
only their course along each shore, but are unable to connect these
curves with the great ridge of the tidal wave.
In the Atlantic the marginal wave travels towards the north, and
impinges upon the coasts of North America and of Europe. In the Indian
Ocean it also pursues a northerly course, and finally washes the shores
of Hindostan, the Bay of Bengal, and the Arabian Gulf: while in the
Pacific, on the contrary, the waves diverge from the equator towards the
poles—but in all they partake also of the westerly course of the moon.
Although such are the directions in which the tides unquestionably
proceed along the _shores_ of those seas, yet observations at islands in
the open sea and towards the centres of the oceans contradict the idea
of corresponding progressive waves throughout the entire area of those
seas.
Upon the coasts of Britain and New Brunswick the tides are high, from
the local circumstances of the coast and bottom of the sea; while in the
centre of the ocean, where they are due to the action of the sun and
moon only, they are remarkably small. The spring-tides rise more than 40
feet at Bristol, and in the Bay of Fundy, in Nova Scotia, they rise
upwards of 50 feet; the general height in the North Atlantic is 10 or 12
feet, but in the open and deep sea they are less; and at St. Helena they
are not more than 3 feet, whilst among the islands in the Pacific they
are scarcely perceptible.
The mean height of the tides will be increased by a very small quantity
for ages to come, in consequence of the decrease in the mean distance of
the moon from the earth; the contrary effect will take place after that
period has elapsed, and the moon’s mean distance begins to increase
again, which it will continue to do for many ages. Thus, the mean
distance of the moon, and the consequent minute increase in the height
of the tides, will oscillate between fixed limits for ever.[110]
The tidal wave extends to the bottom of the ocean, and moves uniformly
and with great speed in very deep water, variably and slow in shallow
water; the time of propagation depends on the depth of the water as well
as on the nature and form of the shores. Its velocity varies inversely
as the square of the depth—a law which theoretically affords the means
of ascertaining the proportionate depth of the sea in different parts;
it is one of the great constants of nature, and is to fluids what the
pendulum is to solids—a connecting link between time and force.
The great oceanic wave that twice a-day brings the tides to our shores,
has occupied a day and a half in travelling from the place where it was
generated. The wave first impinges on the west coast of Ireland and
England, and then passes round the north of Scotland, up the North Sea,
and enters the Thames, having made the tour of Great Britain in about 18
hours.
At the equator the tide-wave follows the moon at the rate of 1000 miles
an hour; it moves very slowly in the northern seas on account of the
shallowness of the water; but the tides are so retarded by the form of
the coasts and irregularities of the bottom of the sea, that a tide is
sometimes impeded by an obstacle till a second tide reaches the same
point by a different course, and the water rises to double the height it
would otherwise have attained. A complete extinction of the tide takes
place when a high-water interferes in the same manner with a low-water,
as in the centre of the German Ocean—a circumstance predicted by theory,
and confirmed by Captain Hewett, who was not aware that such an
interference existed. When two unequal tides of contrary phases meet,
the greater overpowers the lesser, and the resulting height is equal to
their difference; such varieties occur chiefly in channels among islands
and at the estuaries of rivers. When the tide flows suddenly up a river
encumbered with shoals, it checks the descent of the stream; the water
spreads over the sands, and a high crested wave, called a bore, is
driven with force up the channel. This occurs in the Ganges; in the
Amazon, at the equinoxes, where, during three successive days, five of
these destructive waves, from 12 to 15 feet high, follow one another up
that river daily; and in a lesser degree in some of our own rivers.
There may be some small flow of _stream_ with the oceanic tide; but that
does not necessarily follow, since the tide in the open ocean is merely
an alternate rise and fall of the surface: so that the wave, not the
stream, follows the moon. A bird resting on the sea is not carried
forward as the waves rise and fall; indeed, if so heavy a body as water
were to move at the rate of 1000 miles in an hour, it would cause
universal destruction, since in the most violent hurricanes the velocity
of the wind hardly exceeds 100 miles an hour.
During the passage of the great tidal wave in deep water, the particles
of the fluid glide for the moment over each other into a new
arrangement, and then retire to their places; but this motion is
extremely limited and momentary. Over shallows, however, and near the
land, both the water and the waves advance during the flow of the tide,
and roll on the beach.[111]
The friction of the wind combines with the tides in agitating the
surface of the ocean, and, according to the theory of undulations, each
produces its effect independently of the other; wind, however, not only
raises waves, but causes a transfer of superficial water also.
Attraction between the particles of air and water, as well as the
pressure of the atmosphere, brings its lower stratum into adhesive
contact with the surface of the sea. If the motion of the wind be
parallel to the surface, there will still be friction, but the water
will be smooth as a mirror; but if it be inclined, in however small a
degree, a ripple will appear. The friction raises a minute wave, whose
elevation protects the water beyond it from the wind, which consequently
impinges on the surface at a small distance beyond; thus, each impulse,
combining with the other, produces an undulation which continually
advances.
