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Jacques W. (Jacques Wardlaw) Redway.

Elementary physical geography : an outline of physiography online

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is fastened to a circular card subdivided into thirty-two
parts, on which are printed the cardinal directions. These
are called points of the compass. The compass-box is
mounted on gimbals, so that, no matter what may be the
motion of the vessel, the box always swings into a hori-
zontal position.

In going over almost every travelled ocean route,
the variation of the compass changes day by day. On
the regular routes of the
transatlantic liners, the
variation increases from
about eight degrees at
New York to more than
thirty -five degrees at the
crossing of the 40th me-
ridian. It then decreases
to about twenty degrees
at Liverpool.

In arctic regions, where
the horizontal element of
force is so weak, and the
dipping force so strong,
sailing by compass is a very difficult matter. Not only
does the variation change rapidly over short C(Kirsos, but
the needle becomes exceeding sluggish. On whaling ves-
sels it is customary to attach a line to the compass-box so
that the steersman, by occasionally shaking it, may better
judge the course over which the vessel is sailing.

Luminous Phenomena.— Transparent as it seems, the
atmosphere nevertheless does not afford passage to all tlie
light that may be transmitted through it. A ray of light
in i)assing obliquely is not only refracted, or bent out of




MARINKRS CO.Ml'ASS
All ordiiiaiy pattern.



278 PHYSICAL GEOGEAPHY

the direction iu which it started, but possibly it is decom-
posed into differently colored rays. The distortion that
one may observe by looking at an object across the top of
a very hot stove, or a smoke-burning chimney is an ex-
ample of refraction. On the other hand, the color effects
observed when light passes through a glass prism, such as
a chandelier pendant, or even the bevelled edge of plate-
glass, are examples of decomposition — the beautiful dis-
play of the colors red, green, and violet, with their com-
pounds resulting.

A ray of light striking the surface of a highly polished
metal or vitreous substance is refiected, rebounding in the
same manner as does a rubber ball thrown against the floor.
The same thing occurs when the ray strikes the surface of
a body of water, or even that of two layers of air resting
one upon the other.

The air always contains innumerable dust-motes and
particles of matter so fine and light that they seem alwaj's
to float. This is seen when a few rays are admitted into a
darkened room ; the passage of the rays is marked by the
light reflected by the motes ; and it follows, therefore, that
a part of the light emanating from a luminous source is
always scattered. The scattering of the light in this
manner is called diffraction. It is a singular fact, more-
over, that some kinds of floating matter will scatter the
blue, while other kinds scatter the red rays.

The color of the sky is thought to result from diffrac-
tion. The red rays are scattered and the blue rays ordi-
narily reach the eye. At times, however, wdien the air is
heavy with dust, the sky acquires a hue that is distinctly
red. This was very noticeable in 1883, after the eruption
of Krakatoa ; for nearly a year the sunsets were exceed-
ingly lurid. Ordinarily, at sea, "the blueness of the sky is
very marked, and the color is purer than on land ; with ac-



ELECTRICAL AND LUMINOUS PHENOMENA 279

cumulating moisture, however, it may acquire purplish
tints. At very great elevations, also, the blue gives way
to a dead hue that approaches blackness.

Mirages. — When a layer of air rests on another of
different temperature and density, the surface of contact
often reflects so much light that it acts as a mirror. If the
surface is lower than the eye of the observer, the reflection
much resembles that produced by a body of water, and a
mirage results. In deserts and arid regions, the illusion is
so perfect that nothing but experience will enable one to
distinguish the mirage from a lake. The " lake " mirages
of the Colorado Desert have hired both cattle herds and
travellers to their death.

With the reflecting surface above the eye, the character
of the mirage differs. Thus, at times, ofif the lake shore
at Chicago, one may see the lighthouse and the shijiping at
the mouth of the river inverted in the air. If possible,
illustrate this by means of a large mirror held overhead,
face downward.

Still another form of mirage occurs when objects, ordi-
narily hidden by the earth's curvature, are brought in
sight. It sometimes happens that the rays of light reflect-
ed from an object, are refracted so that they are curved
slightly toward the earth, and a distant object is thereby
brought to view. This phenomenon occurs at times along
the Mediterranean and Ked Seas, and it is not unknown
along the Great Lakes. As a rule, a dry, still atmosphere
is essential to tlie formation of the mirage.

