Full text of "Physiography for high schools"

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YOSEMITE FALLS FROM SENTINEL HOTEL.

A flood plain in the foreground.
By permission of Oliver Lippincott.

Frontispiece.

PHYSIOGRAPHY

FOR HIGH SCHOOLS

BY
ALBERT L. AREY, C.E.

GIRLS' HIGH SCHOOL

FRANK L. BRYANT, B.S.

ERASMUS HALL HIGH SCHOOL

WILLIAM W. CLENDENIN, M.S., M.A,

WADLEIGH HIGH SCHOOL
AND

WILLIAM T. MORRF:Y, A.M.

BUSHWICK HIGH SCHOOL

NEW YORK CITY

D. C. HEATH & CO., PUBLISHERS

BOSTON NEW YORK CHICAGO

2F8

Printed in U.S. A.

PREFACE

FOR a number of years the authors have felt the need of a text
that presents physiography from the high school point of view,
both in content and in treatment. Since more than nine tenths
of the pupils who enter secondary schools complete their formal
education in such schools, the needs of this great class cannot be
neglected ; and subjects must be planned with the fact in mind
that secondary pupils should know the scientific explanation of
the common phenomena of nature.

A course in physiography in a college is naturally limited by the
existence of parallel courses in astronomy, geology, and meteor-
ology ; and the fear of overlapping has led college men to omit
many valuable topics from their courses and from the texts which
they have prepared. No such limitation is found in the high
school, and teachers are at liberty to select such topics as will
contribute most to the culture of the pupil.

High school pupils should know of the earth as a whole, its
relation to the other heavenly bodies, and the influence of its size,
shape, and motions upon our daily life. They should know of
the sun and the moon and their influence, and lack of influence,
upon us. We have, therefore, included in our course, such astro-
nomical topics as are necessary to this end. The pupils should
also know of the natural resources of our country and their impor-
tance, and should understand the influence of climate and physical
environment upon a given region as well as upon the history and
the development of our nation and of the civilizations of the world.
We have, therefore, included topics usually treated only in geology,
meteorology, and history.

The abstract discussion of processes, as processes, belongs to
the college rather than to the high school; we have, therefore,
discussed such topics as diastrophism, erosion, and the like in
connection with concrete instances of their work.

iv PREFACE

We are not in accord with those who would make physiography
in the high school a regional subject. The secondary student
easily masters a scientific treatment of a general topic such as
mountains when the mountains of various regions are studied
and contrasted ; but he fails to do so if mountains are discussed
in a fragmentary manner in connection with descriptions of various
regions.

The treatment of the subject here presented has been in success-
ful use in our class rooms many years, and we believe that it will
be found equally satisfactory to others.

This text contains more matter than can be mastered by first-
year pupils, and we have indicated by smaller type the paragraphs
that it is our custom to omit with first-year classes ; it is expected,
however, that each teacher will make his own selection of the
topics to be omitted. Where the subject is taught in the fourth
year, no omissions will be necessary.

The italicized words in the text are intended as a guide to the
teacher and the pupil. Technical terms are italicized when first
introduced, and are to be defined by the pupil. Sentences in
italics are to be memorized. In descriptions of functions and
properties, the important words are italicized for emphasis. This
use of italics makes printed questions on the text unnecessary.

The questions found at the end of each chapter cannot be
answered by quoting the text. Each requires the pupil to draw
an inference from some fact or facts stated in the text, or to exer-
cise his judgment in contrasting facts. Some of them call upon
him to exercise his imagination, and here suggestions from the
teacher may be in order. We have found them particularly valu-
able as a stimulant to independent thought on the part of the pupil.

CONTENTS

PART I
THE EARTH AS A PLANET

CHAPTER FACE

I. THE EARTH IN SPACE 3

II. LATITUDE, LONGITUDE, AND TIME 17

III. THE MOON 30

IV. THE SOLAR SYSTEM . 36

V. MAP PROJECTION .50

VI. TERRESTRIAL MAGNETISM 58

PART II
THE AIR

VII. PROPERTIES AND FUNCTIONS OF THE AIR 65

VIII. TEMPERATURE OF THE AIR 73

IX. WEIGHT AND DENSITY OF THE AIR 91

X. MOVEMENTS OF THE AIR 99

XL MOISTURE OF THE AIR 117

XII. LIGHT AND ELECTRICITY OF THE AIR 129

XIII. WEATHER AND CLIMATE .139

XIV. CLIMATE OF THE UNITED STATES 160

PART III

THE SEA

XV. GENERAL CHARACTERISTICS OF THE SEA . . . .183

XVI. MOVEMENTS OF THE SEA 195

Vi CONTENTS

PART IV
THE LAND

CHAPTER PAGE

XVII. THE MANTLE ROCK 219

XVIII. THE BED ROCK 246

XIX. THE GROUND WATER 270

XX. THE WORK OF RIVERS 284

XXI. LIFE HISTORY OF A RIVER 310

XXII. LAKES, FALLS, AND RAPIDS 315

XXIII. GLACIERS 328

XXIV. PLAINS AND PLATEAUS 351

XXV. MOUNTAINS 376

XXVI. VOLCANOES AND EARTHQUAKES 402

XXVII. SHORE LINES AND HARBORS 423

APPENDIX 435

INDEX 439

PART I

THE EARTH AS A PLANET

PHYSIOGRAPHY

CHAPTER I
THE EARTH IN SPACE

The earth is a ball nearly 25,000 miles around. It is composed
of rock, with about three-fourths of its surface covered with
oceanic waters having an average depth of only 2^ miles. The
whole is surrounded by an envelope of air probably more than
200 miles thick.

On its surface we are unconscious of any motion because of the
steadiness and freedom from jar, still we know that the familiar
phenomena of the rising and the setting of the sun are due to
the turning of the earth on its axis. At night the star dome
appears to revolve about the earth, and gives us further evidence
that we live on a ball that is turning in space uniformly and
regularly.

We also learn that while the earth is whirling, it is rushing
through space with inconceivable velocity. While the seconds'
pendulum of a clock makes one swing, the earth moves i8J^ miles,
which is a thousand times the speed of the fastest express trains.

In going this distance each second, the earth curves about one-
ninth of an inch from a straight line. This slight rate curvature
continued for a year, brings the earth around to the place of start-
ing.

The absolute uniformity of turning of the earth on its axis,
and the regularity of movement as a whole about the sun, are
of great service to mankind. The former affords a convenient
means of measuring the length of the day, and the latter marks
off the year.

4 PHYSIOGRAPHY

The earth is only one member of a family of rotating balls.
This family, together with other bodies, is controlled by the
sun and constitutes a system.

A conception of the earth in space, as a member of the solar
system, and some knowledge of conditions on other worlds, may
give us clearer views of our real insignificance in space, time, and
matter.

Condition of the Interior of the Earth. The interior of the earth is
believed to be solid throughout, although the temperature is undoubt-
edly above the melting point of materials on the earth's surface. The
melting point increases with the pressure, but it is believed that the
pressure raises the melting point faster than temperature rises as the
center is approached, so that the fusion point is never reached. Be-
cause the earth as a whole has a greater density than the material near
the surface, the central portion is thought to be more dense than the
so-called crust.

Careful studies made of the variation in the position of the earth's
axis, the effects of tide-producing forces acting upon the earth, and the
velocity of earthquake waves through the earth, have led to the conclu-
sion that the earth is more rigid than steel.

Form, Size, and Weight. Water and mud fly off from the fast
rotating wheels of wagons and automobiles, when running on wet,
muddy roads. This is due to the tendency of bodies to move in a
straight line. When a body moves in a curved path it appears to
be pulling away from the axis of rotation. This pull is called
centrifugal force. The centrifugal force is greater, the greater the
distance from the axis.

Because of the rotation of the earth, the excess of centrifugal
force developed in equatorial regions causes it to bulge out there
and to flatten in the polar regions. The resulting form is that of
an oblate spheroid.

This does not necessarily indicate a one-time molten condition
of the earth. The sea bulges at the equator and flattens at the
poles, and the land wears down to sea level.

The axis of rotation of the earth, or the polar diameter, is
7,899.76 miles, and the equatorial diameter is 7,926.60 miles, the
latter being nearly 27 miles more than the former. The ends of

THE EARTH IN SPACE 5

the earth's axis are called poles. The average of the different
diameters of the earth is nearly 8,000 miles, and the circumference
is about 25,000 miles.

The surface area of the earth is nearly 197,000,000 square miles,
of which 54,000,000 square miles are land. The earth is 5.6 times
as heavy as a sphere of water the same size.

Problem of Eratosthenes. The first successful attempt to measure the
size of the earth was made about 200 B.C. by Eratosthenes, an astronomer
and geographer of Alexandria, Egypt. He learned that at Syene, the

sun's noon rays
on June 21 St.

FIG. i. METHOD OF ERATOSTHENES

A, a vertical pillar at Syene, is 23 y? north of the equator at E, a point on the Tropic of
Cancer. B, a vertical pillar at Alexandria is 7.2 or 5,000 stadia further north. C is the
center of the earth and CA and CB radii.

most southern city of Ancient Egypt, the gnomon or vertical pillar cast
no shadow at noon on June 21 st. At Alexandria, 5,000 stadia directly
north of Syene, the sun's noon ray on the same day made an angle
of 7.2 degrees with a vertical pillar. Assuming the earth to be a
sphere, this angle of 7.2 degrees is equal to the angle formed at the center
of the earth between radii to Alexandria and Syene. It follows that as
these two places are on the same meridian, an arc of 7.2 degrees equals in
length 5,000 stadia, so that 360 degrees, or the distance around the earth,
equals 250,000 stadia. As the distance between Syene and Alexandria
is about 500 miles, the circumference of the earth would be 25,000 miles,
which is not far from the truth.

6 PHYSIOGRAPHY

EVIDENCES THAT THE SURFACE OF THE EARTH IS CURVED

1. During every eclipse of the moon, that portion of the shadow
of the earth cast on the moon always has a curved edge.

2. The circumnavigation of the earth proves that it is not flat.
It does not prove that the earth has the form of a sphere. It
might, for instance, have the oval form of a football.

3. As ships sail away their hulls disappear first, and as they
come into port their masts appear first. This shows that the
water surface is actually rounded up between us and the distant
ship.

4. At sea, the circle known as the horizon seems both to sink
and to increase in size with an increase in elevation above the
surface of the water. If the water surface were an extended
plane, our area of visibility would not increase with an increase
of elevation.

5. That the weight of a body is about the same everywhere on the
earth's surface shows that the earth is globular in form. The slight
increase in weight noticed as one approaches the poles is in part due to a
flattening of the earth's surface in those regions.

6. That the apparent shifting of the sky position of the stars is directly
proportioned to the distance traveled north or south, shows that the
earth's surface is curved along north-south lines like a sphere.

7. Places not on the same meridian have different times of day as a
result of the curved shape of the earth's surface along east-west lines.
If the earth were flat all places would have the same time.

8. On the shores of a calm lake, away from the tides and swells, the

:"- ::---.- ::-^r~::~':r.i: sJo.:":"."'"."I. ------

- - Wile - > \ B - Iflite -

r ..

- - J curva -

FIG. 2. POST METHOD FOR MEASURING THE CURVATURE OF THE EARTH

curvature of the earth may be measured by erecting in a straight line
three posts, A, B, C, at the same height above the surface of the water.
When looking with a telescope from the top of post A to the top of
post C, the top of post B will be above the line of sight. If the distance
from A to B is a mile, top of post B will be 8 inches above the line of
sight, or the curvature of the water surface is 8 inches to the mile. In
two miles the curvature is 8 inches X 2 squared, or 32 inches; in three

THE EARTH IN SPACE 7

miles, 8 inches X 3 squared, or 72 inches that is, the curvature for any
distance is equal to 8 inches multiplied by the square of the number of
miles.

THE MOVEMENTS OF THE EARTH

Rotation and Revolution. The earth has two principal motions,
a uniform spinning motion called rotation, and a forward move-
ment in its path about the sun called revolution.

The angular rate of motion due to rotation is 15 an hour;
and the absolute rate of motion of a particle on the earth's
surface is greatest at the equator. Here it is 25,000 miles a day,
or more than a thousand miles an hour, and decreases toward the
poles, where it is nothing. The rate of motion of the earth as a
whole, due to revolution, is about a degree a day. This amounts
to 1,600.000 miles a day.

ROTATION AND ITS EFFECTS

1. The side of the earth that at any moment is turned toward
the sun is in sunshine, and the side turned away from the sun is
in darkness. The rotation of the earth from west to east pro-
duces a movement of illumination and shadow around the earth
from east to west.

2. The eastern horizon is really sinking and the western horizon
rising, which has the effect in the lower latitudes of making the
sun, moon and stars appear to rise along the eastern and set along
the western horizon.

3. The period of rotation, in respect to the sun, is 24 hours,
and determines the length of day.

4. The slight bulging of the earth in the equatorial regions is due
to rotation.

5. Winds and ocean currents, because of the earth's rotation, are
deflected to the right in the northern hemisphere and to the left in the
southern hemisphere. This may be illustrated by pouring water upon
a rotating globe.

6. Every particle of matter on the earth's surface describes each day
a circle. These circles are largest at the equator, and particles there
consequently have the greatest velocity. This motion has the effect

8 PHYSIOGRAPHY

of lessening the weight of bodies, due to the tendency of bodies to fly
away from the center of rotation. Bodies therefore, for this reason,
weigh less at the equator than in higher latitudes.

FIG. 3. STAR TRAILS, DUE TO THE EARTH'S ROTATION, MADE ON A
PHOTOGRAPHIC PLATE.

7. Bodies falling from a considerable height fall to the east of a ver-
tical line suspended from the point of starting.

Foucault Pendulum Experiment. In 1851 Foucault, a French physi-
cist, devised a remarkable proof of the earth's rotation by means of a
pendulum. From the dome of the Pantheon in Paris he hung a heavy
iron ball about a foot in diameter by a steel wire more than 200 feet long.
The pendulum was set in motion and the plane of vibration seemed to
rotate slowly toward the right. It can be easily shown by a simple

THE EARTH IN SPACE

FIG. 4. FOUCAULT'S EXPERIMENT

experiment that the plane of vibration of a pendulum remains fixed.
The true interpretation must then be that the floor of the Pantheon was
actually turning under the plane in
which the pendulum was swinging.

If the pendulum were suspended
at the pole, the earth would turn
around under it in twenty-four hours.
The time required for the earth to
shift entirely around under the plane
of the vibrating pendulum increases
as the latitude decreases. At the
equator there would be no tendency
for the earth to shift.

REVOLUTION AND ITS EFFECTS

Stars Shift Westward. The

movement of the earth, in its path

about the sun, causes the sun to

appear to move eastward among

the stars. This has the effect of

making the stars appear to shift westward about a degree a day.

The real path the earth travels each year is called its orbit.

The path which the sun appears to follow around the heaven once a
year, as a result of the annual movement of the earth in its orbit about
the sun, is called the ecliptic. It is so called because all the eclipses of
the sun and moon occur when the moon is in the plane of this path.

The zodiac is the belt of the heavens, 16 degrees wide, 8 degrees on
each side of the ecliptic. It is so called because the constellation or
groups of stars in it are thought to resemble or outline the forms of
animals. The 360 degrees of the zodiac are divided into twelve equal
parts, each called a sign.

The Latin names, with the symbols used to represent them, are as
follows :

The Spring Signs The Autumn Signs

c r Aries, the Ram. =^ Libra, the Balance.

Taurus, the Bull. m Scorpio, the Scorpion.

n Gemini, the Twins. * Sagittarius, the Archer.

The Summer Signs The Winter Signs

S Cancer, the Crab. V3 Capricornus, the Goat.

*l Leo, the Lion. ffff Aquarius, the Waterman,

^l Virgo, the Virgin. * Pisces, the Fishes.

10 PHYSIOGRAPHY

Change of Seasons. As the earth moves forward around the
sun, its axis is always tipped 23^2 degrees from a perpendicular
to the plane of its orbit. The earth's axis is always inclined in
the same direction, so that during a revolution the axis remains
parallel to itself in all positions. It is because of the (i ) inclination
of the earth's axis and its maintenance of (2) parallelism during a
complete (3) revolution, that the change of seasons occurs.

Cause of Unequal Days and Nights. The same causes produce
a shifting of the daily sky path of the sun during the year, and the
consequent variations in length of daylight and darkness.

Ellipse, Perihelion, and Aphelion. The orbit of the earth has
the form of an ellipse, with the sun at the north focus. The earth
is at perihelion, or nearest to the sun, on January 2, when it
is 91,500,000 miles away, and at aphelion, or farthest from the
sun, on July 3, when it is 94,500,000 miles away.