Those beautiful silvery streaks on the surface of a tranquil sea called
cat’s-paws by sailors, are owing to a partial deviation of the wind from
a horizontal direction. The resistance of the water increases with the
strength and inclination of the wind. The agitation at first extends
little below the surface, but in long-continued gales even the deep
water is troubled: the billows rise higher and higher, and, as the
surface of the sea is driven before the wind, their “monstrous heads,”
impelled beyond the perpendicular, fall in wreaths of foam. Sometimes
several waves overtake one another, and form a sublime and awful sea.
The highest waves known are those which occur during a north-west gale
off the Cape of Good Hope, aptly called by the ancient Portuguese
navigators the Cape of Storms: Cape Horn also seems to be the abode of
the tempest. The sublimity of the scene, united to the threatened
danger, naturally leads to an over-estimate of the magnitude of the
waves, which appear to rise mountain-high, as they are proverbially said
to do: there is, however, reason to doubt if the highest waves off the
Cape of Good Hope exceed 40 feet from the hollow trough to the summit.
The waves are short and abrupt in small shallow seas, and on that
account are more dangerous than the long rolling billows of the wide
ocean.
“The sea-shore after a storm presents a scene of infinite grandeur. It
exhibits the expenditure of gigantic force, which impresses the mind
with the presence of elemental power as sublime as the water-fall or the
thunder. Long before the waves reach the shore they may be said to feel
the bottom as the water becomes shallower, for they increase in height,
but diminish in length. Finally, the waves become higher, more pointed,
assumes a form of unstable equilibrium, totters, becomes crested with
foam, breaks with great violence, and continuing to break, is gradually
lessened in bulk till it ends in a fringed margin.”[112]
The waves raised by the wind are altogether independent of the tidal
waves; each maintains its undisturbed course; and as the inequalities of
the coasts reflect them in all directions, they modify those they
encounter and offer new resistance to the wind, so that there may be
three or four systems or series of co-existing waves, all going in
different directions, while the individual waves of each maintain their
parallelism.
The undulation called a ground-swell, occasioned by the continuance of a
heavy gale, is totally different from the tossing of the billows, which
is confined to the area vexed by the wind; whereas the ground-swell is
rapidly transmitted through the ocean to regions far beyond the direct
influence of the gale that raised it, and it continues to heave the
smooth and glassy surface of the deep long after the wind and the
billows are at rest. In the South Pacific, billows which must have
travelled 1000 miles against the trade-wind from the seat of the storm,
expend their fury on the lee side of the many coral islands which bedeck
that sunny sea.[113] A swell sometimes comes from a quarter in direct
opposition to the wind, and occasionally from various points of the
compass at the same time, producing a vast commotion even in a dead
calm, without ruffling the surface. They are the heralds that point out
to the mariner the distant region where the tempest has howled, and not
unfrequently they are the harbingers of its approach. At the margin of
the polar ice, in addition to other dangers, there is generally a swell
which would be very formidable to the mariner in thick weather, did not
the loud grinding noise of the ice warn him of his approach.
Heavy swells are propagated through the ocean till they gradually
subside from the friction of the water, or till the undulation is
checked by the resistance of land, when they roll in surf to the shore,
or dash in spray and foam over the rocks. The rollers at the Cape de
Verde Islands are seen at a great distance approaching like mountains.
When a gale is added to a ground-swell the commotion is great and the
force of the surge tremendous, tossing huge masses of rock and shaking
the cliffs to their foundations. During heavy gales on the coast of
Madras the surf breaks in nine fathoms water at the distance of four and
even four and a half miles from the shore. The violence of the tempest
is sometimes so intense as to quell the billows and scatter its surface
in a heavy shower called by sailors spoon-drift. On such occasions
saline particles have impregnated the air to the distance of 50 miles
inland.
The force of the waves in gales of wind is tremendous; from experiments
made by Mr. Stevenson, civil engineer, on the west coast of Scotland,
exposed to the whole fury of the Atlantic, it appears that the average
pressure of the waves during the summer months was equal to 611 pounds
weight on a square foot of surface, while in winter it was 2086 pounds,
or three times as great. During the storm that took place on the 9th of
March, 1845, it amounted to 6083 pounds. Now, as the pressure of a wave
20 feet high not in motion is only about half a ton on a square foot, it
shows how much of their force waves owe to their velocity. The rolling
breakers on the cliffs on the west coast of Ireland are magnificent:
Lord Adair measured some that were 50 and even 150 feet high.
In the Isle of Man, a block winch weighed about 10 stone was lifted from
its place and carried inland during a north-westerly gale; and in the
Hebrides a block of 42 tons weight was moved several feet by the force
of the waves. The Bell Rock light-house in the German Ocean, though 112
feet high, is literally buried in foam and spray to the very top during
ground-swells when there is no wind. On the 20th of November, 1827, the
spray rose 117 feet, so that the pressure was computed by Mr. Stevenson
to be nearly three tons on a square foot.
The effect of a gale descends to a comparatively small distance below
the surface; the sea is probably tranquil at the depth of 200 or 300
feet; were it not so, the water would be turbid and shellfish would be
destroyed. Anything that diminishes the friction of the wind smoothes
the surface of the sea—for example, oil or a small stream of packed ice,
which suppresses even a swell. When the air is moist, its attraction for
water is diminished, and consequently so is the friction; hence the sea
is not so rough in rainy as in dry weather.
