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Jacques W. (Jacques Wardlaw) Redway.

Elementary physical geography : an outline of physiography online

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apt to have her yards stripped off, even if the masts are not
snapjied.

* The accompanying storm cards are adapted to use in any
cyclones of the northern hennsphere ; the upper diagram is
available for the route between New York and English ports.
The small arrows fly with the wind ; the long arrow represents
the storm track through the belt of latitude to which the dia-
gram applies. For West Indian hurricanes note that tiu' stoini
track recurves as follows ; June and October, latitude 20° to 2'.i°\
July and September, latitude 27° to 29° ; August, latitude '30° to
3:}°. When a falling barometer and other signs indicate the ap-
proach of a cyclone, s«;lect the diagram that applies to the lati-
tude and plot the position of the ship according to the direction
ot the wind. In low latitudes, for instance, the wind is N NK ;



264



PHYSICAL GEOGRAPHY







^'^$ %'-



"3^ "z^l I ^-



i






'w 5W. ^e^"
E SE. Oft'*''*'



the vessel is tlieu in the position that is sliown on the lower
diagram, and is in the dangerous semicircle. If possible it is
best to lie-to (on the starboard tack), and observe the wind ; if
((f) it freshens ivithout shifting, the vessel is certainly in the
storm track. In this ease the navigator keeps off, with the wind
on the starboard quarter, holding to the course, {b) If it shifts
to the r/!/ht, the ship is to the right of the storm track and should

be put on the starboard tack,
making as much headway as
possible until obliged to lie-to.
(c) If it shifts to the left, the
ship is on the left of the storm
track and should be brought
about until the wind is on the
starboard quarter, lying-to on
the port tack if necessary. In
scudding, the wind should be
kept always on the starboard
tack to run out of the storm.
If the vessel is in the latitude
where the cyclone probably
recurves (according to the
month) the middle diagram is
applicable. Suppose that the
wind is S E ; the vessel then
has the position marked in
the middle diagram. It is on
the right of the storm track
and should run out as in (6),
previously noted. In high lat-
itudes the upper diagram is
indicated. Suppose that the
wind is N E. The ship then has the position shown to the left
of the storm track, in the navigable semicircle, and should be
brought about as in (c), previously noted. In any case oil may
be used to prevent the waves from (breaking over the vessel.

* It is unstable because the cold air is resting on a layer of
air that is specifically lighter, and when the latter is pressed up-
ward it soon develops into a whirl. Winter cyclones are not
confined to definite localities, as ai-e tropical cyclones, and in




■vT* ^



1 1:



'- "5- E. ''S



K'^// %^ ^ •=■



*/„












-^w sw. /s sw
STORM CARDS.



CYCLONIC STORMS 265

comparison with the latter their tracks are erratic. Their gen-
eral direction is easterly, however.

^ In very many cases a land-storm may originate at sea and
finally end somewhere at a considerable distance inland. Many
West Indian hurricanes sweep into the Gulf of Mexico and thence
into the Mississippi valley. On the other hand, there are many
— perhaps a majority of north Atlantic storms — that begin far in
the interior of the continent. In many instances storms have
originated somewhere in the Pacific, crossed the United States,
and the Atlantic, finally disappearing in the interior of Eurasia.
Many of the cyclonic storms of the Pacific Coast of the United
States travel southward between the Coast Range and Sierra Ne-
vada Mountains. In some instances the storm is dissipated in
the arid region to the southward, but occasionally a cyclonic
disturbance finds enough moisture to enable it to pass into the
Mississippi Valley.

' Under such conditions a warm wave results. Although with
respect to temperature, the difference between warm waves and
normal Aveather is not so great as that between cold waves and
normal weather, yet the former are far more fatal. In all the
densely populated parts of the country the advent of a warm
Avave is marked by an enormous increase in the death-rate. Dur-
ing several warm waves that, in July, 1881, covered the Missis-
sippi Valley, there were more than one thousand deaths from
sunstroke — probably a greater numI)or than have resulted from
the cold waves of a score of years. Warm spells may result from
other cases. The typical " warm wave " is the result of settled
conditions, and not disturbances. The air resting upon the given
area without being disturbed in the course of two weeks becomes
intolerably hot.