In the sketch (Fig. 5) the earth is shown in four positions as
it makes its annual journey about the sun.

On December 21 the north pole is tipped away from the sun
and in the middle of the long period of darkness. The noon tan-
gent rays just reach the Arctic Circle, thus causing the whole
area within that circle to be in darkness. At this time the sun's
noon ray is vertical over the Tropic of Capricorn. It is winter
in the northern hemisphere and summer in the southern. The
area within the Antarctic Circle is lighted and the south pole is
in the middle of the long period of sunlight. The days are shoiter
than the nights in the northern hemisphere.

On March 21 the noon ray is vertical over the equator and
the rays are tangent at the poles. Day and night are equal all
over the earth.

On June 21 the north pole is tipped toward the sun. The
tangent noon rays just reach the Antarctic Circle, thus causing
the area within that circle to be in darkness. At this time the
sun's noon ray is vertical over the Tropic of Cancer. It is summer
in the northern hemisphere and winter in the southern. The area
within the Arctic Circle is lighted and the north pole is in the mid-

THE EARTH IN SPACE

FIG. 5. FOUR POSITIONS OF THE EARTH CORRESPONDING TO THE FOUR SEASONS

ID the winter and summer positions two different views are shown, one looking along a line
perpendicular to the earth's orbit and the other looking parallel to the earth's orbit.

PHYSIOGRAPHY

die of its long period of sunlight. The days are longer than the
nights in the northern hemisphere.*

Direction of Sunrise and Sunset. The sun rises directly in the
east and sets in the west only twice a year, on March 21 and

FK. 6. SHOWING POSITION OF SUN'S APPARENT DAILY SKY PATHS AT THE EQUATOR
Paths are vertical to the horizon and days and nights always equal.

September 23. At these two dates, called the Equinoxes, the
sun's noon ray is vertical at the equator, or as more commonly ex-
pressed, "the sun is crossing the line," and days and nights are
everywhere equal.

From the March, or Vernal Equinox, to the September, or Au-
tumnal Equinox, in the northern hemisphere the sun rises north
of east and sets north of west, and the days are longer than the
nights.

From the September to the March Equinox, in the northern
hemisphere the sun rises south of east and sets south of west,

See Chap. Vffl for more exact length of days in higher latitudes at different times of year.

THE EARTH IN SPACE 13

and the nights are longer than the days. (Make corresponding
statements for the southern hemisphere.)

The northern journey of the sun culminates on June 21,
called the Summer Solstice. The southern journey culminates on
December 21, called the Winter Solstice.

Nadir

FIG. 7. SHOWING POSITION OF SUN'S APPARENT DAILY SKY PATHS AT LATITUDE 41 N.
The paths are tipped toward the south, showing our long days in summer and our short days
in winter. The direction of sunrise and sunset at different times of year may be read from
the figure.

The Sun's Daily Sky Path. Because of the inclined position of
the earth's axis, the sun's daily sky path (not only the sun's vertical
ray and the sunrise and sunset position, but also each corresponding
position for every moment of the day), migrates northward for
one half of the year and then southward for the other half of the
year. The effect of this is to bring about the regular changes in
the inequality of the lengths of day and night. From a study of
the above sketches, the shifting position of the sun's daily sky path
for the year may be seen in places of different latitudes. The
middle position is the sun's daily path for the March and Sep-
tember equinoxes, and the position farthest north is the sun's

14 PHYSIOGRAPHY

path for the June solstice, and the position farthest south is the
sun's daily path for the December solstice.
The planes of all the sun paths are always inclined from a

Nadir

FIG. 8. SHOWING POSITION OF SUN'S APPARENT DAILY SKY PATHS AT THE ARCTIC CIRCLE,

LATITUDE 66^ N.

On June 21 the sun remains above the horizon and on Dec. 21 below the horizon for
the entire 24 hours.

vertical an amount equal to the latitude of the observer, for the
planes are parallel to each other. See Figs. 6, 7, 8, and 9.

QUESTIONS

1. How do we know that the earth is whirling uniformly in space?

2. Make a sketch of an oblate spheroid and draw in the axis and the
equatorial diameters. Properly letter the sketch and locate the poles.
Where is the centrifugal force due to rotation the greatest? The least?

3. What is the area of the water surface of the earth? What substan-
ces are as much as 5.6 times as heavy as water?

4. Why are not the so-called evidences proofs that the surface of the
earth is curved? Which evidences are strongest? Which weakest?

5. What are the relative positions of daylight and darkness upon the
earth? In what direction do they travel?

6. Try to picture in your mind, by using a globe, the actual path
which a particle at the equator describes due to the combined motion

THE EARTH IN SPACE 15

of rotation and revolution. Make a free-hand sketch to show the
motion and describe it.

7. A degree of longitude in latitude 40 eauals about 53 miles. How

Zenith

Nadir

FIG 9. SHOWING POSITION OF SUN'S APPARENT SKY PATHS AT THE NORTH POLE
Paths are nearly horizontal. The year is divided into two periods of sunlight and darkness.

many miles an hour does a point in that latitude move? How does this
result compare with the distance the earth as a whole moves in an hour,
due to revolution?

8. When riding on a railroad train, in what direction does the outside
view from the window appear to move? What does this illustrate in
respect to the apparent motion of the sun, moon and stars?

9. What effect has centrifugal force, due to rotation, upon the weight
of bodies at the surface of the earth? Where is this effect greatest?
Where the least?

10. The stars that rise and set, rise about four minutes earlier each
night. Why? In a month these stars appear to shift westward. How
far? What effect has this in twelve months? What is the real cause
of the apparent shifting of the star dome?

n. If the earth's axis were perpendicular to the plane of its orbit,
would revolution cause a change of season? Suppose the inclination of
the earth's axis varied during a single revolution, what effect would that
have upon the change of seasons? Is revolution necessary for a change
of seasons? Explain.

16 PHYSIOGRAPHY

12. In what direction does the sun's daily path through the sky shift
from December 21 to June 21? During this period of six months, which
is growing in length, our period of illumination or the period of darkness?
Why? How is it from June 21 to December 21?

13. Why is not the sun always the same distance from the earth?

14. In about what latitude is the noon ray of the sun vertical on Jan-
uary ist? March ist? July ist? September ist? At these different
dates, which is the longer here, the daytime or the night? In what
latitude approximately are the northern and the southern limits of
illumination at these dates?

15. State whether at these different dates the sun rises north or south
of east and sets north or south of west.

1 6. Why are the Tropics placed where they are?

17. What is meant by the expression "sun crossing the line"? How
often does this occur? In what direction is the sun migrating at each
time?

18. Make an estimate of the time of the exposure of the photographic
plate used in making the cut for Fig. 3 on page 8.

CHAPTER II

LATITUDE, LONGITUDE, AND TIME
LATITUDE

The equator is the circle extending around the earth midway
between the poles. Circles parallel to the equator are called
parallels. The planes of all parallels, as well as the plane of the
equator, are at right angles to the earth's axis. The distance ex-
pressed in degrees, north or south of the equator, is called the latitude
of a place.

The axis of the earth extended northward marks the position
of the north pole of the heavens. The elevation of the celestial or
sky pole above the horizon equals the latitude of the observer.
The angle between a vertical line and the plane of the earth's
equator also equals the latitude of the observer.

Because the equatorial bulge makes the curvature of the sur-
face of the earth grow gradually less from the equator toward
the poles, degrees of latitude increase slightly in length toward the
poles. Less curvature of the earth's surface in the higher latitudes
means that the surface has the form of an arc of a larger circle. A
degree, or 1-360 of the length of the circumference of a larger circle,
is evidently longer than a degree of a smaller circle. A degree of
latitude at any place is therefore 1-360 of the circle whose curvature
is that of the meridian at that place.

The circle N E S E f represents a meridian section of the earth,
N S being the axis and E E' the equator. HOE' is the plane of the
horizon with the observer at 0.

O N' extends north and is parallel to the axis N S. The point Z
is the zenith directly over the observer.

PHYSIOGRAPHY

The angle O C E' is the latitude of the observer and equal to
N' O H, the altitude of the north pole of the sky.

Proof. Angle Z N f , the zenith distance of the north pole of the
sky plus the angle H O N' equals a right angle, or 90 degrees, since Z
is the perpendicular to H H f .

Angle N C plus angle E' C O equal a right angle, or 90 degrees,
since the axis of the earth is perpendicular to the plane of the equator.

FIG. 10. THE ALTITUDE OF THE NORTH SKY POLE, ANGLE N' O H, EQUALS THE
LATITUDE OF OBSERVER 0, ANGLE O C E'

Angles N CO and N' Z are equal, being corresponding angles made
by a line crossing two parallel lines.

Therefore the complementary angle E' C O, the latitude of the ob-
server, equals H N, the altitude of the north pole of the heavens.

How to Find a North and South Line. By the following meth-
ods, a north and south line may be located:

(a) On any clear night the direction of Polaris, when it is
directly above or below the sky pole (see Fig. n), is due north.
This occurs twice in every twenty-four hours, when Polaris and
Mizar, the star in the bend of the handle of the Big Dipper, are
in a vertical line.

(b) The direction of a magnetic needle, when corrected for
variation, will enable one to locate a north and south line.

(c) The direction of the shortest shadow cast on a horizontal
plane by a vertical post is north and south. When the sun is at

LATITUDE, LONGITUDE, AND TIME 19

its highest point in the sky, shadows are shortest. This occurs
at solar noon, which is approximately noon, local time.

Latitude Determined by Night. The latitude of an observer may
be found on any clear night by means of the Pole Star (Polaris).

FIG. n. SHOWING THE ROTATION OF THE HEAVENS ABOUT THE NORTH STAR

The number of degrees of a heavenly body above the horizon is
called its altitude. At the equator the North Star appears on the
horizon, and its altitude is consequently zero. At 40 degrees north
of the equator, for instance, the North Star is 40 degrees above the
horizon (altitude 40) ; and at the north pole of the earth it is in
the zenith (altitude 90). The altitude of the North Star in the
northern hemisphere equals, therefore, the latitude of the place

20 PHYSIOGRAPHY

where the observation is made. This is not always absolutely
correct, since Polaris describes daily a circle, ij^ from the north
pole of the sky.

Latitude Determined by Day. Another method of finding the
latitude of a place is to measure the distance of the noon sun from
the observer's zenith. At the time of the equinoxes the sun is
on the sky equator, and the distance of the noon sun from the
zenith equals the latitude of the place where the observation is
made.

To find the latitude of a place at other times of the year by means of
the zenith distance of the noon sun, certain corrections should be made.
The Nautical Almanac gives the position of the noon sun in reference
to the sky equator. This is called the sun's declination. In the north-
ern hemisphere, if the sun is north of the sky equator, the zenith
distance of the noon sun will be that number of degrees less than the
latitude. If the sun is south of the sky equator, the zenith distance of
the noon sun will be just that number of degrees more than the latitude
of the place.

The zenith distance of the sun should be found just as it crosses the
observer's meridian, that is, when it is on a north and south line.

LONGITUDE

The lines that pass from pole to pole on the earth's surface
are called meridians. Meridians are farthest apart at the equator
and converge toward each pole.

The meridian that passes through Greenwich, England, is the
Prime Meridian, and the meridian from which longitude from o
degrees to 180 east and 180 west is reckoned.

Definition, Prime Meridian, Use. Longitude is the distance
expressed in degrees east or west from the prime meridian. A
degree of longitude at any place is 1-360 of the parallel of that place.

The location of a place anywhere on the earth's surface may be
found by determining its latitude and longitude. A place in lat-
itude 40 degrees north and longitude 75 degrees west is on the
parallel 40 degrees north of the equator, at a point where the
meridian 75 degrees west of Greenwich crosses it.

LATITUDE, LONGITUDE, AND TIME 21

How Longitude is Determined. Longitude is determined by
finding the amount by which the noon at Greenwich is earlier or
later than the observer's noon. Since the earth turns eastward
through 360 degrees in 24 hours, it turns 15 degrees an hour, or
i degree in four minutes. An hour slower than Greenwich means
that the place is 15 degrees west longitude, and an hour faster
means that the place is 15 degrees east longitude.

Various methods for the determination of longitude are used:

(a) By the Chronometer, which is an accurate clock that keeps
Greenwich time. Chronometer time is compared with local time
found by taking an observation of the noon sun. At sea observa-
tion is made with a sextant. Before noon the sun's altitude is
increasing. When it ceases to increase the sun is on the meridian
and the time is apparent noon.

(b) By making a direct telegraphic comparison between the clock
set to local time of the observer and that of some station of known
longitude. The difference in time will give the difference in longi-
tude between the two places.

TIME

How Time is Determined. The rotation of the earth furnishes
us with a measure of time. The day is a universal unit of time.
It is the interval between two successive passages across a given
meridian of a given heavenly body. If the sun is the heavenly
body taken for reference, the day is called a solar day, if the moon
a lunar day, and if a star a sidereal day.

The three kinds of days may be better understood from a study
of Fig. 12. E represents the earth in its orbit about the sun S, and
E' is the position of the earth a day later. M represents the moon
in its orbit about the earth, and M r its position a day later. Far to the
left of the diagram is a certain star so far away that lines drawn to it
from any point on the earth's orbit are practically parallel. The moon
M, the sun S, and a star S' are on the meridian with the observer at 0.

The earth rotates as it moves forward in its orbit. The direction of
the motion of revolution of both earth and moon, and the direction of
the motion of the rotation of the earth, when seen from above the north
pole, are counter-clockwise, as indicated by arrows in figure.

?2 PHYSIOGRAPHY

The real movement of the earth of approximately a degree a day
in its path or orbit about the sun causes the sun to appear to move
among the stars eastward about a degree a day. This has the effect
of making the stars rise four minutes earlier and set four minutes
earlier on successive nights. In a year's time the stars come back to
the same position in the sky at the same time of day, for four minutes
each day of the 365 days of the year make about one whole day.

Fio. is. DIAGRAM SHOWING EFFECT OF REVOLUTION OF THE EARTH UPON THE
LENGTHS OF DIFFERENT KINDS OF DAYS

Sidereal Day. During one complete rotation of 360 degrees, the earth
moves from its position at E, Fig. 12, to a new position at E f , and the
observer at is brought to O r . The same star has again come to the
observer's meridian and one sidereal day has ended.

A sidereal day may then be denned as the interval of time between the
passage of a star across a meridian and its next passage across the same
meridian. It is divided into 24 sidereal hours. Astronomical clocks
keep sidereal time and mark the hours from o to 24. The sidereal day,
being about four minutes shorter than the solar day, sidereal noon comes
four minutes earlier each day, so that during a year it occurs at all hours
of the day and night.

Solar Day. Since the forward motion of the earth in its orbit is about
a degree a day, the earth must rotate eastward one degree more than
360 degrees to bring the sun again to the observer's meridian, that is,
the earth turns through 361 degrees from O, Fig. 12, to O" to complete
one solar day.

Lunar Day. The daily motion eastward of the moon in its orbit is
about 13 degrees. The earth must rotate 13 degrees more than 360

LATITUDE, LONGITUDE, AND TIME

FIG. 13. SUN DIAL

degrees to bring the moon again to the observer's meridian, that is, the
earth turns through 373 degrees from to 0'", Fig. 12, to complete one
lunar day.

Mean Solar Time. The apparent motion of the sun being faster when
nearer the earth and slower when farther away, makes the sun a poor
timekeeper.

By taking the average length of all apparent solar days in a year, a
definite length of our day is obtained. Our clocks and watches are regu-
lated to keep this mean solar time. The apparent solar time read on
the sun dial, and the mean solar
time read from our clocks, agree
only four times a year. This
average day is called the mean
solar day, and may be considered
as being regulated by an imaginary
sun that has a uniform motion and
consequently crosses the meridian at
regular intervals.

The attempt to construct clocks
with compensating devices that
would keep real solar time was
made during the eighteenth cen-
tury. The variation in the sun's apparent motion was so complex, that
apparent time clocks were abandoned early in the nineteenth century.

The sun dial consists of two essential parts, a style or gnomon and a
dial. The style is placed parallel to the earth's axis and casts a shadow
on the dial. The different hours of the day are marked on the dial, and
the shadow of the style cast by the sun passing over it, as the sun moves
through the sky, indicates the time of day.

The style is usually a rod or edge of a thin plate of metal, and
being parallel to the earth's axis makes an angle with the horizontal
dial-plate equal to the latitude of the place where the sun dial is
located.

Equation of Time. When the sun does not cross the meridian until
after mean noon time the sun is said to be slow, and when it crosses the
meridian before mean noon the sun is said to be fast. The amount
that the real sun is ahead or behind the imaginary average sun is called
the equation of time.