Currents of various extent, magnitude, and velocity disturb the
tranquillity of the ocean; some of them depend upon circumstances
permanent as the globe itself, others on ever-varying causes. Constant
currents are produced by the combined action of the rotation of the
earth, the heat of the sun, and the trade-winds; periodical currents are
occasioned by tides, monsoons, and other long-continued winds; temporary
currents arise from the tides, melting ice, and from every gale of some
duration. A perpetual circulation is kept up in the waters of the main
by these vast marine streams; they are sometimes superficial and
sometimes submarine, according as their density is greater or less than
that of the surrounding sea.
The exchange of water between the poles and the equator affects the
great currents of the ocean. Although these depend upon the same causes
as the trade-winds, they differ essentially in this respect—that whereas
the atmosphere is heated from below by its contact with the earth, and
transmits the heat to the strata above, the sea is heated at its surface
by the direct rays of the sun, which diminish the specific gravity of
the upper strata, especially between the tropics, and also occasion
strong and rapid evaporation, both of which causes disturb the
equilibrium of the ocean. The rotation of the earth also gives the water
a tendency to take an oblique direction in its flow towards the
equatorial regions, as, in order to restore the equilibrium, deranged by
so many circumstances, great streams perpetually descend from either
pole. When these currents leave the poles they flow towards the equator;
but, before proceeding far, their motion is deflected by the diurnal
rotation of the earth. At the poles they have no rotatory motion; and
although they gain it more and more in their progress to the equator,
which revolves at the rate of 1000 miles an hour, they arrive at the
tropics before they have acquired the same velocity of rotation with the
intertropical ocean. On that account they are left behind, and
consequently seem to flow in a direction contrary to the diurnal
rotation of the earth. For that reason the whole surface of the ocean,
for 30 degrees on each side of the equator, has an apparent tendency
from east to west, which produces all the effects of a great current or
stream flowing in that direction. The trade-winds, which blow constantly
in one direction, combine to give this current a mean velocity of 10 or
11 miles in 24 hours.
It has been supposed that the primary currents, as well as those derived
from them, are subject to periodical variations of intensity occasioned
by the melting of the ice at each pole alternately.
In consequence of the uninterrupted expanse of ocean in the southern
hemisphere, the prevalence of westerly winds, and the tendency of the
polar water towards the equator, a great oceanic current is originated
in the Antarctic Sea. Driven by the prevailing winds, the waters take an
easterly direction inclining to the northward, and one part sets upon
the American coast, where it is divided. A small part doubles Cape Horn,
while the main cold stream flows down the American shore; then turning
suddenly to the west, it loses itself in the great equatorial current of
the Pacific, which crosses that ocean between the parallels of 26° S.
and 24° N. in a vast stream nearly 3500 miles broad. In the north this
stream is interrupted by the coast of China, the Eastern Peninsula, and
the islands of the Indian Archipelago; but a part forces its way between
the islands, and joins the great equatorial current of the Indian Ocean,
which, impelled by the S.E. trade-wind, maintains a westerly course
between the 10th and 20th parallels of south latitude; as it approaches
the Island of Madagascar the stream is divided; one part runs to the
north-west, bends round the northern end of Madagascar, flows through
the Mosambique Channel, and, being joined by the other branch, it
doubles the Cape of Good Hope outside of the Agullhas Bank, and, under
the name of the South Atlantic Current, it runs along the west coast of
Africa to the parallel of St. Helena. There it is deflected by the coast
of Guinea, and forms the Great Atlantic Equatorial Current, which flows
westward and divides upon Cape St. Roque in Brazil. One branch of the
stream setting southward along the continent of South America, becomes
insensible before it reaches the Straits of Magellan; but an offset from
it stretches directly across the Atlantic to the Cape of Good Hope,
having made the circuit of the South Atlantic Ocean, and keeping 150
miles outside of the Cape or Agullhas current, which runs in the
opposite direction, it pursues its course into the Indian Ocean, where
traces of it are met with 2000 miles from the Cape.
The principal branch of the great equatorial current takes a northerly
course from off Cape St. Roque, and rushes along the coast of Brazil
with such force and depth that it suffers only a temporary deflection by
the powerful streams of the river Amazon and of the Orinoco. Though much
weakened in passing among the West Indian islands, it acquires new
strength in the Caribbean Sea. From thence, after sweeping round the
Gulf of Mexico with the high temperature of 88° 52ʹ of Fahrenheit, it
flows through the Straits of Florida, and along the North American coast
to Newfoundland under the name of the Gulf-stream: it is there deflected
eastward by the form of the land and the prevalent wind, and after
passing Newfoundland by a current from Baffin’s Bay. From the Azores it
bends southward, and aided by the north-east trade rejoins the
equatorial current, having made a circuit of 3800 miles with various
velocity, leaving a vast loop or space of water nearly stagnant in its
centre, which is thickly covered with sea-weed. The bodies of men,
animals, and plants of unknown appearance, brought to the Azores by this
stream, suggested to Columbus the idea of land beyond the Western Ocean,
and thus led to the discovery of America. The Gulf-stream is more salt,
warmer, and of a deeper blue than the rest of the ocean, till it reaches
Newfoundland, where it becomes turbid from the shallowness of that part
of the sea. Its greatest velocity is 78 miles a-day soon after leaving
the Florida Strait; and its breadth increases with its distance from the
strait until the warm water spreads over a large surface of the ocean.