* A cold wave that occurred in Jaimary, 1888, is an example
of the effects of the translation of cold air from the extreme
north. At Helena, Montana, the temperature fell fifty degrees
in four and one-half hours, and sixty-four degrees in less than
eighteen houis. At Crete, Nel)raska, the tlicrmouieter fell eigh-
teen degrees in three minutes. This wave covered almost the
whole United States, carrying freezing weather into l^'ioridn.
California, and southern Tcx.is. In March, 1HH7, a cold wave,
extending along th*; valley of the St. i^awrenee River, wa.s
marked by a fall of temperature ranging from fifty to seventy*



266



PHYSICAL GEOGKAPIIY



one degrees in twenty-four hours. In Denver, January 15, 1875,
there was a drop in temperature of forty-eight degrees in one

hour.

• This term was first noted in the records of an exploring party
which, in 1747, wintered on tlie shores of Hudson Bay, at a place
now called York Factory. It was introduced as a technical name
into the weather .service in 1876.

" General A. W. Greely, U. S. A., notes twenty-five tornadoes,
in which the aggregate damage reached tlie sum of $15,000,000,
while the loss of life was nearly fifteen hundred. Concur-
rent with a storm that in February 9, 1884, crossed the United
States, there were sixty distinct tornadoes. On that day eight

hundred people
were killed, twen-
ty-five hundred
were wounded,
and more than ten
thousand build-
ings were de-
stroyed.

" The story il-
lustrated in this
cut is a grcM'^some
summary of hor-
rors. The house
was surrounded
by a grove of trees.
To the east of the house the trees were felled and twisted from
right to left ; those west of the house were untouched. The
house itself was demolished and the debris hurled into the creek-
bed near by. When tlie tornado cloud swooped down upon the
house, the family fled for their lives, but unfortunately in the
wrong direction. At first they ran northward, a direction of
safety. Then, one after another, they turned eastward — first a
little girl, who was instantly killed ; then an older boy and a
girl, who were bruised and partly stripped of their clothing.
The mother ran directly into the whirl and was found crushed
and mangled against the trunk of a tree. The father, with the
babe in his arms, had reached a place of absolute safety, but in
his fright turned eastward and ran into the whirl. They were



w


N

C\NG PCOPLT


fAT HER
AND BABY/


E


^y-' -

li..-
U-.




'■/"■> MOTHER
fclRU


-^ AS N3^




M-tChEn




Q.; .' .-p^


HOUSE


s




^



CYCLONIC STORMS 267

picked up by the wind, thrown several hundred feet, and instant-
ly killed. Ad inspection of the accompanying illustration
shows that the safest path of flight is toward the northwest or
the southeast ; to the southwest or the northeast is one of the
greatest danger.

'^ In 1853 the necessity for a weather bureau was urged by
Commander M. F. Maury, iDut it was not until after his death
that systematic land observations were carried out. Tlie first
organization was effected by General Myer, U. S. A., Chief Signal
Officer, who trained the rank and file of his department to make
weather observations. Since that time the Weather Bureau has
been attached to the Department of Agriculture and placed in
charge of its Secretary. Most of the European nations liave es-
tablished similar bureaus, and daily observations are made on all
transatlantic steamships. So complete are these records that
scarcely a storm occurs in the North Atlantic which is not fol-
lowed and its path jjredicted Avith a high degree of probability.
Flags (or sometimes painted cylinders and cones) are displayed on
public buildings in nearly every town in the United States and
Europe. For land service these flags are commonly used. A
square white flag denotes clear weather ; a blue flag, rain or snow.
Temperature is indicated by a triangular blue flag. Above the
.square flag it denotes higher temperature ; below the square flag,
l(jwer temperature ; its absence denotes no change in tempera-
ture. Whenever the temperature falls twenty degrees or more
(sixteen degrees in the northern States) if the mercury sinks as low
as 32° (P.), it is technically a cold wave, and its approach is indi-
cated by a white flag containing a black square. It is commonly
called the "black flag." A fifth flag is sometimes employed to
indicate local storms. For the benefit of mariners a Monthl.v
Pilot Chart for the North Atlantic is published by tlie United
States Hydrographie Office. This shows storm tracks of the pre-
ceding month, and the position of ice, fog, floating wrecks
(called "derelicts"), and other obstacles, for the current month.






i/



CHAPTER XV

ELECTRICAL AND LUMINOUS PHENOMENA OF THE

ATMOSPHERE

Electkicity is a form of energy that is manifested
chiefly by its effects ; of its actual nature practically noth-
ing is known. The laws pertaining to it are fairly well
known, however; and, like most of the other forces of
nature, it is a most useful servant when under intelligent
control. In the slender thread of the incandescent light
and the carbons of the arc light it appears both as light and
intense heat. Passing through insulated copper-wire that
surrounds a core of soft iron, it converts the latter into a
magnet, and thus harnessed it becomes a generator of great
power. Electrical energy seems to be a form of motion,
and it may be produced by motion. It is manifest not
only in the earth and the air, but in space as well.

