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Jacques W. (Jacques Wardlaw) Redway.

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Avinter temperature is not far from 17° (63° F.). In sum-
mer it extends as far north as the Kuril Islands ; in win-
ter it scarcely reaches the Japan coast. Eecent surveys
show that, contrary to common opinion, no part of the
Kuro Siwo enters the Arctic Ocean through Bering Strait.
In a few instances only has a setting of water into the
strait been observed, and these have resulted from strong
southwesterly winds. The prevailing movement in Bering
Strait is a feeble flow from the Arctic Ocean.

It has been definitely ascertained that much of the cir-
culation of the colder waters of the ocean takes the form
of undercurrents, but no survey of an undercurrent has yet
been made. Two very definite surface currents of water
have been observed, however, and their position is fairly
well known. These are the Arctic Currents. One of them
flows southwards along the east shore of Greenland, finally
turning into Bafiin Bay ; the other flows on the west shore
and, emerging into the Atlantic, meets the Gulf Stream off
Newfoundland.

Oft' the coast of Cape Hatteras, almost in the track of
the Gulf Stream, is an adverse current known on pilot
charts as " Little Hell." It is marked by heavy, choppy
waves, and persists, even in the face of a strong southerly
wind. Its waters are cold, and it is thought to result from
the rising of an arctic undercurrent to the surface.



WAVES, TIDES, AND CURRENTS 205

The Antarctic Current is the chief movement of cold
water iu the southern hemisphere. It is a drift rather
than a definite current, however. Its waters are several
degrees cooler than those with which they finally com-
mingle.

Economy of Ocean Currents. — One of the chief and
most important efi'ects of marine currents is the equalizing
of the temperature of ocean waters. Without this inter-
change the heat of equatorial waters would sooner or later
become fatal to many forms of life, and the polar ice-caps
would intrude far into temperate latitudes. The more
practical effects are seen by comparing the coast of Labra-
dor with that of the British Isles, in the same latitude.
The harbors of the former are blocked with ice for five or
six months of the year ; the latter is open the year round.
The former is bathed by cold waters ; the latter by the
drift of the Gulf Stream. The port of Hammerfest, situ-
ated within the Arctic circle, is an open harbor free from
obstnictive ice all the year round. It is very doubtful if
warm currents have any perceptible efiect on the tempera-
ture of a region at any considerable distance from the
coast, but that they keep the coast free from ice is beyond
question. How does this affect commerce?

Evaporation is V(;ry great along the courses of warm cur-
rents and the moisture borne with the wind adds no little
to the rainfall of the regions. When the moisture is con-
densed the latent heat set free adds warmth to the region.
Cold currents have a chilling effect on the air, and if the
latter has much moisture it is apt to take the form of fog.
The Newfoundland and Labrador coasts probably got their
dense fogs in this way. Ocean currents thus are indi-
rectly factors in climate.

Sargasso Seas. — Within the ovals formed by the
branches of the Equatorial Current and tlunr drifts there



206 PHYSICAL GEOGRAPHY

are extensive accumulations of marine j^lauts. These were
named by Spanish navigators Zm-gazzo, or grassy seas.
The accumulations have been sometimes attributed to the
eddying motion of the current and its drift, but of this
there is little or no evidence. Calm water is necessary for
the growth of these species forming the accumulations, and
they occur most frequently in such localities.

Physiographic Effects of Oceanic Movements. —
So closely related to one another is the work of waves,
tides, and currents, that their physiographic effects can-
not well be separated one from the other. In general the
work of waves is both destructive and constructive — they
not only tear away coasts, but they build them as well.
On the other hand, the work of tides and currents is
mainly transporting — they carry material from one place
to another. Although waves act only at the surface, their
work is none the less effective, and throughout the whole
extent of coast one or the other of two things is con-
stantly going on — material is either being removed from
the shore or else it is being added to it.

The rugged outlines of coasts to a considerable extent
are results of wave action. The softer parts are worn and
broken, while the harder portions that remain largely con-
tribute to the frayed appearance of the coast.^' At first
the harder rock projects in the form of long arms ; then
these are broken, leaving a multitude of rocky islets.

Along the coast of the South Atlantic States, the effects
in places are still more noticeable. The shores of Cape
May, New Jersey, are wasting away at the rate of several
feet a year, and those of Charleston Harbor require almost
constant repair, so destructive is the incessant pounding
of the waves.