An important branch leaves the current near Newfoundland, setting
towards Britain and Norway; which is again subdivided into many
branches, whose origin is recognized by their greater warmth, even at
the edge of perpetual ice in the Polar Ocean; and in consequence of some
of these branches the Spitzbergen Sea is 6° or 7° warmer at the depth of
200 fathoms than at its surface. Though the warmth of the Gulf-stream
diminishes as it goes north, Lieutenant Murray says “that the quantity
of heat which it spreads over the Atlantic in a winter’s day would be
sufficient to raise the whole atmosphere that covers France and Great
Britain from the freezing point to summer heat;” and it really is the
cause of the mildness and of the damp of Ireland and the south of
England.
These oceanic streams exceed all the rivers in the world in breadth and
depth as well as length. The equatorial current in the Atlantic is 160
miles broad off the coast of Africa, and towards its mid-course across
the Atlantic its width becomes nearly equal to the length of Great
Britain: but as it then sends off a branch to the N.W., it is diminished
to 200 miles before reaching the coast of Brazil. The depth of this
great stream is unknown; but the Brazilian branch must be very profound,
since it is not deflected by the river La Plata, which crosses it with
so strong a current that its fresh muddy waters are perceptible 500
miles from its mouth. When currents pass over banks and shoals, the
colder water rises to the surface and gives warning of the danger.
In summer, the great north polar current coming along the coasts of
Greenland and Labrador, together with the current from Davis’s Straits,
brings icebergs to the margin of the Gulf-Stream. The difference between
the temperatures of these two oceanic streams brought into contact is
the cause of the dense fogs that brood over the banks of Newfoundland.
The north polar current runs inside of the Gulf-Stream, along the coast
of North America to Florida, and beyond it—since it sends an
under-current into the Caribbean Sea. Counter-currents on the surface
are of such frequent occurrence that there is scarcely a strait joining
two seas that does not furnish an example—a current running in along one
shore, and a counter-current running out along the other. One of the
most remarkable occurs in the Atlantic: it begins off the coast of
France, and, after sending a mass of water into the Mediterranean, it
holds a southerly direction at some distance from the continent of
Africa; till, after passing Cape Mesurada, it flows rapidly for 1000
miles due east to the Bight of Biafra in immediate contact with the
equatorial current, running with great velocity in the opposite
direction, and seems to merge in it at last.
Periodical currents are frequent in the eastern seas: one flows into the
Red Sea from October to May, and out of it from May to October. In the
Persian Gulf this order is reversed; in the Indian Ocean and China Sea
the waters are driven alternately backwards and forwards by the
monsoons. It is the southwesterly monsoon that causes inundations in the
Ganges, and a tremendous surf on the coast of Coromandel. The tides also
produce periodical currents on the coasts and in straits, the water
running in one direction during the flood, and the contrary way in the
ebb. The Roost of Sumburgh, at the southern promontory of Shetland, runs
at the rate of 15 miles an hour; indeed, the strongest tidal currents
known are among the Orkney and Shetland islands; their great velocity
arises from local circumstances. Currents in the wide ocean move at the
rate of from one to three miles an hour, but the velocity is less at the
margin and bottom of the stream from friction.
Whirlpools are produced by opposing winds and tides; the whirlpool of
Maelstrom, on the coast of Norway, is occasioned by the meeting of tidal
currents round the islands of Lofoden and Mosköe; it is a mile and a
half in diameter, and so violent that its roar is heard at the distance
of several leagues.
Although with winds, tides, and currents, it might seem that the ocean
is ever in motion, yet in the equatorial regions, far from land, dead
calms prevail; the sea is of the most perfect stillness day after day;
partaking of the universal quiet, and heaving its low flat waves in
noiseless and regular periods as if nature were asleep.
The safety and length of a voyage depends upon the skill with which a
seaman avails himself of the set of the different currents, and the
direction of the permanent and periodical winds; it is frequently
shortened by following a very circuitous track to take advantage of them
if favourable, or to avoid them if unfavourable. From Acapulco, in
Mexico, across the Pacific to Manilla or Canton, the trade-wind and the
equatorial current are so favourable that the voyage is accomplished in
50 or 60 days; whereas, in returning, 90 or 100 are required. Within the
Antillas navigation is so difficult from winds and currents, that a
vessel, going from Jamaica to the lesser Antillas, cannot sail directly
across the Caribbean Sea, but must go round about through the windward
passage between Cuba and Haiti to the ocean; nearly as many weeks are
requisite to accomplish this voyage as it takes days to return. On
account of the prevalence of westerly winds in the North Atlantic, the
voyage from Europe to the United States is longer than that from the
latter to Europe; but the Gulf-stream is avoided in the outward voyage
[_i. e._ from Europe], because it would lengthen the time by a
fortnight. Ships going to the West Indies, Central or South America,
from Europe, generally make the Canary Islands in order to fall in with
the N.E. trade-winds.