Coronas and Halos.— The ring or rings aboiit the sun
or the moon are very common phenomena. The small
rings are coronas ; the larger ones, halos. In the case of
the corona, which is not of very common occurrence, there
is usually a series of concentric, colored rings. Those, it
is thought, result from diflractiou, the light being scattered



280



PHYSICAL GEOGRAPHY



by the moisture of the atmosphere. The halo around the
moou is probably caused by refraction, and it appears
when the air is very moist. For this reason it is apt to
portend rain or snow.

The halos of the sun, which are associated usually with
cold weather, probably are caused by the refraction of the
light as the latter passes through the ice crystals of cirrus




HALOS OBSERVED BY GENERAL GREELY

clouds. Frequentl}' there are several circles. Some of
them are concentric ; some are tangent one to another ;
and some intersect one another. At the places of inter-
section and of tangency more light is radiated, and these
spots, therefore, are sometimes very bright ; they form the
sun (hxjs, or mock suns.

Rainbows. — Occasionally, during a summer shower,
when the sun breaks through a rift in the clouds, the light
passes through the falling drops of water in such a way
that it is not only refracted but decomposed. The resulting



ELECTRICAL AND LUMINOUS PHENOMENA 381

decomposition is the arch of colored light that constitutes
the rainbow. The bow is blue and violet on the inner, and
red on the outer side. Sometimes there is a larger second-
ary box in which the order of colors is reversed.

The rainbow is best observed when the sim is near the
horizon. The observer sees the bow when his back is
turned toward the sun. The rainbow is frequently ob-
servable when heavy waves break and send spray high
into the air, and also in the ascending spray of cascades.

QUESTIONS AND EXERCISES.— Verify the statements concern-
ing the mutual attraction and repulsion of electrified bodies, observing
the directions contained in note i.

Verify the statements concerning the laws of magnetism noted on
p. 273, using one or more stout knitting-needles and strands untwisted
from silk thread. For observing inclination the strand of silk had
better be fastened by means of a slip knot to the needle ; for the other
experiments the needle may be thrust through a bit of pap«r to which
the silk is attached. In magnetizing the needles, rub the ends only.

From the chart, p. 274, estimate the magnetic variation of the place
in which you live.

At any time of their occurrence note carefully whatever you may ob-
serve with reference to auroras, mock suns, halos, and coronas.

Observe whether halos of the moon are followed by clear or by rainy
weather.

Occasionally, in very dry weather the disc of the sun is considerably
distorted at the time of setting ; explain why.

Explain the cause of redness that occasionally marks sunrise and sun-
set when the air is smoky.

The sun and the moon seem to be much larger when near the horizon
than at zenith ; is this phenomenon real or apparent ? The use of a
paper or other tube an inch or two in diameter will aid in the solution
of this question.

Explain the phenomenon of the " sun's drawing water."

COLLATERAL READING AND REFERKNCE.

Waldo. — Elements of Meteorology— pp. KKJ-IHO.
(iUKKLY. — Ainericiuii Wciither.
Davis— Elements of Meteorology.



282 PHYSICAL GEOGRAPHY



NOTES

^ Small pieces of cork will answer for these striking experi-
ments, but bits of alder pith are better. In order that they may
be successful the air of the room should be very dry. The pith-
balls may hang from the end of a penholder thrust obliquely
into the cork of a stoppered bottle. For the electriflers, a glass
lamp chimney and a vulcanite comb may be used. Each must
be made absolutely clean, as the slight film of grease from the
hands will interfere with the reaction.

^ The potential of electricity may be also likened to pressure on
water flowing through a pipe. If the pressure be low the water
will flow quietly through the pipe and fall at no great distance
from the end of the nozzle ; on the contrary, if the pressure be
great, it will be projected to a considerable distance. In a single
cell of galvanic battery the potential, about one or two volts, is
so low that the electricity will not jump across a space of one
thousandth of an inch ; the quantity, moreover, is very small.
In an electric-light wire a current of considerable volume will
leap across a space one-tenth of an inch or more ; its potential
is about 1,000 to 1,500 times as great as that found in a cell of
an ordinary galvanic battery, being from 2,000 to 5,000 volts. A
good frictional electric machine will cause sparks to leap between
points ten or twelve inches apart ; the potential is very high, but
the quantity is small. During a thunder-storm a stroke of light-
ning may jump a distance of a mile. Not only is the quantity
enormous, but the potential is so great as to be immeasurable by
ordinary standards.