On the east coast of England, owing both to waves
and swift tidal currents, the yearly waste is considerable,



WAVES, TIDES, AXD CURRENTS 207

and since the time of Henry VIII. a belt about one mile
in width has been shorn from the Kent coast.^^ Along
the west coast of Scotland, and especially among the
Hebrides Islands, are many thonsaud rocky islets rising
from the sea like spectral watch-towers. They are all that
remain of a former coast as witnesses of the destructive
force of the waves.

As its name indicates, a cliff-girt coast is one that is
bordered by steep or by vertical cliffs. The chalk cliffs
of Dover, England ; the cliffs at Newjiort, Rhode Island ;
and almost the whole extent of the California coast are
examples of this type. Generally there is a narrow strip
of sandy beach between the cliff and the water's edge, but
sometimes this is absent. In every case the cliffs are
shaped by the action of waves. On account of a slow sub-
sidence of the coast, the sea has encroached on the laud,
and little by little, the waves have undermined and bat-
tered down the shores.

The constructive and building power of waves is finely
shown along the coast of the South Atlantic and Gulf
States and that of the Netherlands, the most noticeable
feature of which is the multitude of spits, barrier beaches,
and islands that border it.

In the building of shores not a little depends on the
position and direction of tides and local currents. If the
latter strike the shore broadside, or at right angles, the
Ijars and spits take the shape so common along the Gulf
coast. On the other hand, if they impinge upon the
shore obliquely, the sand and sediment are caught by the
swirl of the current, and deposited in curved forms vari-
ously known as sandy hooAs.

Forms of this character are the rule along the Massa-
chusetts coast. Cape Cod, Monomoy Point, and Nan-
tucket Beach are nothing but sandy hooks ; Marthas Vine-



208 PHYSICAL GEOGEAPHY

yard and Nantucket Islands contain half a score of such
examples. Sandy Hook Peninsula, now an island and an
obstruction to the navigation of Lower New York Bay, is
one of the most striking examples. Find similar examjjles
on the shores of the Nortli and Baltic Seas.

The effects of the tide in scouring out estuaries have
already been noted, but there are certain effects of tidal
currents that, at first, are not obvious. Waves are capa-
ble of battering down a cliff, but they are not able to re-
inove the material, and this, in time, lodging at the foot of
the cliff, would protect it from any further assaults of the
waves. But if the tidal currents remove this material, the
Avaves have an unprotected surface upon which to work.

The bars at the mouths of rivers are nearly always the
work of tidal currents, and so are many of the " banks " or
shoals that obstruct straits and sounds. The North Sea
contains many examples, and LoAver New York Bay is so
full of them that only a small part is available for deep-
draught vessels.

Ocean currents undoubtedly transport an enormous
amount of material. The Gulf Stream sweeps the shells of
certain marine organisms from the Caribbean Sea as far
north as the Carolina coast. The icebergs floated by
arctic currents bring down a large amount of gravel and
bowlders which are finally dropped in lower latitudes.
Both the bank on which the Florida Reefs are built, and
that on which the Bahamas have been formed, are thought
to have been the work of marine currents. It is by no
means impossible that constant deposition of matter car-
r^-^d by ocean currents may have resulted in extensive
changes of level in various parts of the earth's surface.



QUESTIONS AND EXERCISES.— If possible, evaporate a small
q^uantity of stream water of any kind in a beaker, or a porcelain dish,



WAVES, TIDES, AND CURRENTS 209

and note the result. Repeat the experiment with rain water. What
inferences can be drawn that are applicable to the second paragraph of
this chapter ?

Prove that ice, bulk for bulk, is lighter than water.

If possible observe the effects of waves on the shore of any conven-
ient body of water. Note the character of the work they do, or that
you find they have done. Explain how waves make beach sand.

If you are near the ocean, find the season of the year when the tides
are highest.

Refer to the map, p. 199, and note the direction of the tide waves in
various parts of the Atlantic Ocean. What is their general direction in
the South Pacific ?

Explain how ocean currents may affect navigation, either favorably
or adversely.

In one of the first chapters of his narrative, Robinson Crusoe speaks
of the great indraught of the Gulf of Mexico ; what feature is meant ?

Of several thousand sealed and registered bottles thrown into the
Gulf Stream, off the Florida Coast, a number were found afterward in
the Caribbean Sea, along the West Indies ; from the current chart, p.
201, explain their movement.