The passage to the Cape of Good Hope from the British Channel may be
undertaken at any season, and is accomplished in 50 or 60 days; but it
is necessary to regulate the voyage from the Cape to India and China
according to the seasons of the monsoons. There are various courses
adopted for that purpose, but all of them pass through the very focus of
the hurricane district, which includes the islands of Rodriguez, the
Mauritius, and Bourbon, and extends from Madagascar to the island of
Timor.
The extensive deposits of coal discovered in the Bay of Talcahuano, in
Chile, in Australia, New Zealand, in the British settlement at Labuan,
and in Borneo, will be the means of increasing the steam navigation of
the Pacific, and shortening the voyages upon that ocean.
Sea-water is a bad conductor of heat, therefore the temperature of the
ocean is less liable to sudden changes than the atmosphere; the
influence of the seasons is imperceptible at the depth of 300 feet; and
as light probably does not penetrate lower than 700 feet, the heat of
the sun cannot affect the bottom of a deep sea. It has been established
beyond a doubt that in all parts of the ocean the water has a constant
temperature of about 39°·5 of Fahrenheit, at a certain depth, depending
on the latitude. At the equator the stratum of water at that temperature
is at the depth of 7200 feet; from thence it gradually rises till it
comes to the surface in S. lat. 56° 26ʹ, where the water has the
temperature of 39°·5 at all depths; it then gradually descends till S.
lat. 70°, where it is 4500 feet below the surface. In going north from
the equator the same law is observed. Hence, with regard to temperature,
there are three regions in the ocean: one equatorial and two polar. In
the equatorial region the temperature of the water at the surface of the
ocean is 80° of Fahrenheit, therefore higher than that of the stratum of
39°·5; while in the polar regions it is lower. Thus, the surface of the
stratum of constant temperature is a curve which begins at the depth of
4500 feet in the southern basin, from whence it gradually rises to the
surface in S. lat. 56° 26ʹ; it then sweeps down to 7200 feet at the
equator, and rises up again to the surface in the corresponding northern
latitude, from whence it descends again to a depth of 4500 feet in the
northern basin.
The temperature of the surface of the ocean decreases from the equator
to the poles. For 10 degrees on each side of the line the maximum is 80°
of Fahrenheit, and remarkably stable; from thence to each tropic the
decrease does not exceed 3°·7. The tropical temperature would be greater
were it not for the currents, because the surface reflects much fewer of
the sun’s rays which fall on it directly, than in higher latitudes where
they fall obliquely. In the torrid zone the surface of the sea is about
3°·5 of Fahrenheit warmer than the air above it; because the polar
winds, and the great evaporation which absorbs the heat, prevent
equilibrium; and as a great mass of water is slow in following the
changes in the atmosphere, the vicissitude of day and night has little
influence, whereas in the temperate zones it is perceptible.
The line of maximum temperature, or that which passes through all the
points of greatest heat in the ocean, is very irregular, and does not
coincide with the terrestrial equator; six-tenths of its extent lies on
an average 5° to the north of it, and the remainder runs at a mean
distance of 3° on its southern side. It cuts the terrestrial equator in
the middle of the Pacific Ocean in 21° E. longitude in passing from the
northern to the southern hemisphere, and again between Sumatra and the
peninsula of Malacca in returning from the southern to the northern. Its
maximum temperature in the Pacific is 88°·5 of Fahrenheit on the
northern shores of New Guinea, where it touches the terrestrial equator,
and its highest temperature in the Atlantic, which is exactly the same,
lies in the Gulf of Mexico, which furnishes the warm water of the
Gulf-stream.
The superficial water of the Pacific is much cooled on the east by the
Antarctic current; it sends a cold stream along the coasts of Chile and
Peru, which has great influence on the climate of both countries; it was
first observed by Baron Humboldt, and is known as Humboldt’s current. It
is more than 14° colder than the adjacent ocean, and renders the air 11°
cooler than the surrounding atmosphere.
In the Indian Ocean the highest temperature of the surface-water (87°·4)
is in the Arabian Sea, between the Strait of Bab-el-Mandeb and the coast
of Hindostan; it decreases regularly from south to north in the Red Sea.
The superficial temperature diminishes from the tropics with the
increase of the latitude more rapidly in the southern than in the
northern hemisphere, till towards the poles the sea is never free from
ice. In the Arctic Ocean the surface is at the freezing point even in
summer; and during the eight winter months a continuous body of ice
extends in every direction from the pole, filling the area of a circle
of between 3000 and 4000 miles in diameter. The outline of this circle,
though subject to partial variations, is found to be nearly similar at
the same season of each succeeding year, yet there are periodical
changes in the polar ice which are renewed after a series of years. The
freezing process itself is a bar to the unlimited increase of the
oceanic ice. Fresh water congeals at the temperature of 32° of
Fahrenheit, but sea-water must be reduced to 28°·5 before it deposits
its salt and begins to freeze: the salt thus set free, and the heat
given out, retard the process of congelation more and more below.