The Civil Day. Our ordinary day, called the civil day, begins
at midnight and ends on the following midnight. Business is gen-
erally suspended at that time, and the change of date can be made

24 PHYSIOGRAPHY

then with the least confusion. The first 12 hours are called a.m.
(ante-meridian), and the second period of 12 hours p.m. (post-
meridian); 12 m. means noon or sun on the meridian. To find the
exact time at which the sun is actually on the meridian the table
for the equation of time must be consulted or an observation
must be made.

For a person who travels around the earth, the number of times
the sun crosses his meridian would be one less if going westward
and one more if going eastward, than it would be if he stayed at
home. It is evident, then, that if the traveler does not add a day
when going westward and drop a day when going eastward, upon
his return his reckoning will differ one day from that at home. It
has been agreed among mariners to make the change of date at
the iSoth meridian from Greenwich.

To avoid confusion of dates on islands crossed by the meridian,
an off-set eastward a few degrees is made about New Zealand and
an off-set westward is made about the Fiji Islands. Another off-
set eastward is made to avoid passing across the extreme eastern
part of Siberia. After passing through Bering Strait the date line
returns to the iSoth meridian.

International Date Line. The iSoth meridian, together with
the off-sets mentioned, constitute the international date line. The
date on the western side of this line being a day later than on the
eastern side, ships, in crossing it, omit a day in their reckoning
when going westward, and repeat a day when going eastward.

The Conventional Day. The day which by international consent
it has been decided that any country has at any moment, is called
the conventional day. The conventional day begins at the inter-
national date line, and moves westward 15 degrees an hour with
the sun. Parts of two different days are on the earth at the same
time. The midnight line, which is just opposite the noon sun,
marks the forward or westward boundary of each advancing day.

Local Time. The mean solar time of any place is called its
local time. Places of different longitude differ in local time four
minutes for each degree. In going around the earth at the equa-

LATITUDE, LONGITUDE, AND TIME 25

tor, a distance of about 25,000 miles, the local time changes at
the rate of one hour for a distance of about 1,038 miles. In lati-
tude 40 degrees, a distance of about 80 1 miles, makes a difference
of one hour in local time, and in latitude 60 degrees, 519 miles.

Standard Time Belts in the United States. Because of the con-
fusion that resulted from each place keeping its own local time,
especially along railroads extending east and west, most railroad
towns readily gave up their own and adopted the time in use by
the railroad. The number of railroads increased until at certain
centers there were many railroads entering the same city, each
with a different local time in use. Much confusion arose from
having different local times used in the same place. A definite
system of keeping time in the United States was decided upon,
and in 1883 the different railroad lines put it into operation.
This system is called Standard Time, and may be defined as the
time based upon a certain meridian that is adopted as the time
meridian for a definite belt of country. Its advantage is that
neighboring places keep the same time, instead of differing a few
minutes or seconds according to their longitude. This is of
especial importance in the operation of railroads and telegraphs,
and with the transaction of any business concerned with contracts
involving definite time limits. The standard time meridians of the
United States, as adopted, are 75 degrees, 90 degrees, 105 degrees,
and 120 degrees west from Greenwich.

This system has been extended to the remote possessions of the
United States, and has spread over the greater portion of the
world.

Eastern Standard Time. The mean solar time of the 75th
meridian is used for places on both sides of that meridian and in a
belt approximately 15 degrees wide, and is called Eastern Standard
Time. This meridian runs through Philadelphia, and there local
and standard time are the same. The time within this belt is five
hours slower than Greenwich time. The so-called time belts have
very irregular eastern and western boundaries, depending upon
the location of cities upon the railroads. Study carefully Fig. 14
and trace the time belt boundary lines.

PHYSIOGRAPHY

LATITUDE, LONGITUDE, AND TIME 27

Central Standard Time. The time of the next belt westward is
the mean solar time of the goth meridian, called Central Standard
Time, and is one hour slower than Eastern time. When it is noon,
Eastern time, at Washington, Baltimore, Philadelphia, New York,
and Boston, it is n a.m., Central time, at Chicago, Minneapolis,
St. Louis, and New Orleans.

Mountain Standard Time. The next time belt westward uses
the mean solar time of the io5th meridian, called Mountain Stan-
dard Time. Denver, Colorado, is on this meridian, so that clocks
in that city indicate both mountain and mean solar time.

Pacific Standard Time. The time belt on the extreme west of
the United States covers the States on or near the Pacific coast,
and has the mean solar time of the i2oth meridian, called Pacific
Standard Time. Time in this belt is three hours slower than in the
eastern belt, and eight hours slower than Greenwich time. In
Alaska, standard time is nine hours slower than Greenwich time.

El Paso, Texas, has the peculiar condition of having four differ-
ent systems of time in use. The mountain standard time belt
tapers southward to a point at El Paso. This allows the Central,
Mountain and Pacific time belts to meet. The standard time
for Mexico, on the south, is 24 minutes later than Mountain time.
The railroads that enter El Paso from the east, south and west
bring their own time. Mountain time is used by the city officials
of El Paso.

Time Signals. The time service of the United States is under
control of the Government. By cooperation of the telegraph
companies, time signals are sent out daily at noon, Eastern time,
from the Naval Observatory at Washington, D. C., to nearly
every telegraph station in the country. These regulate automati-
cally more than 30,000 clocks, and drop time balls in scores of
different ports of the Atlantic, Pacific, Gulf of Mexico, and Great
Lakes coasts. Time signals for the extreme western part of the
United States are distributed from Mare Island Navy Yard, in
California.

28 PHYSIOGRAPHY

THE CALENDAR

The very early calendar, worked out by the Romans, was based largely
on the motions of the moon. As the yearly number of revolutions of the
moon varies, the seasons and festivals did not keep in place, and the
Roman calendar fell into a state of great confusion. The year consisted
of ten months, March being the first and December the tenth and last.
January and February were added later. There were about 29^ days
in a lunar month, so the months were given 29 and 30 days alternately,
beginning with January. The number of days in a week was probably
based upon the number of planets then known, including the sun and
moon. In the year 46 B.C., the Roman calendar was reformed by
Julius Caesar, under the advice of Egyptian astronomers.

The Julian Calendar. The Julian calendar was planned without
reference to the moon. It made three consecutive years of 365 days
each, and the fourth of 366 days. The extra day was added to February,
that month then having only 29 days, and the other months having
alternately 30 and 31 days. The length of the Julian year was 365.25
days, and since the true year has 365.24 days, the Julian year was .01
of a day, or 11.2 minutes too long.

This difference of 11.2 minutes between the length of the Julian year
and the year now in use amounts to a little more than three days in 400
years. As a consequence, the date of the vernal equinox came continu-
ally earlier in the Julian year. In 1582 the vernal equinox occurred on
the nth of March.

The Gregorian Calendar. In that year Pope Gregory XIII directed
that ten days be stricken from the calendar, so that March equinox
might occur on March 21. A further reform was introduced at this
time in order to prevent a similar occurrence. The Pope decreed that
the centurial year should not be counted as a leap year except when
divisible by 400. Thus 1800, 2100, and so forth, are not leap years,
but 1600, 2000, and 2400 are leap years.

The Gregorian calendar is now used in all civilized countries except
Greece and Russia, where the Julian calendar is still in force in spite of
repeated efforts to abolish it. The i4th of every month here is the first
of the month there.

In England it was adopted in 1752. Dates of events occurring before
the Gregorian calendar was adopted are termed Old Style (O. S.), and
those after the adoption New Style (N. S.).

In order to gratify the vanity of Augustus Caesar, the month now
bearing his name, formerly called Sextilis, was given 31 days so as to
have as many as July, formerly called Quintilis, which was named for

LATITUDE, LONGITUDE, AND TIME 29

Julius Caesar. A day was accordingly taken from February, leaving
only 28 days for that month, and given to August. Because of the
superstition of having three months of 31 days each, together, September
and November were reduced to 30 days, and October and December were
given 31.

QUESTIONS

1. How may ships be located at sea? If city streets extend east and
west and at right angles to avenues, how may places be located thereby?
Compare the plan of locating a place in the city with that of locating the
ship at sea.

2. How may the following be determined in the southern hemi-
sphere: (a) Latitude by night? (fy Latitude by day? (c) A north and
south line?

3. At what time of day is longitude usually determined? Why?

4. What is the circumference of the earth at the 6oth parallel, as
compared with the circumference at the equator ?

5. Why is a solar day about four minutes longer than a sidereal day?
Do solar days differ in length? Why?

6. In laying out a north and south line by means of the noon sun,
what besides a watch would be necessary?

7. What are some of the practical advantages of having the civil day
change at midnight? State any difference you may see between the
civil day and the conventional day.

8. How long has every day been on the earth before it reaches you?
At what time by the clock at your place does a new day start on the
earth? If Sunday is just east of the international line, what day is just
west of the line? Explain.

9. By how much does the local time of your place differ from stan-
dard time? Why are the boundaries of the standard time belts so irregu-
lar?

10. At what hour do the noon time signals from Washington reach
Chicago? Denver? Explain.

11. What advantages has the sun over the moon for calendar pur-
poses? State the reason for the present rule for leap year.

CHAPTER III
THE MOON

Distance, Area, and Size. The moon's average distance from the
earth is about 240,000 miles. The actual distance during a single
month varies about 30,000 miles, causing a corresponding variation
in its apparent size.

The diameter of the moon is 2,163 m il es > being about 27 per
cent of the diameter of the earth.

The surfaces of the moon and earth are to each other as the
squares of their diameters, or as one to fourteen. Their volumes
are to each other as the cubes of their diameters, or as one to
fifty.

Real and Apparent Motion of the Moon. The apparent motion
of the moon and stars by night and of the sun by day, is due to
the earth's rotation from west to east. There is a real eastward
motion of the moon, as may be seen by noting the position of
the moon among the stars from night to night.

Since the moon makes one complete revolution about the earth
in about 27^3 days, the eastward motion is about 13 degrees a day;
and as the sun also appears to move eastward among the stars
about i degree a day, the eastward daily gain of the moon is about
12 degrees. This causes the moon to rise about 50 minutes later
each day.

Moon has no Atmosphere or Water. The moon has no appreciable
atmosphere. Its absence is shown by the fact that when the moon
hides a star, the star disappears suddenly and not gradually, as it would
if its light passed through an atmosphere. There seem to be no effects
of erosion on the moon, which also goes to show that there is no at-
mosphere there. If the moon ever had an atmosphere at any stage of
its development it has lost it. If water existed on the moon it would
evaporate during the long day there and form an atmosphere.

THE MOON

Flo. 15. LUNAR TOPOGRAPHY (Stellar Evolution)

32 PHYSIOGRAPHY

Moonlight Surface Markings. Moonlight is but reflected sunlight.
The surface markings on the moon are known to be due to a very uneven
surface. The visible surface of the moon has an area about equal to
that of South America, and nearly one-half of the area is covered with
dark gray patches which were once supposed to be seas. The rest of
the surface consists of mountains, so called volcanoes and craters, and
ringed valleys. Some mountain chains have peaks nearly 4 miles high.

Same Face is Always Toward the Earth. Since the same side of the
moon is always turned toward the earth, it follows that the period of
rotation of the moon on its axis and its period of revolution about the
earth are the same, about 27^ days. Consequently we know nothing
except by inference about the other side of the moon. The side of the
moon that is toward the sun is always brightly illuminated, and the side
turned away from the sun is in darkness. As the moon makes her way
eastward around the earth, varying portions of the illuminated half are
seen. This causes the moon's phases.

PHASES OF THE MOON

New Moon. When the moon and the sun are on the same side of the
earth, the dark side of the moon is turned toward the earth and we have
new moon. New moon, strictly speaking, occurs when none of the bright
surface is visible. Popularly the moon is said to be new when seen as a
very thin crescent. A day or two later, when the moon has moved a
little eastward of the sun, we may see in the early evening in the
western sky a small portion of the illuminated half in the form of a
crescent, convex westward, or toward the sun, with the horns turned
eastward, or away from the sun.

First Quarter. A week after new moon, half of the illuminated hemi-
sphere may be seen. The moon has now reached first quarter, and its
shape is that of a half -circle. A line connecting it with the earth is at
right angles to a line connecting the sun and the earth. As the moon
passes beyond the first quarter the boundary line between the light and
the dark area begins to be convex eastward, and the lighted portion con-
tinues to grow larger.

Full Moon. When the moon and the sun are on opposite sides of the
earth, the whole lighted half of the moon is turned toward the earth,
and we have full moon, about a week after the first quarter. The line
dividing the light and dark areas after full moon changes from the left
side to the right side of the moon's disk.

Third Quarter. The moon reaches the last or third quarter about a
week after full moon. In this phase the half-circle is convex toward

THE MOON 33

the left instead of convex toward the right, as seen in the first quarter.
After third quarter, the moon being west of the sun, the crescent curves
to the left or toward the sun, and horns point to the right away from
the sun.

Waxing and Waning. In its revolution from new to full moon, the
visible illuminated area increases and the moon is said to wax. From
full to new the illuminated area decreases and the moon is said to wane.

NEUQBBOUS ' - - -^ .k NEW CRESCENT

R1LMQON

OLD GIBBOUS Q ^Hfl ^ " OLD CRESCENT

LAST QUARTER

FIG. 1 6. MOON'S PHASES

The real illumination of the moon is shown in the inner eight positions in orbit about the earth
at E. The sun is at the right. The apparent illumination is shown in the corresponding
outer position.

Earth-Shine. The dark portion of the moon is sometimes lighted by
sunlight reflected from the earth, called Earth-Shine. This occurs at
the young and old crescent phases, and makes the entire disk of the
moon visible.

ECLIPSES

Shadows. All of the planets and their satellites are opaque
bodies and cast long, cone-shaped shadows away from the sun.
The length depends upon the size of the sphere and its distance
from the sun. The average length of the earth's shadow is
about 866,000 miles, and that of the moon 232,000 miles.

PHYSIOGRAPHY

Cause of Eclipses. The word eclipse as here used means a
darkening of a heavenly body. This darkening may be real or
apparent. The moon is eclipsed when it passes into the earth's
shadow; the sun is eclipsed when the moon passes between it and
the earth. During a lunar eclipse the moon is really darkened,
light from the sun being cut off by the earth. During a solar
eclipse the sun is only apparently darkened; the moon cuts off

V -Total Solar
Eclipse

t-P<iTtial Solar
Eclipse

FIG. 17. SOLAR AND LUNAR ECLIPSES

light that would otherwise reach the earth. In reality it is the
earth rather than the sun that is eclipsed.

Total Lunar. In the figure, the moon is passing through the
earth's shadow, BCD, and is totally eclipsed. The moon's disk at
this time is usually visible, however, because of sunlight bent into the
earth's shadow by our atmosphere. This gives the moon during a total
eclipse, a dull, copper colored appearance.

Partial Lunar. When the moon passes slightly north or south of the
center of the earth's shadow, and only a part of the moon's disk enters
the shadow, a partial lunar eclipse occurs. The moon in its monthly
revolution about the earth usually escapes the earth's shadow entirely.

Total Solar. When the moon passes between the earth and the sun,
and its shadow, called the umbra, reaches the earth, a total eclipse occurs
in that portion of the earth covered by the shadow.

Partial Solar. Just outside the umbra of the moon's shadow, an
observer in the penumbra or partial shadow would see only a part of
the sun's disk, and would experience a partial solar eclipse.

When the moon's shadow is not long enough to reach to the earth,

THE MOON 35

and the moon passes centrally across the sun's disk, leaving a ring of the
sun's disk exposed, the eclipse is said to be annular. The moon appears
as a black spot covering the central portion of the sun's disk, surrounded
by a ring of light.

Number of Solar and Lunar Eclipses in a Year. There are always
at least two eclipses of the sun in a year, and there may be as many as
four. The largest number of lunar eclipses in a year is three. As every
eclipse of the moon is visible at one time from all points on one-half of
the earth, and eclipses of the sun from a narrow area only, many more
lunar than solar eclipses are visible at a given place.

QUESTIONS

1. Compare the moon with the earth in respect to size and physical
conditions. Where and when do we see the young crescent? The old
crescent? How long is each usually visible? Why?

2. During what phase of the moon do lunar eclipses occur? Solar
eclipses?

3. How many solar eclipses would occur each year if the orbits of the
earth and moon were in the same plane?

4. The time from full moon to full moon, called a lunar month, is
29^/2 days, while the actual time of a revolution of the moon about
the earth is 27^ days. To what is this difference due?

CHAPTER IV
THE SOLAR SYSTEM

Solar System Defined. The sun, together with the bodies
revolving about it, is called the Solar System. The members of
the system are the sun, the planets and their satellites, the plan-
etoids, some comets, and meteors. They may be briefly described
as follows:

1. The sun is near the center of the system, a very large, hot,
self-luminous body giving heat and light to the other members.
Its gravitative attraction controls their motions.