The fundamental laws of electrical energy are not diffi-
cult to understand. If a pith-ball, suspended by a silk
fibre, be brought near a piece of hard rubber, or vulcanite,
that has been briskly rubbed by flannel, the ball will at
first cling to the vulcanite and then immediately be re-
pelled from it. If another ball, electrified in a similar
manner, be brought near the first, the two will vigorously
repel each other.^ If, however, the second pith-ball be
electrified by a piece of glass rubbed with silk, the two
balls will then show a strong attraction for each other.

Such an experiment demonstrates the principal laws of
electricity. Bodies similarly elech'ijied repel ; bodies dif-

268



ELECTRICAL AND LUMINOUS PHENOMENA 269

ferenily electrified attract one anotJier. The electricity de-
veloped when glass is rubbed with silk is called positive ;
that produced by rubbing vulcanite with flannel, negative.
Electricity passes quite freely through metallic sub-
stances, but with difiiculty through such material as silk,
wool, gums and resins, dry wood, and dry air. When,
however, the electric force is so great that it will pass
through these it is said to have a high potential, just as
steam confined within a boiler is at high pressure.'^ The




LIGHTNING

From an imtantancoits photograph by IV. F. Cannon.

" sparks " produced by rubbing sealing Avax or vulcanite
with flannel are of moderately high potential.

To the electricity of the air and the earth many of the
most marvellous phenomena are due. In the simplest
form we see its efi'ects when tiny sparks result from rub-
bing the long knap of woollen cloth or the fur of an animal
pelt ; we see its grandest effects Avhen great flashes of light-
ning forge across the sky. The electricity of the air is usu-
ally of high potential ; that which forms a flash of light-



270 PHYSICAL GEOGEAPIIY

ning is of exceedingly high tension. Next the earth,
however, the electricity of the atmosphere is not commonly
noticeable, especially if the air is moist. At considerable
elevations, or at times when the air is very dry, its pres-
ence becomes marked. The hair of the head crackles as
a comb is drawn through it, and tiny sparks are given oflf
when woollen clothing is rubbed. In the dry summer
climate of deserts, the hair of horses' tails stands out like
bushes, and their manes are like fright wigs ; sparks half
an inch long may be drawn from a metallic body insulated
from the ground.

Ordinarily the electricity of the air is positive, but, with
much moisture present, it may be negative. Just before
the beginning of a gentle shower it often becomes nega-
tive, and during a heavy storm it frequently changes from
positive to negative and vice versa very rapidly. In such
cases the character of the electricity may vary in different
places ; that is, it may be positive at one locality and neg-
ative at another, only a few miles distant.^

Neither physical nor chemical change in a substance
takes place without the development of electric energy.
Friction likewise is a potent factor in its generation. The
flowing of water; the chafing of the winds against the
earth's surface ; even the friction of the air against itself
produces it copiously. Evaporation and condensation are
attended by an electric disturbance ; and inasmuch as an
enormous amount of the vapor of water is constantly aris-
ing from the earth at one place to be condensed,^ at an-
other these changes in physical form, together with fric-
tion, maybe regarded as the chief agents in its production.

Since these factors are constantly at work, it is evident
that electricity is being constantly produced. But the
electricity of the air and that of the earth are unlike ;
the two, therefore, neutralize each other. Because moist-



ELECTRICAL AND LUMIXOUS PHEXOMEXA 271

m-e is a good conductor, if the air be moist the two kinds
of electricity readily pass, one from the earth to the air,
the other from the air to the earth, until the equilibrium
is restored. This transference is quietly but constantly
going on, so that ordinarily there is no great accumulation
of electricity. It is only when the air is very diy that
the transference takes place with difhculty.

Thunder Storms. — When clouds are present in the
air, however, there is often an enormous accumulation of
electricity, either within or upon their surface, and the
transference or exchange, therefore, may become violent
and destructive. Such disturbances are commonly known
as thunder storms.