' At different localities, the character of the electricity may be
so very unlike, that the earth currents are sufficient to operate
telegraph wires without the aid of the batteries. In regions of
dry climate such conditions are more frequent than in areas of
considerable rainfall.

' The vapor of water is not only a good conductor of electricity,
but it is an excellent storage reservoir as well. The small glob-
ules of vapor that compose the cloud mass carry each the charge
of electricity upon the surface. But when a great number of
these globules are condensed to form a drop of water, the surface
of the drop is infinitely smaller than the aggregate surface of the



ELECTRICAL AND LUMINOUS PHENOMENA 283

globules. The potential of the drop, in comparison with that of
the globules, is enormously increased. If an electrified body, such
as a vulcanite rule, is brought near a spi-ayer or a sprinkler the
fine spray immediately gives place to large drops.

*The lightning itself, or rather the electricity, is not necessarily
visible. The flash of light that accompanies the discharge is
due to some extent to the foreign matter in the path of the dis-
charge, heated to whiteness. The air being a poor conductor
offers considerable resistance to the passage of the electricity,
and is therefore intensely heated along the line of discharge.
The thunder is produced in exactly the same manner as is the
noise that accompanies the dischai'ge of a firearm. The air at
the point of discharge is rarefied almost to the extent of being a
vacuum ; the rush of the air to fill the suddenly made vacuum
is accompanied by noise. The rumbling of the thunder is due
partly to echo and reverberation, and partly to the fact that the
sound along the line of discharge reaches the ear at difi'erent in-
tervahs — the greater the distance the longer the time I'equired for
the sound to reach the ear. Discharges of high potential only are
accompanied by thunder.

^ In paintings and illustrations it has always been customary
to depict the electric discharge in the form of a zigzag line of
many sharp angles. In the past few years photographs of the
lightning stroke have been successfully made. One of these on
a preceding page shows the fallacy of former notions on the
subject.

' This reflection is called heat lightning. It is rarely ever ob-
served except at the horizon when the latter is overcast by clouds.
The reflected flashes of light are usually so far away that the
accompanying thunder is not heard.

* While Ciesar was engaged in carrying on his military opera-
tions in Africa, he relates that, during a severe hail-storm, the
spears of his fifth legion were tipped with fire. The phenomenon
was undoubtedly identical with that of St. Elmo's fire. Itisjiot
improbable that the " ignis fatuus," "Jacko' lantern," or "Will
o' the wisp" is a similar electric phenomenon. This is a liazy
indistinct light that appears occasionally in swamps. According
t(j tradition and fiction, the ignis fatuus is a bright light that
moves rapidly from placid to place mainly for the purpose of allur-
ing unsu.specting travellers into dangerous places. As a matter



284 PHYSICAL GEOGRAPHY

of fact, it has no great power of locomotion, and practically is
stationary.

" The aurora is not confined to northern regions ; it occurs in
southern circumpolar regions as well. In the southern hemi-
sphere, however, it is called the aurora australis, but the south-
ern aurora is neither so brilliant nor so frequent in occurrence
as that of the northern regions.

'° It must not be thought that the aurora occurs at night-time
only ; it may take place at any time — day or night. It is not
visible in day-time, however, on account of the greater brilliance
of the sun.

" Professor Balfour Stewart has advanced the opinion that
both auroras and earth currents are secondary currents due to
small but rapid changes in the earth's magnetism. The body of
the earth may be compared to the magnetic core of an induction
coil, the lower strata being the insulating medium, while the
upper strata, which are much better conductors, take the part of
a secondary coil.

" Nearly all the elements are more or less sensitive to magnet-
ism ; iron, cobalt, and nickel possess the force most strongly,
however. Bismuth and copper seem to be repelled and take an
east-and-west position, or a direction at right angles to that of
an ordinary magnet. Such substances are said to be diamagnet-
ic. A piece of soft iron retains its magnetism only while it is in
contact with a magnet or near to it ; a piece of steel, on the con-
trary, once magnetized retains the property permanently. A steel
bar may be magnetized by rubbing its ends with those of another
magnet, or by winding several hundred turns of insulated wire
about it, through which a current of electricity is passing.