2. The planets, eight in number, upon one of which we live,
revolve about the sun in elliptical orbits, in different periods of
time, and at different distances from the sun. Planets are distin-
guished from stars by their changing position among the stars, and
by their visible disk when seen through a telescope. Stars keep
their relative position in the sky, and through a telescope appear
as points of light.

Consult the following table:

PLANETS

Diameter
in
Miles.

Average Distance
from Sun in
Millions of Miles

Period of
Revolution
in Years.

Number of
Satellites
or Moons

Mercury

2,700

0.24

Venus

7,800

0.62

Earth

7.QI3

I.OO

4,300

141

1.88

Jupiter

87 ooo

483

12 OO

Saturn

72 OOO

886

20 OO

Uranus

2 ej OOO

i 782

84 oo

Neptune

32,000

2,792

165.00

3. All except two of the planets have satellites revolving about
them. The satellites are very unevenly distributed among six of

THE SOLAR SYSTEM 37

the planets, as seen in the table above. Our moon is an example of
a satellite.

4. The planetoids (planet-like bodies), about seven hundred
and fifty in number, are small bodies, as compared with any of
the planets, and revolve about the sun between the orbits of the
planets Mars and Jupiter.

5. Comets are bodies that are temporarily visible, of large
dimensions and small mass, unstable in form, usually with long
tails and with uncertain orbits. Some comets revolve about the
sun in closed orbits, have fairly definite periods of revolution, and
are consequently members of the Solar System. Other comets
with open orbits enter and then pass out of the Solar System
without becoming members of it.

6. Meteors are comparatively small masses of stone or metal
that enter the earth's atmosphere from outside space. The light

Jttptter fatam Vranua Neptuntt

FIG. 18. DIAGRAM or ORBITS or THE PLANETS DRAWN TO SCALE.

given out by them is due to their being heated by the friction and
compression of the air. Meteors are popularly called " shooting
stars."

Size of the Solar System. It will give us a better conception
of the size of the orbits of the different planets if we draw to scale
a map of the solar system. The orbits of the first four planets are
so small compared with the orbits of the last four, that it is
difficult to find a suitable scale to use, to represent the whole
upon a single page of this book. The scale, one millimeter,
equivalent to 20,000,000 miles, is used.

Although the orbits of the planets are elliptical, they differ so
little from circles that for this purpose the circle may be said to
represent the planet's orbit.

Space Outside the Solar System. The known bodies occupying
space outside of the orbit of Neptune are comets, meteoric swarms,
large gaseous masses called nebulae, and stars.

38 PHYSIOGRAPHY

In literature the stars are often referred to as " numberless "
and " countless." As a matter of fact, only about 3,000 stars
can be seen without a telescope at any one time, and in the whole
heavens there are fewer than 6,000 stars that may be seen with
the naked eye. With the telescope fainter stars are seen. The
moderate sized photographic telescope, with the modern sensitive
plate, will show stars that are too faint to be seen with the largest
telescopes. It has been estimated that the -photographic plate
has made record of about one hundred million stars. Each of
these stars shines by its own light and is consequently a sun.
Many are more brilliant and larger than our own sun, and may
be centers of other systems.

THE SUN

Diameter, Density, and Temperature of the Sun. The sun is a

huge sphere of incandescent gases and metallic vapors, with a
diameter of 866,000 miles, and is 1.4 times as heavy as a sphere
of water of the same size. Although but a small fraction of the
total light and heat given out by the sun reaches the earth, yet
nearly all life activities and most movements of air and water
are due to this amount.

The difference between conditions on the sun $nd those now on
the earth is due largely to a difference in temp(| ture

THE CONSTITUTION OF THE SU&v

The Photosphere. The visible surface of the sun is called the photo-
sphere (light-sphere). It is cloud-like in appearance and gives forth
most of the light and heat which the sun radiates.

Sun-Spots. Dark spots of irregular outline, called sun-spots, often
many thousands of miles in diameter, mar at times the brightness of
the photosphere. The sun-spots are probably connected with the hidden
circulation in the great body of the sun below the photosphere, and are
dark only in comparison with it. Observers of sun-spots soon found
that the sun turns on its axis from west to east. The earth's magnetism
is disturbed during a period of unusual activity in the sun. A large
number of sun-spots appear and a greater development of solar prom-
inences occurs most frequently at these times. The period of maximum
disturbance occurs on an average about every eleven years.

^ r

K o
T f

THE SOLAR SYSTEM 39

As the sun rotates on its axis in about 26 days, no spot would
remain continuously visible for more than 13 days, being one-half of the
period of the sun's rotation. Some spots last, however, only a few days,
while others persist for months.

Elements in the Sun. By means of an instrument called the spectro-
scope it is possible to tell some of the substances of which the sun is

FIG. 20. GREAT SUN-SPOT OF OCTOBER, 1903 (Stellar Evolution)

composed. About 40 elements, such as iron, carbon, hydrogen, nickel,
silver, etc., familiar to us on the earth, are now recognized in a layer of
gas overlying the photosphere.

Chromosphere. Outside this metallic layer is a deep envelope of
gas, mostly hydrogen, called the chromosphere (color-sphere).

When the moon comes between the earth and the sun, the light
from the photosphere is cut off and the sun is said to be eclipsed.
During the solar eclipse the chromosphere can be se~n as a brilliant
scarlet ring. From its surface tongues of flame called mominences shoot
out to altitudes of many thousands of miles.

The Corona. The outermost portion of the sun is the corona
(crown), a halo of pearly light extending out many thousands of miles,
with streamers reaching out millions of miles. It is believed that the

40 PHYSIOGRAPHY

light of the corona is due to the reflection of light from dust particles,
liquid globules, and small masses of gas.

How the Sun's Heat is Maintained. The theory that the sun's heat
is largely maintained by the gradual shrinkage of its volume is generally
accepted. The fall of matter toward the center would continuously
generate heat, as the blow of a hammer on a nail would heat both nail
and hammer. Other sources of heat may add to the total amount that
the sun sends out into space, such as that resulting from combustion,
the falling of meteors, and radioactivity.

THE PLANETS COMPARED

The characteristics common to all of the planets may be briefly
enumerated as follows:

1. The planets move in the same direction about the sun from
west to east. The sun rotates in this direction. The direction of
movement as seen from above the north pole of the earth is oppo-
site to the hands of a clock.

2. The paths or orbits of all the planets are ellipses, with the
sun at one of the foci.

3. The other planets are non-luminous, like the earth; conse-
quently the light that comes from them to us is reflected sun-
light.

4. Most of the planets are known to rotate in the same direction
as the earth rotates, from west to east.

THE PLANETS AS INDIVIDUAL BODIES

Mercury. So far as is known at present, Mercury is the smallest
planet, the nearest to the sun, and the swiftest in its movements about
the sun. It can be seen only in the direction of the sun during early
twilight or late dawn. Mercury has a thin atmosphere, if any at all,
has surface markings of permanent streaks, and a known rotation
period equal in length to its year of 88 days. Since the periods of rota-
tion and revolution are the same length, Mercury always turns the same
side to the sun. This side is always heated and has perpetual daylight,
while the side turned away from the sun is always cold and in darkness.

Venus. Venus shines in the sky with peculiar brightness. It has
a diameter considerably more than double that of Mercury and only a
little less than that of the earth. The period of rotation is now known
to be 255 days, and equal to its period of revolution. Venus and Mer-

THE SOLAR SYSTEM 41

cury are the only planets that have equal periods of rotation and revo-
lution. They pass between the earth and the sun, and consequently are
the only planets that present all phases similar to those of our moon.
The passages are called transits, and occur at irregular and relatively
long intervals of time. During these passages Venus and Mercury
look like small, round, black spots passing across the sun.

The Earth. Although we know that the earth is a planet moving
about the sun like the other planets, the earth seems to us to be a
center about which the other heavenly bodies move. The earth has the
general form of the other planets, that of a spheroid. It is the third in
distance from the sun, and the largest of the four smaller planets
whose orbits lie within those of the planetoids. The earth makes 366
rotations during one revolution.

Mars. Mars, though having only a little more than one-half the
diameter of the earth, resembles it in more respects than any of the other
planets. Its period of rotation is 24 hours, 37 minutes, or a little more
than our day. The inclination of its axis is about 24 degrees. There-
fore, except for its greater distance from the sun, the days and change
of seasons resemble those of the earth.

Surface markings on Mars indicate to some astronomers snow fields
and canals. There seems to be little doubt about the white polar
caps that appear and disappear according to the season. It is not cer-
tain, however, that they are fields of snow.

Although we have as yet no foundation from which to make any
positive statement concerning the inhabitants of Mars, it may be
claimed that if any planet other than the earth is inhabited, it is
probably Mars. Mars appears in our sky shining with a steady, pale
red light.

Jupiter. Jupiter is the largest of all the planets, and, with the excep-
tion of Venus, often the brightest in the sky. Surface markings on
Jupiter are described as parallel belts and spots. Because of the lack of
the permanency of the markings, they are thought to be due to a deep
atmosphere surrounding the planet. From observations of the spots,
it has been found that Jupiter has a rotation period of about ten
hours, which is the shortest known of any of the planets.

The circumference of Jupiter is about 11 times the circumference of
the earth, and with a rotation period less than half of that of the
earth; the rate of rotation at the equator of Jupiter is about 30,000
miles an hour, nearly 30 times the rate of rotation at the equator of the
earth.

The outer four planets comprising the major group Jupiter,
Saturn, Uranus, and Neptune are supposed to be of a higher temper-

42 PHYSIOGRAPHY

ature, of less density, and in not so advanced a stage of development as
the four planets Mercury, Venus, Earth, and Mars, comprising the
minor group.

Saturn. Saturn is distinguished from all the other planets by three
thin, flat meteoric rings, easily visible through a small telescope, which
surround it in the plane of its equator. The rings are together about
40,000 miles wide, and the inner edge less than 6,000 miles from the
planet. At distances ranging from a hundred thousand miles to nearly
eight million miles from Saturn, are ten satellites, more than have yet
been discovered belonging to any other planet in the Solar System.

The surface markings on Saturn are not seen nearly so well as those
on Jupiter, because Saturn is nearly twice as far from us. There are
bright and dark belts, and at times faint spots. Saturn rotates on its
axis in about 10^ hours. Because Saturn has a density of about three-
quarters that of water, it is believed to be largely in a vaporous con-
dition. It may be seen shining in the sky with a steady yellowish
light, with about the same degree of brightness as the brightest star.

Uranus and Neptune. Uranus was discovered in 1831, and Neptune
in 1846. All the other planets were known to the Ancients. Uranus is
a very faint object in the sky, and Neptune is invisible to the naked eye.
Neptune is the most remote of the planets now known, and has the
longest period of revolution, one year there being 165 earth years. It
may be inferred that the physical condition of Uranus and Neptune is
probably much the same as that of Jupiter and Saturn. The rotation
period of Uranus, as indicated by surface markings, is between ten and
twelve hours. These planets, being so far from the sun, receive a
small amount of heat per unit area compared with that received by
the earth.

The Satellites of the Solar System Compared. Previous to 1610
the only satellite known was our moon. In that year Galileo first
pointed his telescope to the sky and saw four large moons of Jupiter.
Our moon is more than 2,100 miles in diameter, but not as large as any
of three of the eight moons of Jupiter, and one of the ten moons of
Saturn. The largest satellite of Jupiter is 3,558 miles in diameter, and
considerably larger than the planet Mercury. The smallest satellites
known are the two belonging to the planet Mars, both of which are
probably less than ten miles in diameter. One of the two is only 5,800
miles distant from Mars, and makes a revolution in less than eight
hours, one-third of the time it takes Mars to rotate.

The earth's satellite is about 240,000 miles distant from the earth,
and makes a complete revolution in about 27^ days. The most dis-
tant satellite of Saturn takes considerably more than an earth year to

THE SOLAR SYSTEM 43

make a revolution. The mass of our moon as compared with the mass
of the earth is probably greater than the mass of any other single
satellite, compared with the mass of its planet. Mercury and Venus
have no satellites, Uranus has four, and Neptune one.

The Planetoids. The planetoids, sometimes called asteroids, move
about the sun just as the planets do. They are so small that they
are invisible to the naked eye. Not until the beginning of the nine-

FIG. 21. HALLEY'S COMET (Evening Sky Map)

teenth century were any of these bodies discovered. There was an
early belief that an undiscovered planet revolved between the orbits
of Mars and Jupiter. This ; no doubt, led to the discovery of the
first and largest planetoid, Ceres, 485 miles in diameter. There are
several whose diameters are more than a hundred miles, but the majority
are much smaller, ranging down to about ten miles in diameter. New
ones are now being found every year by the method of photography.

Comets. Comets are in strong contrast with planets in appearance
and physical condition. Most of them enter the Solar System with
orbits in the form of open curves, make one turn about the sun, and
pass away, probably forever.

PHYSIOGRAPHY

Of the few comets that belong permanently to the Solar System, all
have definite periods of revolution about the sun, varying from 3.3 years
(Encke's Comet) to about 76 years (Halley's Comet). Halley's Comet
last appeared during May, 1910.

The typical comet is largely self-luminous, and is composed of a
head and a tail. In the center of the head is a bright, star-like
nucleus surrounded by faintly luminous matter, called the coma. The

FIG. 22. PEARY METEORITE
In American Museum of Natural History, New York

tail acts like your shadow when you walk around a lamp. It always
points away from the light. Some astronomers maintain that it is the
pressure of sunlight that drives the gaseous molecules from the nucleus
and thus forms the comet's tail.

The head may have a diameter greater than that of the sun, with
a nucleus as large as the earth, and the tail equal in length to the dis-
tance of the earth from the sun. The amount of matter in a comet is
very small, in most cases less than one millionth of that of the earth.

The orbits of the planets are slightly elliptical and all are approx-
imately in one plane; those of the comets are greatly elongated and

THE SOLAR SYSTEM 45

lie in every possible position. With the unaided eye it is a rare sight
to see a comet.

Halley's Comet has been pursuing its fixed orbit about the sun
since the dawn of history, and undoubtedly long before. The accounts
of many of its earlier appearances seem to indicate that it has been a
conspicuous object. The last appearance, during May, 1910, was dis-
appointing. This tends to show that the great comet has for ages been
slowly disintegrating.

Under the most favorable conditions the nucleus of Halley's Comet
was brighter than stars of the first magnitude, the coma was a faint
light, and the tail was a band of light about 8 degrees wide at its
widest place and 1 20 degrees long. Stars were plainly visible through
the comet's tail. It is believed that the earth passed through the tail
on May 18, 1910. At that time there were no unusual manifestations
seen, such as the falling of an unusual number of meteors, a glow of the
sky, or the appearance of deadly gases, all of which had been predicted.

Meteors. The earth in its path about the sun encounters daily
many millions of small bodies which enter its atmosphere from out-
side space. On a clear, moonless night, one may see several an hour.
They often appear at altitudes of a hundred miles, move many miles a
second, give out light and heat, and are usually consumed before they
reach the surface of the Earth. These bodies are called Meteors.

The appearance of an unusual number of meteors, usually in August
and November, is known as a Meteoric Shower. Sometimes bodies
weighing from a few pounds up to several tons fall to the earth's surface
unconsumed. Such bodies are known as Meteorites. Some are com-
posed of nearly pure iron, with a little nickel. Most meteorites are
composed of stone, often with traces of iron in them. About thirty
of the different elements found in the earth have been found in
meteorites.

THEORIES CONCERNING THE ORIGIN AND
DEVELOPMENT OF THE EARTH

The Nebular Hypothesis of Laplace. Many hypotheses have
been proposed, but the one that has exercised the greatest influ-
ence upon thinking people is the Nebular Hypothesis, as formu-
lated by Laplace. This hypothesis maintains :

1. That the matter of the Solar System was once a highly heated
mass of gas called a nebula.

2. That the form was a vast spheroid extending beyond the
orbit of the farthest planet.

4 6 PHYSIOGRAPHY

3. That the nebula was in process of cooling, and the cooling
caused shrinkage. An effect of shrinkage was to increase the rate
of rotation, and this increased the equatorial bulge.

4. That when the rotation increased to a certain speed, the
centrifugal force at the equator of the spheroid equaled the
attraction of the gravitation. Upon further cooling and contrac-
tion, the equatorial portion separated from the great rotating
mass, forming a ring resembling the rings of Saturn.

5. That as the cooling and contraction of the spheroid con-
tinued, additional rings were separated. The first ring gave rise
to the outermost planet, and the later ones to the other planets
in turn.