"WTien large masses of cloud hover over the earth it
sometimes happens that they are differeutly electrified.
Under such circumstances the two clouds are mutually at-
tracted. The potential of the electricity is very high and
the transference takes place in the form of blinding Hashes
of lightning.^ Usually the interchange takes place between
the two clouds, but not infrequently it is between the
clouds and the earth. The form of lightning varies. The
interchange takes place always along the line of least re-
sistance, and as this is seldom, if ever, a straight line, it
has taken the name, zig-zag lightning.^

Another form is known as sheet lightning. This inter-
change takes place, not along a line, in the form of a
chain, but simultaneously over a large area. The dis-
charge is not attended by a crash of thunder nor by a
blinding flash of light. On the contrary there is nothing
but a quivering, bluish glow that lasts sometimes for eight
or ten seconds. A sheet-lightning discharge takes place
usually between the earth and the clouds. The electricity is
of low potential and therefore not destructive. This name
is also applied to flashes of lightning that, occurring at



273 PHYSICAL GEOGRAPHY

a considerable distance, are reflected from the under sur-
faces of clouds.' Still another form is commonly called
hall lightning. Of this kind of discharge but little is
known, and although its occurrence has been alleged for
more than two hundred years, its existence is somewhat in
doubt.

Occasionally the discharge takes unusual forms. Among
them, but rare in occurrence, is the phenomenon known as
St. Elmo's fire. This discharge, though best known at sea,
is also occasionally observed on land. At the time of its
occurrence there is usually a considerable electrical disturb-
ance, though not necessarily a thunder-storm. Owing to
the feebleness of the light emitted, it is rarely if ever
noticed in the daytime. It consists of a pale, shimmering
light, at the tips of the yards, spars, and from every
pointed part of the ship's rigging. The glow lasts for a
few moments and then the phantom light disappears.^ In
all probability the St. Elmo's fire is identical with the
bluish glow that is seen when a frictional electrical
machine is worked in the dark — a phenomenon commonly
known from its shape as the " brush " discharge.

The Aurora Borealis. — This magnificent display,
commonly called the " northern lights," » is without doubt
an electrical phenomenon that possibly is similar in nature
to the brush discharge. It is most common in high lati-
tudes, though it is occasionally observed between latitudes
30° and 40° N. In appearance the aurora is an arch of
light stretching across the sky fifteen or twenty degrees
above the horizon. It has a tremulous motion, and the
upper streamers sometimes mount to the zenith.

In color the aurora varies between pale green and crim-
son. Sometimes it closely resembles a green curtain edged
and lined with gold. Auroras are most frequent during
sun-spot periods ; they are usually coincident with mag-



ELECTRICAL AND LUMINOUS PHENOMENA 273

netic storms also. In circumpolar regions they are of
daily occurrence. '° The cause of auroras is not with cer-
tainty known, but they are thought to be an exchange be-
tween the electricity of the atmosphere and that of the
earth. The arch of the aurora nearly always surrounds
the earth's magnetic pole."

Magnetism. — A bar of steel, iron, or nickel, or a piece
of lodestone '~ that has the property of attracting and
holding to its surface small pieces of similar metals is
called a magnet. Steel retains its magnetism permanently,
and for all practical purposes the magnet is a flat bar of
polished steel, eight or ten inches in length. Sometimes,
however, it is bent into a U-shaped form called a horse-
shoe magnet.

When a bar of steel is magnetized, it is found that the
magnetic force is not uniformly distributed throughout the
bar, but is most intense at or near the euds.''^ These are
the poles of the magnet ; they are designated as positive
+ , and negative — , according to the direction they take
when the magnet is suspended at the centre of gravity.

If a slender bar of ordinary steel be suspended by a hair
from its centre of gravity, it Avill lie indifierently in any
direction in which it is placed. If the bar be magnetized,
however, it takes new properties. It no longer remains in-
differently in any position ; on the contrary it turns until
its direction is nearly or quite north and south. It no
longer remains balanced, but the north-pointing end dijis
toward the earth.

If now another bar magnet bo brought near it, the latter
shows no little sensitiveness. If the -f- end of the bar bo
presented to the + end of the suspended magnet, the latter
will instantly turn away ; if the two — ends be brought to-
gether the same thing will be noticed. On tli«' contrarv if
-f and — poles be brought together they are strongly at-



274



PHYSICAL GEOGRAPHY



tracted. From these experiments the laws of magnetism
are deduced. Like magnetic poles repel ; unlike poles at-
tract. Either pole of the magnet, however, will attract
alike an unmagnetized piece of iron or steel.