The ice from the north pole comes so far south in winter as to render
the coast of Newfoundland inaccessible: it envelops Greenland, sometimes
even Iceland, and always invests Spitzbergen and Nova Zembla. As the sun
comes north the ice breaks up into enormous masses of what is called
packed ice. In the year 1806 Captain Scoresby forced his ship through
250 miles of packed ice, in imminent danger, until he reached the
parallel of 81° 50ʹ, his nearest approach to the pole: the Frozen Ocean
is rarely navigable so far.
In the year 1827 Sir Edward Parry arrived at the latitude of 82° 45ʹ,
which he accomplished by dragging a boat over fields of ice, but he was
obliged to abandon the bold and hazardous attempt to reach the pole,
because the current drifted the ice southward more rapidly than he could
travel over it to the north.
The following considerations have induced some persons to believe that
there is sea instead of land at the north pole. The average latitude of
the northern shores of the continent is 70°, so that the Arctic Ocean is
a circle whose diameter is 2400 geographical miles, and its
circumference 7200. On the Asiatic side of this sea are Nova Zembla and
the New Siberian islands, each extending to about 76° N. latitude. On
the European and American sides are Spitzbergen, extending to 80°, and a
part of Old Greenland, whose northern termination is unknown. Facing
America is a large island—Melville Island—with some others not extending
so far north as those mentioned; consequently all of them may be
considered continental islands. As there are no large islands very far
from land in the other great oceans, there is reason to presume that the
same structure may prevail here also, and, consequently, it may be open
sea at the north pole. Possibly also it may be free from ice, for
Admiral Wrangel found a wide and open sea, free from ice and navigable,
beginning 16 miles north of the island of Kotelnoi, and extending to the
meridian of Cape Jackan. In fine summers the ice suddenly clears away
and leaves an open channel of sea along the western coast of Spitzbergen
from 60 to 150 miles wide, reaching to 80° or even to 80-1/2° N.
latitude probably owing to warm currents from low latitudes. It was
through this channel that Captain Scoresby made his nearest approach to
the pole. A direct course from the Thames, across the pole to Behring’s
Straits, is 3570 geographical miles, while by Lancaster Sound it is 4660
miles. The Russians would be saved a voyage of 18,800 geographical miles
could they go across the pole and through Behring’s Straits to their
North American settlements, instead of going by Cape Horn.
Floating fields of ice, 20 or 30 miles in diameter, are frequent in the
Arctic Ocean: sometimes they extend 100 miles, so closely packed
together that no opening is left between them; their thickness, which
varies from 10 to 40 feet, is not seen, as there is at least two-thirds
of the mass below water. Sometimes these fields, many thousand millions
of tons in weight, acquire a rotatory motion of great velocity, dashing
against one another with a tremendous collision. Packed ice always has a
tendency to drift southwards, even in the calmest weather; and in their
progress the ice-fields are rent in pieces by the swell of the sea. It
is computed that 20,000 square miles of drift ice are annually brought
by the current along the coast of Greenland to Cape Farewell. In stormy
weather the fields and streams of ice are covered with haze and spray
from constant tremendous concussions; yet our seamen, undismayed by the
appalling danger, boldly steer their ships amidst this hideous and
discordant tumult.
Huge icebergs and masses detached from the glaciers, which extend from
the Arctic lands into the sea, especially in Baffin’s Bay, are drifted
southwards 2000 miles from their origin to melt in the Atlantic, where
they cool the water sensibly for 30 or 40 miles around, and the air to a
much greater distance. They vary from a few yards to miles in
circumference, and rise hundreds of feet above the surface. Seven
hundred such masses have been seen at once in the polar basin. When
there is a swell the loose ice dashing against them raises the spray to
their very summits; and as they waste away they occasionally lose
equilibrium and roll over, causing a swell which breaks up the
neighbouring field-ice; the commotion spreads far and wide, and the
uproar resounds like thunder.
Icebergs have the appearance of chalk-cliffs with a glittering surface
and emerald-green fractures: pools of water of azure-blue lie on their
surface or fall in cascades into the sea. The field-ice also, and the
masses that are heaped up on its surface, are extremely beautiful from
the vividness and contrast of their colouring. A peculiar blackness in
the atmosphere around a bright haze at the horizon indicates their
position in a fog, and their place and character are shown at night by
the reflection of the snow-light on the horizon. An experienced seaman
can readily distinguish by the blink, as it is termed, whether the ice
is newly-formed, heavy, compact, or open. The blink or snow-light of
field-ice is the most lucid, and is tinged yellow; of packed ice it is
pure white: ice newly formed has a greyish blink, and a deep yellow tint
indicates snow on land.