6. That the central body was the sun.

7. That each ring parted at its weakest point, and the matter
was collected into a planet, which was hot and gaseous.

8. That the cooling of the planet caused a contraction, which in
turn increased the rate of rotation, and consequently the amount
of bulging. Some of the planets followed the example of the
parent nebula, and formed rings which became satellites.

9. That as the cooling and shrinkage went on, the gases changed
to a liquid and then to a solid state. In the case of the earth, the
volume changed from a rotating mass extending to the orbit of
the moon to its present size.

10. That the more volatile material of the earth remained in a
gaseous state, and formed our atmosphere, originally much deeper
and of a higher temperature than now. As the atmosphere cooled,
the water vapor condensed and formed clouds. As cooling con-
tinued, rain fell and the ocean formed.

THE PLANETESIMAL HYPOTHESIS

During the last few years the planetesimal hypothesis has been
formulated, and may be stated as follows:

1. The hypothesis starts with a cold nebula, spiral in form,
which is the most common type now seen.

2. The spiral nebula consists of a central portion or nucleus,

THE SOLAR SYSTEM 47

which became our sun, with two arms starting from opposite
sides and curved spirally about the nucleus or center.

3. A significant feature of the spiral nebulae is the presence of
numerous nebulous knots in the arms. These knots are the

FIG. 23. SPIRAL NEBULA
From Stellar Evolution

denser portions of the nebula, and the nuclei of future planets
and satellites.

4. The knots or nuclei are surrounded by a nebulous haze,
which is composed not only of gaseous particles but also of in-
numerable solid or liquid particles. These particles revolve about
the center of the nebula like little planets, and are called

48 PHYSIOGRAPHY

planetesimals. The nuclei grow and become planets and satellites
by the in-fall of planetesimals. The earth and moon were two
companion nuclei of unequal size.

5. The earth developed from a knot in an arm of a spiral nebula
by the capture of outside planetesimals. The increasing gravita-
tional compression of the interior produced the internal heat of
the earth.

6. Gases were held in the solid planetesimals as they are held in
meteorites that now fall to the earth. As the growing earth be-
came heated by internal compression, the gases were given forth
gradually, thus forming an atmosphere about the earth. Until
the earth had attained a mass greater than that of the moon
(-^Y of the earth), its gravity was probably insufficient to enable
it to hold the gases of an atmosphere such as we now know. The
gases now issuing from volcanoes were occluded in the original
planetesimals which formed the earth.

7. When the earth had reached such size that water vapor was
held in the atmosphere in sufficient quantity to reach the satura-
tion point, the water vapor began to condense, and then the ocean
began to form.

THE TWO HYPOTHESES CONTRASTED

* Nebular Hypothesis Planetesimal Hypothesis

1 . Nebula, hot and large, formed i . Nebula, cold, formed two
rings around central mass or arms around central mass or
sun.' sun.

2. Rings became planets. 2. Nuclei or knots became plan-

ets and satellites.

2. Smaller rings separated from 3. Smaller knots were captured
planets and became satellites. by larger knots and became

satellites.

4. Planets and satellites origin- 4. Planets and satellites origin-
ally hot and large, gradually ally cold and small, gradually
cooling and growing smaller. heating and growing larger.

5. Outermost planet, Neptune, 5. Planets and satellites forming
formed first and others at at same time.

later periods.

6. Earth always had an atmos- 6. Earth when small without an
phere. atmosphere.

THE SOLAR SYSTEM 49

QUESTIONS

1. Name points by which each class of bodies comprising the solar
system differs from all the other classes.

2. Compare the diameter of the sun with the diameter of the orbit of
the moon about the earth. Compare the periods of rotation of the sun
and moon.

3. As far as is known, which planet has the shortest period of revolu-
tion? How do you account for this?

4. Briefly compare physical conditions on each planet with those on
the earth. What planets have you seen? What are the difficulties in
rinding favorable opportunities for seeing the planets?

5. What purposes do the satellites seem to serve? What are some of
the superstitions connected with our moon? What was the first dis-
covery made by the telescope?

6. Why are the stars generally invisible by day? How can we dis-
tinguish stars from planets?

7. Why are planetoids never seen with the naked eye? What dis-
tinguishes meteorites?

8. What are some of the peculiarities of comets? Describe a comet
you have seen.

9. According to the Nebular Hypothesis, what planet was formed
first? Why are the outer planets larger than the inner planets?

10. According to the Plane tesimal Hypothesis, why are some planets
so much larger than others? Which theory would require the longer
time for the development of the earth? Why?

1 1 . What are two real motions of the sun? Describe two apparent
motions of the sun and point out cause of each.

12. Which of the heavenly bodies are self-luminous?

13. Are any of the planets repeating a portion of the earth's history?
What ones? Have any of the planets reached a more advanced stage in
their development than the earth? Which ones? Explain.

CHAPTER V
MAP PROJECTION

Map making is one of the most important arts, and every great
nation has a body of men engaged in surveying and map making.
In the United States the General Land Office has mapped most of
the country in order to allot and sell the public domain. The
United States Geological Survey is making an accurate large scale
map to show geological and relief features, our navigable rivers,
our lakes, and our coasts. On maps, then, we depend for the sale
of our public lands, and the navigation of our rivers, lakes, and
seas.

A map is the representation of a portion of the surface of the
earth on a plane. The portion represented is indicated by its lati-
tude and longitude. The scale of a map is the ratio between the
length of a line on the map and the actual distance the line repre-
sents. The scale one mile to the inch is also a scale of -B^B^
because i mile equals 63,360 inches. On the U. S. Topographic
Maps the scale most frequently used is -g^iffir, about i mile to
the inch. The mapping of large areas with their curved surfaces
and poleward converging meridians presents difficulties that are
met by certain devices called projections.

Projection, in map making, is a method of representing the curved
surface of the earth on a plane. One method is illustrated by pro-
jecting (throwing) upon a screen the shadow of the frame of a
half globe, with wires for meridians and parallels. The point from
which the rays of light proceed, where the eye may be placed to
view the globe to get the same effect, is called the point of projec-
tion; the screen is the plane of projection, and the rays of light
the lines of projection.

Orthographic Projection. When the point of projection is dis-
tant and the lines of projection parallel and at right angles to the

MAP PROJECTION 51

plane of projection, the orthographic projection is formed. If the
plane of projection is parallel to the axis of the globe the ortho-
graphic equatorial projection is formed with the equator as a diam-
eter, the parallels as straight lines nearer together toward the
poles, the central mericUan straight, but the other meridians curv-

Method

Equatorial

Polar

Orthographic;

FIG. 24. ORTHOGRAPHIC PROJECTION

ing and nearer together toward the margin. If the plane of pro-
jection is at right angles to the axis, bringing a pole to the center,
the orthographic polar projection is formed. In it the parallels
are concentric circles nearer together toward the equator, which
becomes the outer circle. The meridians are straight lines radiat-
ing from the center. This projection, accurate at the center only,
becomes increasingly inaccurate toward the margins, where dis-
tances are much shortened. It shows the actual appearance of the
globe.

Stereographic Projection. When the point of projection is at one
end of a diameter of the globe and the plane of projection is at right

FIG. 25. STEREOGRAPHIC PROJECTION

angles to that diameter, the stereographic projection is formed. An
equatorial diameter gives an equatorial stereographic and a polar diam-
eter, a polar stereographic. This projection, if accurate at the margin,

5 2

PHYSIOGRAPHY

becomes increasingly inaccurate toward the center. Areas are repre-
sented more accurately than in the orthographic projection and less
accurately than in the equidistant.

Globular or Equidistant Projection. When the point of projec-
tion is taken about 1.7 radii from the center of the globe instead of at
the surface of the globe as in the stereographic, the globular projection

Cquic/istant. ^>/'

FIG. 26. GLOBULAR OR EQUIDISTANT PROJECTION

is formed. It is also called the equidistant because the meridians are
equidistant along a given parallel and the parallels are equidistant along
a given meridian. It has a polar as well as an equatorial form. It is
more accurate than the stereographic projection and much more
accurate than the orthographic.

TC K draHn 3 f Kmt 9 ifae. Itnyffi. of dig me&r CD.

45 1

FIG. 27. CYLINDRICAL PROJECTION

Cylindrical Projection. The cylindrical projection is made upon
a cylinder touching the globe at the equator only. The center of
the globe is the point of projection. When the paper is slit along
a meridian and unrolled the meridians and parallels appear as
straight lines at right angles to each other, but at their true dis-

MAP PROJECTION

tances apart at the equator only. The advantage of this projection
is that nearly the whole earth is shown. The disadvantage is that
there is excessive exaggeration of distances toward the poles and
no uniform scale. This projection is often confused with the
Mercator projection which has supplanted it.

Mercator's Projection. This is the cylindrical projection so
modified that at every place the degree of latitude and the degree
of longitude have the same ratio to each other as on the globe itself.
This projection is used to show the whole surface of the earth.
Mariners have adopted it because it shows directions correctly.
Its disadvantages are that distances near the poles are greatly
exaggerated and the scale is not uniform. (See Fig 32, page 60.)

All

\\\

\ \\\

T\ \ \\X\\\\\\\

\\\\\\\\

I I I I III/

I I I I I //////////

FIG. 28. MOLLWEIDE PROJECTION

Mollweide Projection. In this projection the equator and a merid-
ian are laid off their true relative lengths at right angles to each other
at their midpoints. The true relative distances between parallels are
laid off along the meridian and the true relative distances between
meridians along the equator. Ellipses are then drawn passing through
the poles and the proper points on the equator to represent the meridians.
The parallels are then drawn parallel to the equator. This projection
is being used more and more. It is pleasing to the eye and has the great
advantage of showing the entire earth. There is a slight exaggeration
in polar regions, and quite a distortion of shape.

Conical Projection. In the conical projection the point of pro-
jection is the center of the globe and the projection is made upon a cone
touching the globe along any desired parallel. The cone is then slit

54 PHYSIOGRAPHY

along a meridian and spread out. It is evident that this projection is
accurate at the parallel of contact, becoming very inaccurate toward
the poles, one of which cannot be shown.

By using different parallels of contact as bases, it is possible to map
large areas accurately on a large scale. This polyconic projection is
used in the United States Topographic Maps. On such maps the top
parallel is slightly shorter than the bottom one.

North Pole

FIG. 29. CONICAL PROJECTION, BASED ON PARALLEL 30 NORTH

GLOBES AND MODELS

The surface of the earth can, in some ways, be best represented
by maps, in other ways best by models or by globes. Globes have
the advantage of representing the whole earth, in its exact shape,
and with all regions in their true relative positions and in their
true relative sizes. Globes are generally on too small a scale to
show much detail.

Elevations and depressions of the surface of the earth are tech-
nically known as relief. Relief is best represented by models. The
relief of the earth is relatively so slight that models of large areas
fail to give a correct idea of the surface unless an exaggerated
vertical scale is used. This is because horizontal distances on the
landscape are foreshortened, whereas vertical distances are not.

MAP PROJECTION
TOPOGRAPHIC MAPS

Maps on which the physical features are represented are called
topographic maps. On the United States Topographic Maps water
features are represented in blue; culture features, the work of man,
in black; and relief features in brown, by means of contours.

NW,-
C

FIG. 30. CONTOUR MAP OF AN ISLAND WITH THREE PROFILES

The number of spaces between contours shows that point Z is 3 contour intervals above sea
level.

From C toward N E the contours are close together. The profile indicates that the slope is
steep.

From C toward 5 E the contours are/ar apart, indicating a gentle slope.

From C toward 5 W the contours are equidistant, indicating a uniform slope.

From C toward W the slope is gentle, becoming steep, and is convex to sky.

From C toward E the slope is steep, becoming gentle, and is concave to sky.

The re-entrant contours along line C V indicate a valley.

The outcurving contours along C W indicate a ridge or spur.

Contours are lines connecting places of equal elevation. Each
one shows where the new shore line would be if the sea level should

56 PHYSIOGRAPHY

change a certain distance vertically. The difference in level be-
tween adjacent contours is called the contour interval. On most of
our Government maps the contour interval is 20 feet; but it is only
5 feet on certain portions of the flood plain of the Mississippi
River, and is sometimes 250 feet in mountains that are very high
and steep. The significance of contours is brought out by cross
sections called profiles.

Hachures. Relief is also represented on maps by differences in color
and by means of hachures. Hachures are lines drawn to represent the
path water would follow in flowing down a slope. There are many differ-
ent systems of hachures; in one much used the lines are short and thick
where the slope is steep; and long, fine and far apart where the slope is
gentle. (See Fig. 142, page 292.)

QUESTIONS

1. Compare the advantages and disadvantages of maps, models,
and globes.

2. Why are map projections necessary?

3. Compare the advantages and disadvantages of three projections.

4. Name the projections used in the various maps of this book.

5. Note carefully the method of projecting and draw, with a 6-inch
diameter, an orthographic equatorial projection of the globe you use,
numbering the meridians and parallels. Trace in one of the continents,
as South America or Africa, from the globe.

6. Proceed similarly for orthographic polar projection.

7. Proceed similarly for cylindrical projection.

8. Choose those pupils who have done the best work to construct on
large sheets of manila paper large scale maps to be hung on school-
room walls when needed. The backs of maps already mounted may be
used for this. Waxed crayons or colored chalk crayons dipped in melted
wax are cleaner than ordinary colored chalk.

9. If in Fig. 30 the contour interval is 20 feet, how high is the point C?
How long is the island if the vertical and horizontal scales are the same?
Draw a profile through the center from N N W to S S E, and another
from W N W to E S E.

10. Put into a basin a stone shaped like a mountain and fill the basin
so that the tip of the stone just shows. Draw the location of this point
very carefully on a piece of paper placed beside the basin. Lower the
level of the water an inch and draw very carefully the shoreline of the
Stone. Remove another inch and so continue. Draw to the same scale

MAP PROJECTION 57

as in drawing a view of the stone from one side. Label the view and
the contours.

11. Trace your contours very lightly on another piece of paper, using
carbon paper or holding the papers against a window. Change the con-
tour map to an hachure map.

12. Using the same color scheme as on a United States Topographic
Map, show by contours, etc., two peaks of different height, a river, a
lake, a steep slope, a gentle slope. Label properly and locate two points
A and B in sight of each other, and two other points X and F not in
sight of each other.

CHAPTER VI
TERRESTRIAL MAGNETISM

Space about magnets is known as the Magnetic Field. If a
small magnet, known as a magnetic needle, is carried into the
magnetic field of a large steel magnet or an electro-magnet, the
needle will turn and set itself in a definite position in relation to
the magnet. It has been found that the whole earth is surrounded
by a magnetic field, and that magnetic needles set themselves
in definite directions in relation to the earth. If we should follow
the direction in which the magnetic or compass needle points,
we would be going along a magnetic meridian. These magnetic
meridians converge and meet in a locality north of Hudson Bay,
latitude 70 N., and longitude 97 W., known as the North Mag-
netic 'Pole; and also in the Antarctic regions in latitude 72 S.,
and longitude 150 E., known as the South Magnetic Pole.

The north magnetic pole of the earth being 20 degrees from the
geographic north pole and the south magnetic pole about 18
degrees from the geographic south pole, it is seen that the magnetic
meridians do not have the same direction as the meridians of
longitude. It follows that the north-seeking end of the compass
does not indicate true north in most places on the earth. The
departure or variation of the needle from a true north is called
magnetic declination.

Lines connecting places having the same declination are isogonic
lines, and lines connecting places of no declination are agonic
lines. There are many isogonic lines drawn on magnetic charts
of the world, but only three agonic lines. One agonic line crosses
the United States from Lake Superior, through Ohio and Kentucky
to South Carolina. On this line the compass needle points due
north. At all places in the United States east of this line, the

TERRESTRIAL MAGNETISM

needle points west of north. West of this agonic, at all places in
the United States, the compass needle points east of north.

In the state of Maine the variation of the needle is more than
20 west; in the state of Washington more than 20 east, and
in Alaska more than 30 east.

By consulting map (Fig. 32) for the magnetic variation of any
place and then making the necessary correction, the compass may

FIG. 31. LOCATION or THE NORTH MAGNETIC POLE

be used for determining true north. Explorers find the magnetic
needle of little value in pointing out direction in unmapped re-
gions, such as areas about the North and South Poles.

The Mariner's Compass. This instrument consists usually of
several magnetic needles placed side by side, fastened together,

PHYSIOGRAPHY

TERRESTRIAL MAGNETISM 6l

and placed under a circular card. The needle and card are placed
in a basin and supported at the center upon an agate point. The
whole is suspended in such a way that it is always in a horizontal
position, nothwithstanding the rolling of the ship.