It is upon these laws that the whole science of naviga-
tion by the compass depends, for the earth behaves as a
magnet" and the essential part of the mariner's compass is
also a magnet.

Magnetic Variation. — The earth's magnetic poles are
not situated at the geographical poles. The magnetic north
pole is situated Avest of Boothia Land, a few miles north
of the crossing of the 97th meridian and the 70th parallel.




LINES OF EQUAL MAGNETIC VARIATION



Its position is not fixed, and it is moving in a westerly di-
rection.'^ The position of the magnetic south pole is not
known, although roughly approximated.

Because the magnetic poles are not situated at the geo-
graphic poles, it is evident that the magnetic needle can



ELECTEICAL AXD LUMIXOUS PHEXOMEXA 275

point due north and south in but few places. In the ac-
companying chart, a heavy black line passes through
these points. This line, called the agonic, is the line of
no variation. West of this line the north-pointing end of
the needle turns toward the east, and east of it it swerves
to the west. Along each of the lighter lines the needle
has the same deviation at all points, and these lines, there-
fore are called isogonics or lines of equal variation. This
deviation from the true meridian is called declination.

Trace the course of the line of no variation.

Besides that element of magnetic force that causes the
needle to lie in a nearly north-and-south direction, there
is another that causes it to dip or incline one end toward
the earth. This is called the vertical force, or inclination.
Along an irregular line passing around the earth, some-
times north of the equator and sometimes south of it,
the needle has an absolutely horizontal position. North
of this line the negative, or north-pointing end, dips
toward the earth. The farther the observer goes north-
ward, the stronger becomes the vertical force, and when
the magnetic north pole is reached the needle has a ver-
tical position, the — pole being next the earth.

South of the magnetic equator, or aclinal, the conditions
are reversed. The + pole dips more and more, until, at
the magnetic south pole, the needle is again vertical with
the + pole next the earth. A line on which the dip is
everywhere the same is called an isoclinal.

Not only does the position of each isogonic vary from
time to time, but the rate of variation is not uniform ; even
at the same place the rate varies from year to year. lu
the northwestern part of the United States the amount of
variation is at present from 3' to 7' ; in the southwestern
part it is, at jn-esent, notliing ; in tlic <>asteru and central
parts it varies from 5' to 3'.



2T6 PHYSICAL GEOGRAPHY

The deviation from the true geographical meridian also
varies from day to day. Most of these variations are
periodical. Some are daily, some monthly, and some
yearly ; the}^ are probably caused by the daily rotation of
the earth, the passage of the moon, and the annual motion
of the earth. There are also irregular changes in variation
which cannot be accounted for.

Such changes in variation are rarely great ; in temperate
and in low latitudes they cannot well be detected except
by close measurements. In the vicinity of the magnetic
pole, however, they are more marked. At Point Barrow
and at Lady Franklin Bay, during a period of twenty-four
hours, a change of nearly eleven degrees Avas recorded.'^

Magnetic Storms. — Not infrequently the irregular vari-
ations of the needle are so violent that they have been
called magnetic " storms," and during the progress of one
of these disturbances the needle is in a constant tremor.
Magnetic storms seem to be closely associated with the
spots that at times are visible on the surface of the sun.
The sudden formation or change in the position of a sun-
spot is nearly always attended by great magnetic disturb-
ances. The period when they are most frequent, more-
over, corresponds to the period when sun spots are most
numerous.'^

The Mariner's Compass.— The compass is a slender
bar of magnetized steel, so constructed as to balance on a
pivot and turn freely upon it as well. Usually it is armed
with a sliding weight, so adjusted that it exactly counter-
balances the dip or vertical force, thereby keeping the
needle in a horizontal position.

On land the compass is of but little practical use except
in rough surveys. On the sea, however, it furnishes the
only means by which a vessel may be kept continually on
her course. For this reason the mariner's compass is



ELECTRICAL AND LUMINOUS PHENOMENA 277



constructed with tlie greatest care and precision.'^ The
needle, which consists of one or more slender bars of steel,


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Online LibraryJacques W. (Jacques Wardlaw) RedwayElementary physical geography : an outline of physiography → online text (page 18 of 25)