From any available cyclopedia, or other work of reference, prepare
an account of one or more of the following : the Gulf Stream, the Mael-
strom, the bore of the Amazon, the tides of the Bay of Fundy, the
Hell Gate, or the effects of storm-waves.

COLLATERAL READING AND REFERENCE.

PiLLSBURY. — The Gulf stream. United States Coast Survey.

Mill. — Realm of Nature, pp. 154-184.

Shaler. — Sea and Land, pp. 1-74, 187-222.

U. S. Hydrographic Office. — Use of Oil in Storms.



NOTES

' Color names are of frequent occurrence in the nomenclature
of the arms of the sea. The color of sea-water is both apparent
and real. The ai)pan'nt liue is often due to reflection from tlie
sky ; the real color to the substances insolation. Shallow water
is commonly greenish ; deep water a dark blue. The water of
the Gulf Stream has a peculiar blue color and is instantly dis-
tinguished from the lighter colored water on either side. The



210 PHYSICAL GEOGRAPHY

phosphorescence of sea- water, usually observed in warm regions,
is due to a microscopic organism, Noctiluca miliaris, that, like
the common firelly, has the power of emitting light. At times
the wake of a vessel seems like a track of fire.

^ Bulk for bulk, ice is lighter than water. Solid sea-ice floats
with about one-eighth of its mass above the surface. If it con-
tains air-bubbles, however, a greater proportion is out of water.

^ In a few instances the formation known as anchor ice takes
place. It results from the freezing of fresh water at the bottom
of an estuary into which salt water flows. The ice accumulates
on the bottom until its buoyancy overcomes the force with which
it adheres to the bottom ; then the whole mass rises to the surface.
It receives its name from the fact that it is very apt to begin
forming about anchors or other metallic substances lying at
the bottom. In certain cases these have been lifted from the
bottom and floated. In some instances large areas of anchor-ice
have become suddenly detached from the bottom, and the estu-
ary, a few minutes previously free from ice, becomes filled with
sludge. This form of ice is also called ground-ice.

* The formation of the pack is sometimes sudden and frequently
violent. The ci-unching from side-pressure is so great that not
only is the ice piled up in huge blocks, but the blocks, often
weighing many tons, are shot up into the air ten or twenty feet.

" The difference in the form of the Greenland and the south
polar icebergs is due to the character of the glaciers from which
they are broken. Antarctic glaciers are derived from sheets of
land ice ; Greenland bergs, on the contrary, are derived mainly
from the hunmiocky ice of glaciers that flow in ravines. It is
commonly asserted that most of the icebergs floating down
through Davis Strait come from Humboldt Glacier. As a matter
of fact scarcely a single one comes from this quarter ; they nearly
all come from Disko Bay.

'A breeze of two miles an hour throws the surface of still
water into ripples two or three inches broad and not far from an
inch in height. The slope of the wave is rarely the same on both
sides. The wind pushes the crest forward so that the front of the
wave is considerably steeper than the back. Large waves as a
rule result from the union of smaller ones, and this process goes
on until, finally, the accumulation is the greatest mass that the
breeze of the given velocity can move.



WAVES, TIDES, AND CURRENTS 211

' Strictly speaking, it is a matter of adhesion rather than fric-
tion, and wlien the wind blows over the surface of still water the
lower surface of the air actually remains in contact with the
water. In a stiff gale the dragging force exerted on the surface
of the water by the wind amounts to a little more than one
ounce on each square yard of surface.

' The effect of the wind is to push their crests forward rapidly,
practically flattening them.

' A stanch vessel with her head to the wind need fear but little
from the waves. The latter may smash everything above deck,
but the hull will ride tlie waves safely so long as they do not
board her. Riding so that the waves strike broadside, however,
is a different matter, and no vessel can accomplish it without
danger of foundering. The danger from waves arises not so much
from their height but from the possibility of their breaking upon
and boarding the vessel. Otherwise a ship can ride waves of sixty
feet as safely as tho.se of six.