Inside the compass box is a black line called the Lubber Line,
placed in the direction of the ship's bow. The compass card con-
tains 32 rays, each indicating a direction or point of the compass.
Naming the 32 points is called " boxing the compass."

Fte. 33. MARINER'S COMPASS

PART II

THE AIR

CHAPTER VII
PROPERTIES AND FUNCTIONS OF THE AIR

Introduction. No part of his environment is of more immediate
concern to man than the air he breathes. If it is pure he is strong.
Vitiate it and he sickens. Withdraw it, but for a single hour, and
he dies.

No other part of his environment has had so great an influence
in helping or retarding him in his struggle for existence or in his
effort to improve his condition. How he dresses, what he produces,
and what he eats are matters chiefly of weather and climate. Too
great heat and too great cold are alike prohibitive of higher aspira-
tions for better things.

The savage Blacks of equatorial Africa and the Eskimo of
the frozen North are both low in the scale of civilization; the first
because the enervating climate destroys ambition; the second be-
cause providing for mere physical needs exhausts his energies,
leaving no opportunity for cultivation of the higher qualities.
Both must adapt themselves to their climatic environment;
neither can change it.

Definition. The earth's atmosphere, or air, is the outer gaseous
part of the earth. It envelops the solid and liquid parts, extend-
ing to a height of probably more than two hundred miles, and fills
all mines, caves, and underground passages. As ground-air it
penetrates all soils, and by the movements of the water it is car-
ried to the greatest depths of rivers, lakes, and seas.

Properties. Pure air is an invisible gas, colorless, odorless, and
tasteless; very compressible and perfectly elastic. It is very
mobile, and like all matter, has weight. Though under ordinary
conditions gaseous, it may easily be made to assume the liquid
state.

66 PHYSIOGRAPHY

The compressibility and elasticity of the air make possible its
substitution for steam in driving machines. This use is par-
ticularly important in deep mines, where the long distances it must
be carried results in condensation of the steam.

The inertia of the air causes a resistance to motion through it,
retarding the speed of the runner, the automobile, and the express
train. When the air is in motion its inertia causes pressure on
objects not moving with it, which varies as the square of the
velocity.

Composition. Air is essentially a mechanical mixture of nitro-
gen, oxygen, carbon dioxide, and argon. Water vapor, water
particles and dust are usually present in it. The relative amounts
of the first four are nearly constant, while the last three are ex-
tremely variable.

Nitrogen and oxygen bear to each other about the ratio of 78
to 21 by volume, and 76 to 23 by weight. Carbon dioxide con-
stitutes about three hundredths of one per cent of the air, varying
slightly with locality and season. Of argon little is known, its
existence not being known until within recent years. Argon con-
stitutes about one per cent of the air. It was formerly included
with nitrogen.

Essential Composition of the Air

Nitrogen 78 . 00%

Oxygen 21 . 00%

Argon i . 00%

Carbon Dioxide 03%

Distribution of Components. In obedience to the principle of
diffusion (ready and spontaneous mixing) the gases of the air
make a fairly uniform mixture. Local conditions may tempo-
rarily disturb this adjustment, but on the whole the air of one
region of the earth is like that of any other.

Carbon dioxide, being one of the products of volcanic action, is
most abundant in regions of active volcanoes. Being likewise a
product of decomposition and combustion, it is more abundant in
cities, especially in manufacturing cities, than in the country; and
more abundant in winter than in summer. The use by growing

PROPERTIES AND FUNCTIONS OF THE AIR 67

plants of carbon dioxide tends to decrease still further its summer
percentage.

Water vapor, though always present in the air, is not an essen-
tial component. It is one of the most variable constituents of the
air, and is in general more abundant over the sea than over the
land, in low than in high altitudes, and in summer than in winter.

Water and ice particles in the air, known as cloud, fog, mist,
rain, snow, hail, and sleet, are limited to the lower air, reaching
an altitude of only a few miles.

Dust in the air is of two kinds, organic and inorganic. Organic
dust includes microscopic animals and plants, pollen, fibers of
wood and cloth, and the soot of smoke. Inorganic dust consists
chiefly of powdered minerals and rocks derived from the land and
caught up by the winds. Dust is more abundant over the land
than over the sea, and is confined to the lower air. It is more
abundant in cities than in the country, and in dry than in rainy
weather, the dust particles being carried down by the falling rain
drops.

Mountain health resorts are sought partly because of the greater
dryness of the air, and partly because of its freedom from dust and
the disease germs that constitute part of the organic dust of the
air at lower altitudes.

Ozone, sometimes considered a constituent of the air, is really oxy-
gen under peculiar conditions. By passing an electric spark through
air the oxygen is in part changed to ozone, which, however, changes
back to the more stable condition of oxygen.

The invigorating quality of the air after a thunderstorm is thought
to be due, in part at least, to the ozone produced by the passage of
lightning flashes through it. The percentage of ozone increases with
the altitude.

Function of the Air. Although the most important uses of the
air are those of its individual components, yet the air as a whole
has important functions. By virtue of it flight of birds and man
is made possible, and sounds are transmitted. By air in motion
ships and wind-mills are driven, life-giving and disease-producing
germs are carried, and the seeds of many plants are fertilized and
distributed. Rain is distributed over the lands, and waves and

68 PHYSIOGRAPHY

ocean currents are produced. Tornadoes and hurricanes, with all
their destructive power, are but air in violent motion.

As a carrier of waste from higher to lower levels, thereby wear-
ing down the lands, and in the accumulation of sand dunes and
loess deposits, the air is an important geological agent. Its
presence in the mantle rock promotes disintegration of the min-
erals and the production of soil.

One of the chief purposes in cultivating crops is to increase the
amount of ground-air. When the surface is packed by rains and
remains unbroken by cultivation, air penetrates the soil with diffi-
culty, and growing crops languish.

Function of Oxygen. The oxygen of the air is the supporter of
combustion. By its chemical union with other elements heat is
evolved. This process, called oxidation, may be slow, as in the
rusting of metals, in which case the heat radiates as rapidly as
produced, and there is no perceptible increase of temperature; or
it may be rapid, as in the burning of wood, coal, or oil, resulting
in an increased temperature, and often in the production of light.
By combination with carbon in the blood of animals oxygen sup-
plies the heat necessary to animal life.

The readiness with which oxygen unites with most other chem-
ical elements makes it active in promoting the disintegration of
rocks and minerals. It is an important agent in the decomposi-
tion of dead animal and vegetable matter, thus serving as a purifier
of the air. In the form of ozone its activity is increased.

Oxygen is more soluble in water than are the other constituents of
the air. The percentage of oxygen in air enmeshed in water is therefore
greater than in ordinary air, its ratio to nitrogen by volume being 34
to 66 instead of the ordinary ratio of 21 to 78. It is this enmeshed air,
obtained at the surface and carried by currents to the greatest depths
of all lakes and seas, that makes life possible, even in the profoundest
deeps.

Function of Carbon Dioxide. The carbon dioxide of the air,
though of no direct use to animals, is essential to the life and growth
of plants. Through the action of sunshine and the chlorophyl, or
the green matter, of the plant, carbon dioxide, absorbed mainly

PROPERTIES AND FUNCTIONS OF THE AIR 69

through the leaves of the plant, is broken up, the carbon retained
and the oxygen returned to the air. The carbon thus obtained
unites with other substances brought in solution in the sap,
thus manufacturing plant food and contributing to the plant's
growth. Dissolved in water, carbon dioxide contributes to the
growth of aquatic plants. It is the most effective of the gases
of the air in decreasing the intensity of the sun's rays, and in
checking radiation of heat from the earth.

Since plants use carbon dioxide in the day time it is well to
have growing plants in the living room, the air on their account
containing a slightly increased per cent of oxygen. On the other
hand they should be excluded from sleeping apartments at night,
since they use some of the oxygen and none of the carbon dioxide.

When plants decay, or are burned, the carbon stored up in their tissues
is returned, usually, to the air in the form of carbon dioxide. Under
certain conditions, however, as submergence in water, or burial out of
contact with the oxygen of the air, the carbon of the decaying plant may
contribute to a future store of mineral fuel in the form of coal, oil, or gas.

Function of Nitrogen. Since nitrogen constitutes more than
three-fourths of the weight of the air, without it the air would be
less than one-fourth its present density. Flight for most forms
would then be impossible, and moving air as an agent for driving
machinery and wearing down the land would be correspondingly
weakened.

Another important function of nitrogen is its use as a plant
food. It is a necessary element of the food of all plants, and like
most other elements is taken through the roots in solution. If the
soil is lacking in this element no plant will thrive.

Unlike oxygen and carbon dioxide, nitrogen is not taken by the
plant directly from the air as nitrogen, but comes by way of the
soil from some soluble compound of nitrogen. Some nitrogen is
obtained in the form of nitric acid, carried down from the air by
falling rain drops. This supply was formerly supplemented chiefly
by the application of fertilizers, often in the form of expensive
nitrates imported from distant regions.

We have learned, however, that certain plants, of the family to

70 PHYSIOGRAPHY

which the clovers belong, are " nitrogen gatherers." These plants
serve as hosts for minute organisms, which, attaching themselves
to the roots of the plant, gather and store upon the roots in little
nodules the nitrogen from the ground-air.

Cowpeas, clovers, vetches, beans, and alfalfa are now extensively
grown, alike for their value as forage crops and for the nitrogen
they add to the soil. The entire growth above ground may be
removed and yet the soil be left richer in nitrogen than before the
crop was grown.

Function of Water Vapor. The water vapor of the air is the
source of clouds, fogs, and of all forms of precipitation. Without
it the earth would become parched, and life impossible. It is
lighter than dry air, and its presence makes the air lighter. Like
carbon dioxide it absorbs insolation (radiant energy from the sun)
and heat radiated from the earth. Condensed as cloud it is more
effective in protecting the lands from the direct rays of the sun
and in checking radiation of heat from the earth. Precipitated as
rain it supplies growing plants with necessary water; and as snow
retards radiation and protects crops from the intense cold of
winter. This function of snow is very important in the wheat-
growing regions of the Northwest.

Function of Dust. Perhaps the most important function of dust
is its diffusion (irregular scattering) of light. Without such diffusion
objects would be visible only by reflection, as they now are at
night; and the change from day to night and from night to day
would be sudden, without twilight or dawn.

At certain seasons a considerable part of the dust of the air is
plant pollen. Many plants require pollen from other plants, and
without the wind-borne pollen these would not be propagated.

Putrefaction and fermentation are largely due to the organic
dust of the air. Flesh and vegetables in high altitudes do not
decay readily but simply dry out and shrivel up. This is due to
the freedom of the air from dust germs. Some Indian tribes in
these regions mummify their dead by simply exposing them in the
air and sunshine.

PROPERTIES AND FUNCTIONS OF THE AIR 71

The germs of many diseases are distributed as air-dust; and
flesh wounds heal more readily when the dust germs are washed
away and excluded from the wound.

Every dust particle in the air is a nucleus about which water
vapor may condense; consequently dust in the air promotes cloud
formation and rainfall. Some have even taken the extreme view
that without dust in the air no rainfall would be possible; but
this has been disproved by experiment.

Of esthetic interest is the fact that the sky owes its beautiful
and varying colors, for the most part, to the dust in the air. Gor-
geous sunrises and sunsets occur when the air is laden with, inor-
ganic dust, or with the smoke from forest fires.

Origin of the Air. If the Nebular Hypothesis concerning the origin
of the Solar System be accepted, the air may be considered a remnant of
a formerly denser atmosphere. In this earlier atmosphere many of the
elements which now make up the lands and seas existed as gases in an
intensely hot condition. With loss of heat by radiation these elements
changed to the liquid or solid state. Many of the elements of the
primitive atmosphere were thus withdrawn, leaving the present rem-
nant, the air as we know it.

If we accept the Planetesimal Hypothesis of the origin of the Solar
System, we believe that the air has been driven out from the interior of
the earth by the increasing temperature and pressure. The gases thus
driven out escaped to outer space while the earth was small and its
gravitative attraction weak, and remained as part of the earth only after
the earth's attraction became strong enough to hold its gaseous envelope.

Future of the Air. Whatever the origin of our earth or of its gaseous
envelope, the earth is continuously losing heat. We may therefore look
forward to the time when it will have the temperature of outer space,
excepting only the surface that is turned toward the sun.

Experiment proves that most gases can, with sufficiently low temper-
atures, be liquefied and solidified. We also learn, from a study of the
other members of our system besides the earth, that the smaller ones,
such as the earth's moon, seem to have no atmosphere. These smaller
members have cooled most, and if they ever had atmospheres their
present low temperatures have probably resulted in making their
atmospheres part of their solid masses.

We may therefore infer that with further loss of heat by the earth
the terrestrial seas must in time become solid; and eventually the air

72 PHYSIOGRAPHY

itself become in turn liquid and solid. Upon such an airless earth
life, as we know it, could not exist; and the earth would then appear, to
an observer upon another planet, the lifeless globe that our moon now
appears to us.

QUESTIONS

1. Why is it not correct to say "the air surrounds the earth"?

2. How can you show that the air has weight? That it penetrates the
soil?

3. In what particulars is country air usually purer than city air?

4. In what sense does rain purify the air?

5. Why will plants thrive better than animals in hot, marshy lowlands?

6. Why are trips to the mountains and sea voyages recommended for
convalescents?

7. How can you prove that there is dust in the air; and how can you
decrease the dust in your bed-chamber without stopping ventilation?

8. Why will milk that has been heated before bottling not sour as
quickly as that which is bottled without heating?

9. Why do dairymen cool their milk before shipping; and why is ice
used to keep milk sweet? What is the principle of "cold storage"?

10. Why will a candle lighted and lowered into a narrow deep bucket
so quickly be extinguished? Why are lamp burners ventilated?

11. Why do we ventilate our houses? What would be the result if
we did not? Explain the horror of the "Black Hole" of Calcutta.

12. How do you know there is water vapor in the air?

13. Why should a wound be thoroughly cleansed before binding up?
What is the principle of disinfection and sterilization?

14. Why is it necessary to thoroughly dry our steel cutlery, and not
so necessary with our silverware and china?

15. Why do fires in open fireplaces and in stoves connected with flues
burn better than fires built in the open air?

16. How can the nitrogen of the soil be increased most economically ?

CHAPTER VIII
TEMPERATURE OF THE AIR

Sources of Heat. So evident is it that the sun is the chief
source of heat that the statement of the fact seems to need no
demonstration. The temperature of our days increases with in-
creasing length of the period of sunshine and with the nearer
approach of the sun to our zenith, whereas our coldest season is
that in which the nights are longer than the days, and the sun's
noon position is low above the horizon. The hot belt of the earth is
that which receives nearly vertical rays, while the frozen regions
near the poles have only slanting rays.

At first thought the sun appears to be the only source of heat;
yet we know there are other sources. One of these minor sources,
the interior heat of the earth, is of considerable importance,
notably in deep mines, and in the production of volcanos and
hot springs.

The surface of the land varies in temperature from day to
night and from summer to winter; but if we descend below the
surface the variation is less and less. A depth is finally reached,
varying with the latitude, at which the temperature does not
change, and below this depth the temperature grows warmer the
deeper we go. On this account we conclude that the interior of
the earth is intensely hot. On the other hand, if we ascend in
the air we find that the temperature grows colder, and at the height
of only a few miles freezing temperatures, even in summer, are
reached. Reasoning from this basis we conclude that outer space
is intensely cold.

From our knowledge of cooling bodies we know then that the
earth must be a cooling body, sending its heat in every direction
into outer space, and bringing about equal amounts to every part
of the surface of the land.

PHYSIOGRAPHY

Unimportant amounts of heat are received from the stars, and
reflected from the other planets and the moon.

Insolation. The radiant energy that comes to us from the sun
is called insolation. It does not come to us as heat, but manifests
itself in many ways, e. g. as light and electricity. Only the
insolation which is absorbed by any body is changed to heat and
warms the body.

As solar energy passes out from the sun-center in all directions, it is
evident that only a very minute fraction of it will be intercepted by so
small a body as the earth, at an average distance of about ninety- three
millions of miles. Of the amount thus intercepted but a small portion
is absorbed and transformed into heat; yet upon this minute part of the
total solar energy all of our life-interests and activities depend.

Disposal of Insolation. When insolation is received, it is dis-
posed of in three ways: by reflection, by transmission, and by
absorption. As before stated, it is only the absorbed insolation that
affects the temperature of the body.

Each kind and condition of matter disposes of insolation in a
distinct way. Some substances are good reflectors, some good
transmitters, and some are good absorbers. Experiment has shown
that in general, gases are the best transmitters, liquids the best
reflectors, and solids the best absorbers.