" If the bar be a long one, or if the quality of the steel is not
uniform, there are usually several supplemental poles scattered
about the surface. For the same reasons a light slender bar is
better than a stout one.

'■' The shape of the earth is not such that its magnetic force can
possess much intensity. Several magnetic poles are known to ex-
ist, but only the two north poles of great intensity are usually
charted. The pole of greatest intensity is the one commonly
known as the magnetic north pole. Since its discovery by Ross,
it has moved about forty miles westward. In 1879 it was ap-
proximately located by Lieutenant Schwatka in the open space



ELECTRICAL AND LUMINOUS PHENOMENA 285

between Victoria and Franklin Straits. Its exact position has
not been determined since 1831, and it is doubtful if its location
at that date was so precise as might be inferred from the figures,
which are expressed in minutes of arc. At that time there were
no instruments sufficiently delicate for such precise determina-
tion. In 1884 the position of this pole was again approximately
determined to be in lat. 70° 30' N. ; long, 96° 40' W. The po-
sition of the magnetic south pole has not been with certainty
discovered.

'* Observations made at Paris on the movement of the magnetic
north pole cover a period of more than three hundred years. In
1580, the declination at the city was 11° 30' East. It decreased
until in 1683 it was nothing, after which time the variation be-
came west. The westerly variation increased until, in 1814, it
amounted to about 22° 30' W. Since that time it has dropped to
about 22°, and, it is thought, is slowly decreasing. In 1790 the
variation at Norfolk, Va., was nothing ; in 1893 it was about 3°
16' W. In New York City the variation in 1686 was 9° W. ; in
1790 it had decreased to 4° 15' W. ; after this time, however, it
gradually increased until, in 1893, it was about 8° 25' W.

'^ In order better to study these variations, magnetic observa-
tories have been established in various parts of the world. The
essential part of such an observatory is a series of magnets each
carrying a small mirror, mounted in such a manner that a spot
of light is thrown on a sheet of photographic paper. The sheets
of paper are fastened each to a cylinder revolved by clockwork,
so that the spot of light traverses the whole length of the sheet
in twenty-four hours, thus drawing a line upon it. If the mag-
net were motionless the line Avould be straight, but if the mag-
net turns even a .small fraction of a minute, the .spot is thrown
out of position and the line becomes irregular. Usually three
magnets are employed — one to measure variations in horizontal
force ; one for variations in vertical force ; and one to measure
the strength of the horizontal force.

" This period recurs every eleven years. In 1882 the formation
of a sun spot was attended by a magnetic storm that was reconh d
at Point Barrow, Lady Franklin Hay, Los Angeles, Kew (Loii(l<.ii),
Cape Horn, and Paris. Telegraph instniiiicrits were alTcctcd, and
in some instances, long circuits were worked by ground currents.
At the magnetic observatory tlven in Los Angeles, California,



286 PHYSICAL GEOGIUPHY

the trtmior of the magnets was so great that for several hours one
of tlie instruments failed to make a legible record.

'" In the Ritchie compass, now generally used in the United
States Navy, the compass-box is filled with alcohol in which the
card and needle almost float. The object being to relieve the
bearing of the weight of the card, and thus make the needle more
sensitive. It is a most excellent compass and is vastly superior
to the ordinary compass formerly used. The compass of Sir
William Thomson (Lord Kelvin), consists of a battery of six or
more very slender magnets held in a skeleton frame. The latter
is so light that the friction on the bearing is imperceptible. This
compass is used in the English Navy, and by most of the trans-
atlantic liners. As an efficient instrument it has no superior.
The use of steel in the construction of vessels has added materi-
ally to the difficulties of sailing by compass. The hull of a steel
or iron vessel has poles of intensity peculiar to itself, and these
are apt to change in time, so that frequent tests of the compasses
are necessary. There are various devices for obtaining the
proper correction for the compass on steel vessels ; a very effec-
tive method is to swing the vessel, stem and stern, along a
geographic meridian and then compare the observed with the
normal variation. On battle-ships either the addition or the
removal of the armament, or the substitution of a steel for a
wooden mast, is apt to make readjustment of the compasses
necessary.



CHAPTER XYI
CLIMATE AND ITS FACTOKS

The conditions of a region with reference to its liabita-
bility constitute its climate, and these, in general, are the
results of heat and moisture ; climate, therefore, includes
all the modifications of environment due to heat and cold,
rain and drought. It is modified by many conditions, of
which the principal are latitude, altitude, position of high-
lands, direction and prevalence of winds, and the inclina-
tion of the earth's axis, together with its constant parallel-
ism to itself.'