Icebergs come to a lower latitude by 10° from the south pole than from
the north, and appear to be larger; they have been seen near the Cape of
Good Hope, and are often of great size; one observed by Captain
D’Urville was 13 miles long, with perpendicular sides 100 feet high;
they are less varied than those on the northern seas, a tabular form is
the most prevalent. The discovery ships under the command of Sir James
Ross met with multitudes with flat surfaces, bounded by perpendicular
cliffs on every side, from 100 to 180 feet high, sometimes several miles
in circumference. On one occasion they fell in with a chain of
stupendous bergs close to one another, extending farther than the eye
could reach even from the mast-head. Packed ice too is often in immense
quantities: these ships forced their way through a pack 1000 miles
broad, often under the most appalling circumstances. It generally
consists of smaller pieces than the packs in the comparatively tranquil
North Polar seas, where they are often several miles in diameter, and
where fields of ice extend beyond the reach of vision. The Antarctic
Ocean, on the contrary, is almost always agitated; there is a perpetual
swell, and terrific storms are common, which break up the ice and render
navigation perilous. The floe pieces are rarely a quarter of a mile in
circumference, and generally much smaller.
A more dreadful situation can hardly be imagined than that of ships
beset during a tempest in a dense pack of ice in a dark night, thick
fog, and drifting snow, with the spray beating perpetually over the
decks, and freezing instantaneously. Sir James Ross’s own words can
alone give an idea of the terrors of one of the many gales which the two
ships under his command encountered:—“Soon after midnight our ships were
involved in an ocean of rolling fragments of ice, hard as floating rocks
of granite, which were dashed against them by the waves with so much
violence, that their masts quivered as if they would fall at every
successive blow; and the destruction of the ships seemed inevitable from
the tremendous shocks they received. In the early part of the storm the
rudder of the Erebus was so much damaged as to be no longer of any use;
and about the same time I was informed by signal that the Terror’s was
completely destroyed and nearly torn away from the stern-post. Hour
passed away after hour without the least mitigation of the awful
circumstances in which we were placed. The loud crashing noise of the
straining and working of the timbers and decks, as they were driven
against some of the heavier pieces of ice, which all the exertions of
our people could not prevent, was sufficient to fill the stoutest heart,
that was not supported by trust in Him who controls all events, with
dismay; and I should commit an act of injustice to my companions if I
did not express my admiration of their conduct on this trying occasion.
Throughout a period of 28 hours, during any one of which there appeared
to be very little hope that we should live to see another, the coolness,
steady obedience, and untiring exertions of each individual, were every
way worthy of British seamen.
“The storm gained its height at 2 P.M., when the barometer stood at
28·40 inches, and after that time began to rise. Although we had been
forced many miles deeper into the pack, we could not perceive that the
swell had at all subsided, our ships still rolling and groaning amidst
the heavy fragments of crushing bergs, over which the ocean rolled its
mountainous waves, throwing huge masses upon one another, and then again
burying them deep beneath its foaming waters, dashing and grinding them
together with fearful violence.” For three successive years were these
dangers encountered during this bold and hazardous enterprise.
The ocean is one mass of water, which, entering into the interior of the
continents, has formed seas and gulfs of great magnitude, which afford
easy and rapid means of communication, while they temper the climates of
the widely expanding continents.
The inland seas communicating with the Atlantic are larger, and
penetrate more deeply into the continents, than those connected with the
great ocean; a circumstance which gives a coast of 48,000 miles to the
former, while that of the great ocean is only 44,000. Most of these
internal seas have extensive river domains, so that by inland navigation
the Atlantic virtually enters into the deepest recesses of the land,
brings remote regions into contact, and improves the condition of the
less cultivated races of mankind by commercial intercourse with those
that are more civilized.
The Baltic, which occupies 125,000 square miles in the centre of
northern Europe, is one of the most important of the inland seas
connected with the Atlantic, and, although inferior to the others in
size, the drainage of more than a fifth of Europe flows into it. Only
about a fourth part of the boundary of its enormous basin of 900,000
square miles is mountainous; and so many navigable rivers flow into it
from the watershed of the great European plain, that its waters are
one-fifth less salt than those of the Atlantic: it receives at least 250
streams. Its depth nowhere exceeds 167 fathoms,[114] and generally it is
not more than 40 or 50. From that cause, together with its freshness and
northern latitude, the Baltic is frozen five months in the year. From
the flatness of the greater part of the adjacent country, the climate of
the Baltic is subject to influences coming from regions far beyond the
limits of its river-basin. The winds from the Atlantic bring warmth and
moisture, which, condensed by the cold blasts from the Arctic plains,
falls in rain in summer, and deep snow in winter, which also makes the
sea more fresh. The tides are imperceptible; but the waters of the
Baltic occasionally rise more than three feet above their usual level
from some unknown cause—possibly from oscillations in its bed, or from
changes of atmospheric pressure.
The Black Sea, which penetrates most deeply into the continent of all
the seas in question, has, together with the Sea of Azov, an area of
190,000 square miles: it must at a remote period have been united with
the Caspian Lake, and must have covered all the Steppe of Astracan. It
receives some of the largest European rivers, and drains about 950,000
square miles, consequently its waters are brackish and freeze on its
northern shores in winter. It is very deep, no bottom having been
reached with a line of 140 fathoms: on the melting of the snow, such a
body of water is poured into it by the great European rivers that a
rapid current is produced, which sets along the western shore from the
mouth of the Dnieper to the channel of Constantinople.