The absorptive power of a body may be materially modified by
a change of color or of surface. Dark colors and irregular surfaces
generally promote absorption, while light colors and smooth sur-
faces promote reflection. By increasing the reflecting power of a
body we decrease its absorbing power.

The following table sets forth, comparatively, the treatment of
insolation received by land, water, and air:

Land

Water

Air

Reflector

Fair

Good

Very poor

Transmitter

Poor

Fair

Very good

Absorber. .

Good

Poor

TEMPERATURE OF THE AIR 75

Loss of heat by radiation is in direct ratio to the absorbing power
of a body; a good absorber being a good radiator, and a poor ab-
sorber a poor radiator. If the reflecting power of a body be in-
creased its radiating power will be lessened. Radiation is con-
tinuous.

Heat in a body may be distributed by passing from particle to
particle in contact; this process is called conduction. Solids are
mainly heated in this way, but differ widely in their power of
conduction.

In liquids and gases, e. g. water and air, the most important
method of distributing heat is by convection. By this process parti-
cles in contact with a heated surface are warmed and expand, and
after expansion are lighter, volume for volume, than the surround-
ing particles. The heavier particles then sink, under the greater
pull of gravity, and the lighter are crowded away from the heating
surface, the heavier being heated in turn. This process, depending
as it does upon gravity, requires that the heating surface be below
the substance to be heated.

The principle of convection is applied in the heating of our
houses and in the construction of flues and chimneys.

The land and water, being heated from above, are never warmed
to very great depths; while the air, being chiefly heated at the
bottom, by contact with the land and water, is warmed more
rapidly and through a much greater thickness.

How the Air is Heated. The power of absorption of the air,
though small, increases with increase in density, increase in car-
bon dioxide and water vapor, and in the number of dust and
liquid water particles present. Each dust and water particle,
being a better absorber than air, becomes itself a center of warm-
ing. Therefore, when insolation comes to earth it passes through
the rare upper air with little loss by absorption. As it penetrates
farther into the denser and dustier air more and more of it is
absorbed, and the air is more and more heated. The air absorbs
from one-half to three-fifths, depending upon its cloudiness, of ver-
tical insolation passing through it.

The air is heated most at the bottom, not only because of the

76 PHYSIOGRAPHY

increased absorbing power of the lower layers, but also because of
their contact with the warmer land and water surfaces.

Another very important aid in the heating of the lower air is its
convectional mixing. The air in contact with the warmer land or
water surfaces is warmed and expands. The cooler, heavier air
above sinks and takes its place, to be in turn warmed and re-
placed by cooler air from above. This mixing is for the most part
confined to a stratum of air five or six miles in thickness. As long
as the land and water are warmer than the air resting on them,
convectional mixing will continue a factor in the warming of the
lower air.

The convectional ascent of heated air may be observed above a
lighted gas jet, a hot stove, or a bonfire. Our rooms may be ven-
tilated by admitting cool air at the bottom and permitting the
escape of the heated air above.

How the Air is Cooled. When insolation ceases, as at night,
conditions are reversed. Absorption, in excess of radiation during
the day, is at night exceeded by radiation, and the air is cooled.
Not that radiation does not continue during the day, for it is
greatest when the temperature is highest, but the air does not
begin to cool until radiation is more rapid than absorption.

Since a good absorber is a good radiator, that part of the air
which was most heated during the day is most cooled when inso-
lation ceases. As a consequence, the rare, upper air is but little
cooled, while the lower air is cooled most. Each dust and water
particle, a center of warming during insolation, becomes a center
of cooling when insolation ceases.

One important factor in the warming of the air, convection, is
wanting when the air begins to cool. Being most cooled at the
bottom, the lower layers of air are heaviest, hence there is no
tendency toward convectional mixing. In order to have cooling
by convection it would be necessary to have the air cooled most
at the top. On this account the lower air warms up faster than it
cools down.

The coldest hour of the day is from 4 to 6 a. m., and the warm-
est from i to 3 p. m., depending upon the season. Thus it takes

TEMPERATURE OF THE AIR 77

from seven to nine hours for the air to warm up, while from fifteen
to seventeen hours are required for it to cool down.

Temperatures Determined. The temperature of the air, with
reference to certain chosen temperatures, is determined by the
thermometer. The temperatures of reference are those at which
pure water freezes and boils under a pressure of approximately
14.7 pounds to the square inch. This is the average pressure of
the air at sea level.

The action of the thermometer is based on the fact that most
substances expand uniformly with heating, and contract uniformly
with cooling. The measure of expansion or contraction may be
taken as a measure of the amount of heating or cooling.

Two general classes of thermometers are made,
liquid and non-liquid. Almost any liquid or metal
may be used. In the United States and other
English-speaking countries, two scales for thermom-
eters are in common use: the Fahrenheit (F), and the
Centigrade (C). Their relation to each other and the
method of converting readings of one to readings of
the other are shown in the accompanying figure and
table.

Fig. 34 shows both Fahrenheit and Centigrade scales.
It will be observed that the two scales agree at 40.
Freezing point is 32 on the F., and o on the C., and
boiling point 212 and 100 respectively. It will thus ;
be evident that a change from 32 to 212 degrees on :
the F. thermometer is equivalent to a change o to
100 on the C. This relation may be thus expressed:
180 F = 100 C
9 F = 5 C
i F = % C
1.8 F= iC Flo ' S4

History of the Thermometer. The thermometer was invented by
Galileo, early in the seventeenth century. Soon after its invention it
was graduated into 360 parts, corresponding to the number of degrees
in a circle, hence the name degrees applied to these divisions. The name

78 PHYSIOGRAPHY

has been retained for the divisions of modern thermometers, though very
differently and variously graduated. It was never significant.

Fahrenheit was the first to adopt definite temperatures as a basis for
graduation. According to his scale the boiling point of water was found
to be 212, and the freezing point 32. In the Centigrade thermometer
100 is taken as the boiling point and the freezing point.

The accuracy with which the instrument may be read depends upon
the length of the degree, and this in turn depends upon the relative
capacities of bulb and tube. It is essential to accuracy that the tube
be of even bore. Why?

Mercury and alcohol are commonly used in liquid thermometers, partly
because of their even expansion at all ordinary temperatures, and partly
because of their low freezing points. Mercury freezes at 40 F., and
alcohol at about 200 F. In the winter in high latitudes the temper-
atures are too low to be recorded by mercury thermometers. On the
other hand mercury is the better suited for high temperatures, since its
boiling point is 660 F., while that of alcohol is only about 173 F., or
lower than the boiling point of water.

Maximum Thermometer. It is often desirable to know the
highest temperature attained during a given period. For this
purpose the maximum thermometer is used. This is a modifica-
tion of the ordinary liquid thermometer by a slight constriction in
the bore just above the bulb. This narrowed bore, though wide
enough to allow the expanding liquid to press through, is too nar-
row for the liquid column, of its own weight, to pass back as the
temperature falls. The thermometer thus continues to indicate
the highest temperature attained.

The clinical thermometer used by physicians is a maximum
thermometer. To set the instrument for a new reading the col-
umn of liquid must be made to unite by swinging or jarring the
instrument.

Minimum Thermometer. The minimum thermometer, for reg-
istering lowest temperatures, is simply an ordinary alcohol ther-
mometer, with colorless liquid, containing a short double-headed
pin. The heads of the pin are slightly smaller than the bore, in
order that the alcohol may pass by the pin.

For registering a minimum temperature the tube is placed in an in-
clined position, so that gravity cannot pull the pin down the tube; but

TEMPERATURE OF THE AIR

when gravity is assisted by the surface tension of the liquid, when the
upper end of the contracting column comes in contact with the upper head
of the pin, the pin is pulled down the tube. When, with rising temper-
ature the liquid column begins to lengthen, it passes over and by the
pin, but cannot push the pin against gravity up the tube. The upper
end of the pin thus registers the lowest or minimum temperature at-
tained.

To set the instrument for registering a new minimum the thermometer
is held, bulb upward, until the pin sinks through the liquid to the end of
the column. The instrument is then placed in the inclined position in
which it ordinarily rests.

Thermograph. To obtain a continuous record of the tempera-
ture a self-registering thermometer, or thermograph is used. The

FIG. 35. THERMOGRAPH

varying temperature is recorded by a pen, moved by a system
of levers. The pen rests against a disc or cylinder of paper which
is moved by clock-work. A continuous trace of the pen is made

FIG. 36. THERMOGRAPH RECORD FOR ONX WEEK
Note daily variation of temperature, and hour of highest and lowest temperature.

which, by reference to two sets of lines ruled upon the disc or
sheet, temperature lines and time lines, shows the temperature at

80 PHYSIOGRAPHY

any time. The thermograph takes the place of the maximum and
minimum thermometers.

The record made by the thermograph is a temperature curve for
the period of time covered by the record. (See Fig. 36.)

Approximately accurate temperature curves may be made from ob-
servations of the thermometer taken every two hours. From the daily
averages monthly curves, and from the monthly averages annual tem-
perature curves may be constructed.

Distribution of Insolation. The amount of insolation received
by a given area of land or water in a given time, as during one
complete rotation of the earth, depends mainly upon the following
variables:

1. Length of insolation period, or the number of hours of sun-
shine;

2. The angle at which the insolation rays are received;

3. Condition of the air as regards dust and cloudiness;

4. Distance from the source of insolation, the sun ;

5. The length of the path of the rays through the atmos-
phere.

Length of Insolation Period. Because the earth's axis is in-
clined to the plane of its orbit the insolation period is not the
same for all places, nor for the same place at all times. Most
places upon the earth have the period of rotation unequally
divided between sunshine and shadow.

At the equator the period of insolation is always twelve hours.
In all other latitudes it is only at the equinoxes that the insolation
period is twelve hours; being longer than twelve hours when the
sun is on the same side of the equator as the observer, and shorter
than twelve hours when the sun and observer are on opposite
sides of the equator.

The higher the latitude the greater the length of the continuous
insolation period. Within the polar circles it varies from no
insolation in mid-winter to twenty-four hours of insolation in mid-
summer, for each rotation.

TEMPERATURE OF THE AIR 8l

Relation of Latitude to Greatest Length of Day or Night

Latitude

Greatest Length of
Day or Night

Latitude

Greatest Length of
Day or Night

12 hrs. oo mins.

1 6 hrs. 04 mins.

12 " 16 "

17 " oo "

12 " 4 "

18 " 15 "

12 " 5 2 "

20 " 4 8 "

I 3 " 12 "

66.5

24 " oo "

13 " 34 "

64 days

13 54

I 4 " 20 "

133 "

14 " 48 "

160 "

IS " 20 "

187 "

Other things being equal the amount of insolation received varies
as the length of the insolation period. There is, therefore, at
summer solstice a constantly increasing amount of insolation,
during one rotation, from the equator to the polar circle of the
summer hemisphere; and a constantly decreasing amount from
the equator to the polar circle of the winter hemisphere.

Angle of Insolation. Since the earth's shape is globular, the
angle at which the sun's rays strike at any place varies with the
latitude and with the time of day. This angle is zero at sunrise
and sunset at any station, and is a maximum at noon.

Because of the inclination of the earth's axis and revolution the
angle of the sun's rays varies at any station from day to day.
Vertical noon insolation occurs at the equator at the times of the
equinoxes; and at the tropics, alternately, at the times of the
solstices. During the year vertical noon insolation occurs twice
at every station within the belt, forty-seven degrees wide, lying
between the Tropics. This belt is sometimes called the torrid
zone. No place outside this zone ever receives vertical insola-
tion, the maximum angle being less and less with increase of
latitude, reaching 23^ at the poles.

PHYSIOGRAPHY

Hence, in so far as the angle of insolation determines the amount
of insolation received during one rotation, the maximum amount is
always received upon or between the Tropics. The average for the
year is greatest at the equator and least at the poles.

FIG. 37. SHOWING RELATION OF ANGLE OF INSOLATION TO INTENSITY OF INSOLATION
Surface AB, which receives 100% of insolation when vertical, receives but 25.5% when the angle

of insolation is 15.

Condition of the Air. The two most variable constituents of
the air are likewise those which most intercept insolation. These
are, in the order of their importance, cloud-particles and dust.
Clouds and dust in the air intercept insolation, and thus prevent
land and water surfaces from being as much heated as they would
otherwise be. For this reason those places where cloudiness pre-
vails have a more constant temperature than places with prevail-
ingly clear skies. Cloudy days are less warm in summer and less
cold in winter than are clear days; and the insolation on the
mountain top is more intense than in the valley.

TEMPERATURE OF THE AIR

Distance From Sun. The amount of insolation received varies
inversely as the square of the earth's distance from the sun. While
this factor has a scarcely perceptible value as between any two
places upon the earth, at any given time, the difference in dis-
tance being never as much as four thousand miles, yet as between
winter and summer the value is considerable. The earth is about
three million miles nearer the sun at perihelion, about January
first, than at aphelion, about July first. In consequence a place
receiving vertical insolation January first receives about 5% more
insolation than one receiving vertical insolation July first.

Length of Path Through Air. Oblique rays pass through a
greater thickness of air than do vertical rays; and whereas verti-
cal rays lose half of their intensity, rays approaching tangency lose
more than 90%.

Intensity of Insolation at Different Angles

Altitude of the
Sun

Relative Length of Path
of Ray Through
Atmosphere

Intensity of Insolation
on Surface Perpen-
dicular to Rays

Intensity of Insolation
on a Horizontal
Surface

44.70

0.00

5-70

0.31

0.05

2.92

0.51

0.17

2.0O

0.62

0.31

1.56

0.68

0.44

I-3I

0.72

o-55

I-IS

o-75

0.65

I. 06

0.76

0.72

1.02

0.77

0.76

I .00

0.78

While the poles alternately receive more insolation than any
other portion of the earth, for a brief period about the summer and
winter solstices respectively, owing to continuous insolation there,
all the conditions combine to give -to places at the equator about
two and one-half times the amount of insolation annually received
at the poles.

PHYSIOGRAPHY

Distribution of Heat Over the Earth. The distribution of heat
over the earth does not agree with the distribution of insolation,
though in general following it, since the same factors govern the
distribution of both. It should be remembered that heat is caused
by absorbed insolation, and whatever factors enter into the control
of absorption to that extent affect the temperature of the absorb-
ing substance.

Distribution of Insolation

LATITUDE

Vernal equinox. .....

1. 000

0.881
0.984
0.942

0-934
1.040
0.938
0.679

0.763
I.I03
0.760
0.35 2

0.499
1.090
0.499
0-053

o.ooo

1.202
0.000
O.OOO

0.000
0.000
0.000

1.284

Summer solstice

Autumnal equinox. . .
Winter solstice ......

Increasing obliquity of the sun's rays is accompanied by a more
rapid decrease of heat developed than of insolation received. It
results in an increased per cent of insolation reflected and conse-
quently a decreased per cent absorbed. It is found that while water
reflects only 2% of -vertical insolation, it reflects about 65% when
the sun is only ten degrees above the horizon. On this account
the early morning and late afternoon rays, and the rays received
in high latitudes, have little effect in increasing temperatures. For
this reason alone the polar regions could never be warm; and the
low minus temperatures reported by our Arctic and Antarctic
explorers as occurring there in mid-summer are in part accounted
for.

When we consider also the fact that in the polar regions the lands
and frozen seas are for much of the year covered with snow and ice,
both very poor absorbers, and that the heat produced by absorp-
tion must first be used to melt the ice-cap, we may better appre-
ciate the low temperatures which prevail there.

The northern hemisphere, where the continents are massed, is
warmer in summer and colder in winter than the southern hemi-

TEMPERATURE OF THE AIR 85

sphere, which is mostly water. This is due to the fact that land
is a better absorber and better radiator than water, and the fur-
ther fact that it requires more heat to warm the water than the
land.

Dark colored rocks and soils, being better absorbers than light
colored ones, are warmer under sunshine and colder when insola-
tion is withdrawn. This, in a measure, controls the character and
amount of plant growth, and affects the distribution of heat.

The direction and character of winds and ocean currents, to be
explained, are likewise important factors in the distribution of
heat over the earth.

All things combine to give to regions along the equator the
greatest total amount of heat, and to make its distribution through
the year most equable there.

Shifting of Heat Equator. This zone of greatest heat near the
geographical equator, and of varying width, is known as the doldrum
belt, or simply the doldrums. The line in the midst of this belt,
passing through places having the highest temperatures, is called
the heat equator.

Since the sun's vertical ray shifts during the year over a zone
forty-seven degrees wide, so the doldrums and heat equator shift,
though over a narrower zone.

The temperature of a place continues to increase so long as
more heat is received than is lost by radiation. The change from
warming up to cooling down occurs, during the day, ordinarily an
hour or two past noon, though most heat is received at noon; and
the highest temperature of the year occurs usually some weeks
after the longest day, although most heat is received on that
day.