'" In the use of oil and similar substances two results must be
studied — namely, to prevent the groictli of waves, and to prevent
their breaking. In the great ma.iority of instances, however, the
problem before the sailing master is to prevent the breaking of
waves. For this purpose it is found that sperm oil and oil of
turpentine are the best. In use, the oil is poured into a coarse
canvas sack and the latter is floated to the windward of the ves-
sel, being held in position by any conv^enient outrigging. The
oil oozing through the canvass spreads raj)idly over the surface of
the water. Instantly the waves, though they may run high, cease
to break. The following from the log of the Swedish brigantine
Droit is one of a great many similar testimonials gathered dur-
ing the past few years by the United States Hydrographic Office :
" I had seen upon the pilot chart that oil had been used with
good effect in calming heavy seas. I started to try it and had two
bags made of the capacity of two gallons each. These bags were
stuffed full of oakum, and then one gallon was poured into each,
half fish oil and liali petroleum. A very small hole was cut in
the bottom of each bag which allowed the oil to drop out freely.
One of these bags was suspended from each cathead, just out of
the water, and tlie result was sim[)ly a wonder to me, so much
so that I could hardly believe my senses. No more seas were
shipped and all hands turned to secure the main hatchway prop-



213 PHYSICAL GEOGRAPHY

erly, Avliich was impossible to do before on account of the risk of
being washed overboard. The former combers were now great
rollers only, not a sea breaking nearer than thirty feet from the
vessel. The crew were now able to pump out the ship and clear
up the decks in perfect safety. About 11 p.m. the sea broke over
the starboard side and smashed in one of the boats, but this was
found to be due to the loss of one of the oil bags, and as soon as
another was put out and kept supplied with oil no more waves
came on board."

" This theory of the tides is not accepted by all astronomers,
^ee Appendix, Table VI.

'^ At Lady Franklin Bay, Lieutenant ( now General ) Greely
observed that the tide came from the north.

'^ During pleasant weather the eddy of the Maelstrom is hard-
ly noticeable during slack water, or at the time of neap tides.
When the flood or the ebb of spring tides is strong, however, the
current is strong, and, with a hard northwest wind, it is a dan-
gerous locality.

" According to Herschel and Carpenter the winds themselves
pile up the waters in equatorial latitudes, thereby bringing about
a condition of inequilibrium. Lieutenant Maury held that the
difference in specific gravity between the saltier waters of equa-
torial and the fresher waters of polar regions is competent to
account for ocean cui-rents. That each is an important factor
cannot be denied.

'^ Owing to the turning of the earth on its axis, a point on the
equator travels 25,000 miles in twenty-four hours— a speed of
about 1,000 miles an hour. In latitude 60° it is only half as
much. Consequently water flowing from latitude 60° toward the
equator, every point of which has a greater velocity, has a ten-
dency to lag behind.

'" The velocity varies not only with tlie season, but also with
Mie age and the passage of the moon — that is, the valuations are
yearly, monthly, and daily. The velocity is greatest during sum-
mer and least in winter. The position of the axis of the stream,
or line of swiftest flow, changes also with the season. An adverse
wind will retard ; a favorable wind will increase its velocity.
A quartering wind or one blowing athwart is apt to push some of
the surface water out of the track of the stream, at the same time
pushing colder water into it. The fact that Gulf Stream water



WAVES, TIDES, AXD CURREXTS 213

is occasionally pushed against the coast has more than once
giv'en rise to the statement that its position is subject to change.

" Glaciers and glacial action have also had much to do with
the shaping gf surface features of these coasts . The coasts of Ire-
land, Norwaj', Alaska, and Chile much resemble that of Maine.
Their general outline, however, is due to submergence ; with the
lowering of the level of the land, the waters cover the valleys.

'" During the reign of Henry VIII. the church of Reculver stood
at the distance of a mile from the shore, but the sea now laves
its foundation stones. The famous Goodwin Sands, a shoal about
twenty square miles in extent, southeast of Kent, was formerly a
part of the mainland. In the twelfth century, during a severe
storm, this area was washed away by the sea, and has been cov-
ered with water ever since. The channels through this shoal
shift with every storm.



CHAPTER XII

THE ATMOSPHERE AND ITS PROPERTIES : WINDS

The atmosphere, or air, is the gaseous substance that
forms the outer envelope of the earth. It rests on the
land and the water, and probably penetrates both to a
considerable distance. Being a part of the earth, the
atmosphere partakes of all the general motions of the lat-
ter, but it has also certain movements of its own, and
these are very closely connected with life and its envi-
ronment.