Of all the branches of the Atlantic that enter deeply into the bosom of
the land, the Mediterranean is the largest and most beautiful, covering
with its dark blue waters more than 760,000 square miles. Situate in a
comparatively low latitude, exposed to the heat of the African deserts
on the south, and sheltered on the north by the Alps, the evaporation is
excessive; on that account the water of the Mediterranean is salter than
that of the ocean, and for the same reason the temperature at its
surface is 3-1/2° of Fahrenheit higher than that of the Atlantic: it
does not decrease so rapidly downwards as in tropical seas, and it
becomes constant at depths of from 340 to 1000 fathoms, according to the
situations. Although its own river domain is only 250,000 square miles,
the constant current that sets in through the Dardanelles brings a great
part of the drainage of the Black Sea, so that it is really fed by the
melted snow and rivers from the Caucasus, Asia Minor, Abyssinia, the
Atlas, and the Alps. Yet the quantity of water that flows into the
Mediterranean from the Atlantic, by the central current in the Straits
of Gibraltar, exceeds that which goes out by the lateral currents.
Near Alexandria the surface of this sea is 26 feet 6 inches lower than
the level of the Red Sea at Suez at low water, and about 30 feet lower
at high water.[115]
On the shore of Cephalonia there is a cavity in the rocks, into which
the sea has been flowing for ages.[116]
The Mediterranean is divided into two basins by a shallow that runs from
Cape Bon on the African coast to the Strait of Messina, on each side of
which the water is exceedingly deep, and said to be unfathomable in some
parts. M. Bérard has sounded to the depth of more than 1000 fathoms in
several places without reaching the bottom. At Nice, within a few yards
of the shore, it is nearly 700 fathoms deep; and Captain Smyth, R. N.,
ascertained the depth to be 960 fathoms between Gibraltar and Ceuta.
This sea is not absolutely without tides; in the Gulf of Venice they
rise three feet, and at the Great Syrte to five feet at new and full
moon, but in most other places they are scarcely perceptible. The
surface is traversed by various currents, two of which, opposing one
another, occasion the celebrated whirlpool of Charybdis, whose terrors
were much diminished by the earthquake of 1783. Its bed is subject to
violent volcanic paroxysms, and its surface is studded with islands of
all sizes, from the magnificent kingdom of Sicily to mere barren rocks—
some actively volcanic, others of volcanic formation, and many of the
secondary geological period.
Various parts of its coasts are in a state of great instability; in some
places they have sunk down and risen again more than once within the
historical period.
Far to the north the Atlantic penetrates the American continent by
Davis’s Straits, and spreads out into Baffin’s bay, twice the size of
the Baltic, very deep, and subject to all the rigours of an arctic
winter—the very storehouse of icebergs—the abode of the walrus and the
whale. Hudson’s Bay, though without the Arctic Circle, is but little
less dreary.
Very different is the character of those vast seas where the Atlantic
comes “cranking in” between the northern and southern continents of
America. The surface of the sea in Baffin’s Bay is seldom above the
freezing-point; here, on the contrary, it is always 88°·5 of Fahrenheit,
while the Atlantic Ocean in the same latitude is not above 77° or 78°.
Of that huge mass of water, partially separated from the Atlantic by a
long line of islands and banks, the Caribbean Sea is the largest; it is
as long from east to west as the distance between Great Britain and
Newfoundland, and occupies a million of square miles. Its depth in many
places is very great, and its water is limpid. The Gulf of Mexico, fed
by the Mississippi, one of the greatest of rivers, is more than half its
size, or about 625,000 square miles, so that the whole forms a sea of
great magnitude. Its shores, and the shores of the numerous islands, are
dangerous from shoals and coral-reefs, but the interior of these seas is
not. The trade-winds prevail there; they are subject to severe northern
gales, and some parts are occasionally visited by tremendous hurricanes.
By the levelling across the peninsula of Panama by Mr. Lloyd, in 1828,
the mean height of the Pacific above that of the Atlantic was found to
be three feet six inches.
The Pacific does not penetrate the land in the same manner that the
Atlantic does the continent of Europe. The Red Sea and Persian Gulf are
joined to it by very narrow straits; but almost all the internal seas on
the eastern coast of Asia, except the Yellow Sea, are great gulfs shut
in by islands, like the Caribbean Sea and the Gulf of Mexico, to which
the China Sea, the Sea of Japan, and that of Okhotsk are perfectly
analogous.
The set of the great oceanic currents has scooped out and indented the
southern and eastern coasts of the Asiatic continent into enormous bays
and gulfs, and has separated large portions of the land, which now
remain as islands—a process which probably has been increased by the
submarine fires extending along the eastern coast from the equator
nearly to the Arctic Circle.
The perpetual agitation of the ocean by winds, tides, and currents, is
continually, but slowly, changing the form and position of the land—
steadily producing those vicissitudes on the surface of the earth to
which it has been subject for ages, and to which it will assuredly be
liable in all time to come.