Latitude. — Latitude afi"ects climate chiefly with refer-
ence to temperature. The greater the distance from the
equator, the lower will be its average temperature. The
sun's rays are never vertical beyond the tropics, and in
polar regions they fall so obliquel}' that they impart but
very little heat to the surface which they strike. Illustrate
this by means of the diagi'am on p. 294.

In going from the equator to polar regions, therefore,
one will pass through about every degree of warmth from
perpetual summer to the coldest winter. Within thirty
or thirty-five degrees of the equator the change in tempera-
ture is not great, but beyond the forty-fifth ])arallel the
winter climate grows rapidly cooler for every few degrees
of increase.

Latitude also exerts a considerable influence on rainfall.
As a rule the rainfall is greatest within the torrid zone.

387



288 PHYSICAL GEOGRAPHY

lu the region of tropical calms, on the contrary, the rainfall
is usually deficient. These calms are regions of descend-
ing currents of the air, and the air being warmed by its
descent, instead of chilled, but little rain falls.

Altitude. — The efi'ect of altitude is much the same as
that of latitude. On an average the temperature is lower
by about one degree for every three hundred feet of ascent.
Thus, even in equatorial regions, one may find on the
slopes of snow-clad highlands all the intermediate degrees
of temperature between perpetual summer and eternal
Avinter. In Mexico the effects of altitude are finely illus-
trated. The city and seaport. Vera Cruz, is intolerably
hot and moist, yet less than two hundred miles away, the
City of Mexico enjoys a climate that is dry, cool, and in-
vigorating. The difference is due almost wholly to its alti-
tude — about 7,000 feet above the sea-level.

A still more striking example is found among the pla-
teaus of the Colorado River. Hurricane Ledge is an
almost vertical escarpment, 2,500 feet high, that forms
the boundary betw^een two plateaus. On the upper mesa
the products are those of a temperate climate ; in the
lower they are distinctly sub-tropical. It is scarcely more
than a stone's throw from the former to the latter.

Position of Mountains. — The existence of high moun-
tain-ranges often determines the quantity of rain precip-
itated upon the surface of a given region. In tropical lati-
tudes rain-bearing winds blow from the east, and the eastern
slopes of high ranges are therefore well watered, while
the western slope is dry. In the temperate zones, on the
other hand, the rain winds are from the west ; and the
western slopes in consequence receive most of the rain,
while the eastern side is comparatively dry. Thus, in the
Peruvian Andes, the rain winds deluge the eastern slope,
leaving the western side practically a desert. In the



CLIMATE AND ITS FACTORS ' 289

southern Audes, the conditions are reversed ; the rain
falls on the western side while the eastern slope is arid.

The effect of the absence of mountains is observable in
Australia. Partly because of its latitude, but mainly for
want of a high range, the greater part of the continent is
a desert, and about the only rain that falls is precipitated
on the highlands of the eastern side. In the great African
desert, the few isolated rauges receive considerable rain
on their siinmiits, but none falls elsewhere.

Distance from the Sea. — The proximity of the sea
exerts a marked effect on climate, both with respect to tem-
perature and moisture. The climate of a coast region is
always more equable than that of a far inland or continental
area. The reason therefor is apparent ; the air over the
ocean has a much more uniform temperature than that
over the land. The result is seen Avhen the extremes of
temperature are noted. For example, San Francisco and
Leavenworth, Kan., have nearly the same mean temper-
ature for the year. But while the difference between the
summer and winter temperature of San Francisco is less
than ten degrees (F.), that of Leavenworth is almost fifty
degrees.^

Not all coast regions, however, enjoy a maritime cliniate.
Because the winds of the temperate zones are, as a rule,
westerly, in the eastern coast of such regions land wimls
are prevalent. The coast region of the northeastern part
of the United States is an example. Its climate is dis-
tinctively continental, and the influence of the sea pene-
trates only a very few miles inland.

The climate of islands at a distance from any large body
of land is always equable. The Philippines and the Ha-


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Online LibraryJacques W. (Jacques Wardlaw) RedwayElementary physical geography : an outline of physiography → online text (page 19 of 25)