The air is not a simple, or elementary substance ; as
noted on p. 22, it is a mixture of several elements. The
chief constituents, nitrogen and oxygen, have the propor-
tion of about four parts of the former to one of the latter,
and the proportion does not change materially. The re-
maining constituents, water vapor, carbon dioxide, and float-
ing matter vary greatly. The vapor of water rarely exceeds
one part in one hundred of air. It is nevertheless a most
important constituent, for it is in this form that the water
is borne from the sea and shed upon the land. The floating
particles of smoke, dust, and other matter are also essen-
tial, for they aid materially in condensing the water vapor.

The air is highly elastic. Stop the nipple of a bicj'cle
pump and push the piston quickly; note what occurs.
Pressure, therefore, decreases the volume, making the air
denser. When the pressure is relieved, the air again
expands and is less dense or rarefied} Air next the
ground is denser than that above, because of the pressure

214



THE ATMOSPHERE AXD ITS PROPERTIES 215



or



weight



of that overlavmc^ it.
the sea:



The density decreases
with the distance above the sea; at an altitude of two
miles the density is only two-thirds that at sea-level. At
sea-level a cubic foot of air weighs a Httle more than one
Troy ounce.

The force with which the air presses upon a given sur-
face is called its tension ; and, practically, the tension is
a form of expressing its pressure ^ on the
rock envelope. At sea-level, the column
of air rests upon the surface with a press-
ure of about fifteen pounds on every square
inch, or a little more than a ton on each
square foot of surface. The tension varies
slightly in different latitudes, being a little
greater near the tropics than elsewhere.

It is most convenient to estimate the
tension of the air by observing the height
of a column of mercury, or quicksilver, that
will just balance it. The instrument used
for this purpose is called a barometer. It
consists of a glass tube closed at one end,
and filled with mercury ; the open end is
placed in a small cup filled with mercury.
The pressure of the air on the surface of
the mercury in the cup keeps the column
in the tube in place. If the column in the
tube rises it signifies that the pressure of
air overhead is increasing ; if it falls, the
pressure is decreasing. The weight of the
mercury in the tube is just equal to that
of a column of air, having an equal base, and the two
Vjalance each other.

The atmosphere is warmed partly by the direct rays of
the sun and partly by the heat radiated from the earth.



y



THE BAROMETER.



210 PHYSICAL GEOGRAPHY

It is also heated by compression and cooled by expan-
sion. When a volume of air is compressed, it becomes
greatly heated. Thus, air that descends from higher to
lower levels, becomes heated because it moves into a re-
gion where the density and tension are greater. In the
same way, a volume of rising air expands and is cooled,
because it goes into a region where the tension and density
are less. Heat causes the air to expand and, bulk for bulk,
warm air is therefore lighter than cold air.^ If a volume
of air is warmed from freezing temperature to that of in-
tense summer heat its volume is increased nearly one-fifth.

The temperature of the air varies both with latitude
and with altitude. In equatorial latitudes the mean tem-
perature of the air over the sea is not far from 32°
(90° F.) ; in polar regions it ranges much below 0° (32° F.).
With respect to altitude there is a fall of temperature at
the rate of about one degree for every three hundred feet
of ascent. The effect is very noticeable in the equatorial
Andes. At the base of tlie mountains the heat is intense ;
at an altitude of ten thousand feet the air is mild and
pleasant ; at seventeen thousand feet one lives in a region
of perpetual snow.

Movements of the Atmosphere.— Like the waters
of the sea, the air is everywhere in motiou. The move-
ments are both general and local. The attraction of the
sun and the moon undoubtedly causes atmospheric tides
something like the tides of the sea. Their effects, how-
ever, are very slight, and practically nothing is known
about them.

Sensible movements of the air are called ivinds, and
they are caused by changes of temperature. When the
air at some locality or other is heated to a temperature
higher than that surrounding, it expands and, becoming
lighter, bulk for bulk, it is pushed upward by the heavier



THE ATMOSPHERE AND ITS PROPERTIES 217



air that flows in. In this way winds originate. Such
movements of the air are everywhere taking phice, and it
is evident that they a,re examples of the force of gravity.

Equatorial and polar regions are not equally heated.
The former receives the almost vertical rays of the sun ;
the latter only oblique rays. The air in low latitudes,


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Online LibraryJacques W. (Jacques Wardlaw) RedwayElementary physical geography : an outline of physiography → online text (page 14 of 25)