The doldrum belt and heat equator, therefore, do not attain
their extreme positions north and south at the times of the sol-
stices, but weeks after. Places between the Tropics, having
vertical insolation twice a year, have two maxima and two minima
during the year, and experience their highest maximum tempera-
ture shortly after vertical insolation upon the sun's return toward
the equator.

PHYSIOGRAPHY

Average Position of Heat Equator. The heat equator shifts far-
ther, and remains for a longer time, north of the terrestrial equator than
it does south of it. This is in part because the sun is seven days longer
north of the equator than south of it; and in further part because of the
forms of the continents and ocean basins. Owing to the positions and
outlines of the continents more of the warm ocean currents are turned
into the northern oceans than into the southern, and these make the
northern hemisphere on an average the warmer.

Moreover, the Pacific basin, being almost closed at the north, thus
practically shutting out the cold polar currents that freely enter the
North Atlantic, makes the North Pacific a warmer ocean than the North
Atlantic. The average position of the heat equator is, therefore, more
northerly in the Pacific than in the Atlantic.

Shifting Most Over Atlantic. Being a better absorber and better
radiator than water, land has a higher temperature in summer and a
lower temperature in winter than the sea in the same latitude. This
excessive warming and cooling is most pronounced in its effects in the
northern hemisphere, where the great land areas are; and is also more
pronounced over the relatively narrow Atlantic than over the broader
Pacific.

The accompanying table shows the approximate widths and extreme
positions of the doldrum and trade wind belts during the year in both
the Atlantic and Pacific oceans:

ATLANTIC OCEAN

PACIFIC OCEAN

March

September

March

September

N E Trades

26 N- 3 N
3N-o
o -25 S

35N-nN
11 N- 3N
3 N-2 5 S

25N- 5 N
5 N- 3 N
3 N-28 S

30 N-io N
10 N- 7 N
7 N-2o S

Doldrums

S E Trades ....

Isotherms. Lines drawn through places having the same tem-
perature are called isotherms. They may represent the distribu-
tion of temperatures at any given time, or they may represent the
averages for any desired period, as a week, a month, or the entire
year. Such lines, while very irregular, have in the main a general
east-west direction. This is as we should expect, inasmuch as
length of insolation period and angle of insolation, the most impor-
tant factors in determining the distribution of heat, are constant

TEMPERATURE OF THE AIR 87

along any given parallel. The minor factors in the distribution of
heat are responsible for the departure of isotherms from the
parallels.

Isotherms are continuous lines, and for a limited area may appear
upon the map as closed curves. From their definition two isotherms
cannot intersect. The heat equator is not an isotherm, though it extends
around the earth in the same general direction as isotherms. It may
cross isotherms.

Temperature Gradient. If we pass from one isotherm to the
next of higher or lower temperature, we must pass through all
intermediate temperatures. While we may pass along an indefi-
nite number of routes, it is evident that the shorter the route the
more rapid the change of temperature. The shortest route, which
gives the maximum rate of change, is the direction of the tempera-
ture gradient.

Temperature gradient may be defined as: The rate of change of
temperature measured in F, degrees, in a distance of one latitude
degree, or about seventy miles.

The more closely the isotherms are crowded the more rapid the
change of temperature, or as we say, the steeper the gradient; while
widely separated isotherms indicate gentle gradients.

Isothermal Charts. If the isotherms of any region be drawn
the result is an isothermal chart. Daily, monthly, seasonal, and
annual charts are commonly made.

Isothermal charts are graphic representations of temperature
readings where time is constant and place variable; whereas tem-
perature curves are records with place constant and time -variable.

Vertical Distribution of Heat. If we ascend through quiet air,
as in a balloon, we shall find that, as a rule, the temperature of the
air decreases ; descending, the temperature increases. This change,
due to difference of altitude, is about i F. for every 300 feet, and
is known as the vertical temperature gradient.

PHYSIOGRAPHY

TEMPERATURE OF THE AIR

90 PHYSIOGRAPHY

QUESTIONS

1. If the interior heat of the earth were the chief source of heat, what
part of the earth's surface would be hottest?

2. What reason have you for thinking that the sun, rather than some
other outside source, is the chief source of our heat?

3. What per cent of the sun's radiant energy is received by the earth?
Does Mercury, Venus or the Earth receive the largest per cent ?

4. Why are dark shades of clothing better suited to winter than to
summer? Why are dark colored soils earlier ready for seeding than
light colored?

5. Why do we heat our kettles from below; and why place the radia-
tors that heat our rooms near the floor rather than near the ceiling?

6. Aviators find the air at the height of a few thousand feet always
cold; why is this?

7. The higher we ascend in the air the more intense the insolation;
then why are the tops of high mountains always cold?

8. Why do lakes and rivers cool down so much more quickly than they
warm up? Why do shallow lakes freeze over more quickly than deep?

9. Why is the temperature of Denver more equable than that of St.
Louis? Chicago more equable than Minneapolis?

10. Why is a uniform bore necessary in the tube of an accurate ther-
mometer? Why is the tube expanded at the bottom?

11. How can you use the thermometer to determine altitude?

12. Why do flowers bloom and the trees put forth their leaves so much
earlier on the south than upon the north slopes of mountains and hills?

13. Why is a cloudy day in winter warmer, and in summer cooler, than
a clear day?

14. Why is it warmer in summer in the latitude of St. Louis than
at the equator?

15. Why is the warmest hour of the day later in summer than in
winter, and why is the coldest hour earlier?

1 6. Why is there less difference between the two maximum tempera-
tures during the year than between the two minimum, at places over
which the vertical ray of the sun shifts?

CHAPTER DC
WEIGHT AND DENSITY OF THE AIR

Pressure and Weight. It is a well-known fact that at any point
within a liquid or a gas pressure is equal in all directions. On
this account one moves freely about in the air, although it is press-
ing upon every square inch of the body with a pressure of almost
fifteen pounds, or more than a ton to the square foot. Neverthe-
less this great pressure causes us no inconvenience, because it is
balanced by an equal pressure from within. Pressure of the air
is pressure per unit area.

The pressure of the air sustains, at sea level, a vertical column of
water about 34 feet high, and a vertical column of mercury about
30 inches high. This fact is applied in the lifting pump, the
siphon, and the barometer.

A cubic foot of air at sea level weighs about 1.25 ounces. In a
room 14 ft. long by 12 ft. wide by 10 ft. high there are more than
125 pounds of air; and the weight of the air above an acre of ground
is almost 50,000 tons. The weight of the air above any horizontal
surface is equal to the pressure upon that surface, weight being
simply pressure downward.

Density of the Air. In gases, pressure, density, and volume
bear a definite relation to each other. As the pressure increases
the density also increases, and the volume decreases in the same
ratio. This is not true of liquids or solids. As a result of this
relation the air is densest at the bottom.

So rapidly does the density of the air decrease as we ascend in
it, that at an altitude of about 3.6 miles the air is only half as
dense as at sea level. This means that half of the air is within
3.6 miles of the surface of the sea; and since many mountains are
more than three miles high, their summits reach above one-half of

PHYSIOGRAPHY

the entire mass of the air. Withing the next three miles we pass
through almost one-half of the remaining half of the air; so that
three-quarters of the air is within 6.8 miles of the surface of the
sea. If the air were of the uniform density -of the lower air, it
would extend only about five miles above sea level.

Measurement of Pressure. For the purpose of measuring the
pressure of the air the barometer has been devised. Its construc-
tion depends upon the principle that a given weight of air will
balance an equal weight of any other fluid; or Counterbalance an
equal pressure exerted in any other way.

Two types of barometer are in common use, the liquid and
non-liquid. In the liquid barometer the air sustains a column of
liquid, commonly mercury, in a tube from which the air has been
withdrawn. In the non-liquid or aneroid barometer, the pressure
of the air is counterbalanced by the resistance of a thin metal
diaphragm.

The simple mercury barometer consists essentially
of a glass tube at least thirty-four inches long, closed
at one end, filled with mercury and placed vertically,
open end down, in a cistern of mercury. The tube is
graduated in some linear unit, as the millimeter or
tenth of an inch, the surface of the mercury in the cis-
tern being the zero of the scale.

The mercury sinks in the tube, leaving a few inches
of the upper end of the tube a vacuum, that is, with
no air pressure on the mercury column. The column
of mercury is sustained by the pressure of the air upon
the open surface of mercury in the cistern. When this
pressure increases the mercury rises in the tube, and
when it decreases the mercury sinks.

At sea level the length of the mercury column is
about thirty inches; hence we commonly say the air
pressure at sea level is thirty inches, understanding
that the pressure is measured by the weight of the col-
umn of this length, as pressure cannot be measured

BAROMETER in inches.

WEIGHT AND DENSITY OF THE AIR 93

As it is the vertical length of the column of mercury that
measures the pressure of the air, it is necessary, when tak-
ing a reading, to hold the instrument in a vertical position.
For this purpose, and to protect the instrument, the tube
and cistern are firmly bound together to a rigid frame,
arranged for suspension.

The aneroid barometer consists essentially of a pile
of hollow metallic discs, from which the air is ex-
hausted, and to which an index is attached. This
index moves over a surface upon which there are
graduations to represent the various pressures. The
discs are made of very thin metal, supported by coiled
springs within, and respond to slight changes in air
pressure. Because of its convenience the aneroid is
much used in taking altitudes. Both pressure in
inches and altitudes in feet are usually shown.

Variation in Barometer Reading. At sea level, as we
have seen, the average reading of the barometer is
about thirty inches. As the instrument is carried up
through the air, in a balloon or in ascending a moun-
tain, it is found that the barometer reading is lower
by about one inch for each thousand feet of ascent.
This is because of the air that is left below, only the
air above affecting the barometer.

This is only approximately true, for with increase in altitude there is
a decrease in the density of the air. Whereas a fall of one inch results
from carrying the instrument from sea level up 910
ft., a fall of two inches requires an ascent of 1,850 ft.
The higher the altitude the greater the distance
through which the instrument must be carried to
register a fall of one inch.

Height of the Air. Estimates of the thick-
ness of the air envelope, based upon barometer
readings, are unreliable, inasmuch as we do not
know at what rate the density of the upper
air changes. While one-half of the air lies
ANEROID BAROMETER within 3.6 miles above sea level, we have reason

94 PHYSIOGRAPHY

to know that the air in considerable density exists at a height
of about two hundred miles. Meteors have been observed at
that height.

Lows and Highs. If a stationary barometer be read from hour
to hour it will be noted that its readings vary continuously. This
seems to be due chiefly to a succession of surges in the air, called
lows or highs as the barometer falls or rises. Lows are also called
cyclones and highs anti-cyclones.

As we go outward from the center of a low we observe that the
barometer readings are higher in all directions; and in passing out
from the center of a high the readings are lower. It follows that
about lows and highs systems of lines may be drawn through
places having the same barometer reading. Lines drawn through
places having the same barometer reading are called isobars.

Because of the mobility of the air, and the many conditions
that affect pressure, isobars are not apt to be either regular or
parallel, although about lows and highs that are strongly devel-
oped isobars are closed curves.

The isobars about a high may be aptly likened to the contours of
a hill in a topographic map, the high by analogy being an atmos-
pheric hill; and those about a low to the contours of a depression,
the low being an atmospheric hollow.

Inasmuch as the density varies directly as the pressure, the air
is denser about a high than about a low.

Pressure Gradient. Just as the temperature gradient line is
the shortest distance from one isotherm to the next, so we may
get the pressure gradient line at any place by taking the shortest
distance between the isobars at that place. Numerically expressed,
the pressure gradient is the number of hundredths of an inch
change of pressure, in a distance equal to one latitude degree, or
about seventy miles. Crowded isobars, therefore, signify steep
pressure gradients, and widespread isobars gentle gradients. We
shall see that the direction and strength of the wind are closely
related to the pressure gradient.

If a continuous record of the air pressure is desired, an instru-
ment called the barograph is used. It is usually an aneroid ba-

WEIGHT AND DENSITY OF THE AIR

rometer with a pen-bearing arm in the place of the index. The
pen-point rests against a disc or sheet of paper that moves at a
constant rate, as in the thermograph. The two systems of lines
are time and pressure lines.

The record of the barograph is a pressure curve, and the charted
pressures of any region, as shown by
the isobars, make an isobaric or pres-
sure chait.

Pressure Belts. The distribution
of pressure over the earth is intimately
associated with the distribution of
heat; and as the equatorial regions are
regions of high temperature, they are,

as a result, regions of low pressure. The air being excessively
heated, is pushed away from over these regions, leaving them
deficient in pressure. On either side, in the region of the Tropics,
the pressure is increased, thus giving a high pressure belt in
each hemisphere.

Poleward from the tropical high pressure belts the pressure, as

FIG. 43. BAROGRAPH

Bm^rej^fmreffrtrT^MHH^V

FIG. 44. BAROGRAPH AND THERMOGRAPH RECORDS FOR ONE WEEK

Note their Relation. As the barometer rose the thermometer fell, and as the barometer fell
the thermometer rose. Daily variation of temperature obscured by the variation due to
the passing high and low pressure areas.

PHYSIOGRAPHY

WEIGHT AND DENSITY OF THE AIR

98 PHYSIOGRAPHY

a rule, decreases; and the polar areas are thought to be relatively
low pressure areas.

As a result of this arrangement of pressure, the isobars of the
world have a general east-west trend, and shift with the shifting
belt of equatorial heat.

Uses of the Barometer. As before stated, the aneroid barom-
eter is used in the determination of altitudes, because of its con-
venience in carrying. Aviators and balloonists carry aneroidSj
this being often the only means by which the altitude reached by
them can be known, as they are obscured by the clouds from the
view of observers upon the land.

But a much more important use of the barometer is in fore-
casting the weather. Pressure is one of the factors which deter-
mine the weather; and in forecasting the weather a knowledge of
the distribution of pressure over the country is necessary.

QUESTIONS

1. In a closed vessel, filled with air, the pressure upon the inner sur-
face decreases with decrease of temperature, whereas in the open air
pressure increases with decrease of temperature; why is this?

2. What is meant when we say the pressure of the air is 30 inches?

3. Why does the air at any place vary in pressure?

4. How may the barometer be used to measure altitude?

5. Why must a liquid barometer be held in a vertical position when
read? Why is it not necessary to hold an aneroid in a definite position?

6. Why is it not necessary that the bore of the barometer tube be
regular as in the thermometer?

7. What is the general relation between barometer change and change
of thermometer?

8. Why is mercury so generally used in the construction of liquid
barometers? What is the objection to using water?

9. Why do standard barometers have a thermometer attached?
10. Why do the high pressure belts of the " horse " latitudes shift?

CHAPTER X
MOVEMENTS OF THE AIR

Winds Defined and Explained. The air, being part of the
earth, by necessity partakes of the earth's motions of rotation
and revolution. Entirely distinct from these motions are those
sometimes regular, but more often fitful and irregular movements
of the lower air, called winds.

A wind may be defined as an approximately horizontal natural
movement of the lower air. Winds should be sharply distinguished

Kr-

x ^**

^^" N Ox S'/'^*

\ /

S /N

(30-5) '->

tea) '

HEAT

''(30-5)

FIG. 47. SHOWING How WINDS ARE STARTED

Numbers below A B indicate barometer reading before heating; those above A B barometer
reading after heating.

from vertically moving air; likewise from the upper-air movements,
both of which are called currents.

The air is so mobile that any object moving through it sets a
considerable volume in motion; and the least inequality of pres-
sure disturbs its equilibrium. The most important cause of the
unequal distribution of pressure, and therefore of winds, is the
unequal distribution of heat over the earth.

100 PHYSIOGRAPHY

Figure 47 and its explanation are easily applicable to earth
conditions, and to the explanation of winds.

A B represents any surface, above which the air extends to the height
G H. Suppose a limited area, as C D, is heated in excess of the surround-
ing surface. The air above C D expands, and if we consider only the
expansion upward, the column of air above C D lengthens to K L.
Being unsupported laterally the air in the column above the level of G H
flows away until the upper surface of the air is again level at the height
E F. Some air above C D having flowed away, the pressure upon this
surface is less than before heatiny whereas the pressure upon A C and
B D is greater.

As a result of the rearrangement of pressures, A C and B D are
highs, away from which the lower air moves laterally; and since C D
is a low the movement is more pronounced toward this area. Above
C D the inflowing currents meet and rise, thus completing the circulation
The circulation continues just so long as C D is heated in excess of the
surrounding areas.

If we consider the separate steps in the development of the circulation
described they may be stated as;

1. Local excessive heating;

2. Expansion of the column of air above the heated area;

3. Overflow aloft from, and inflow at the bottom toward, the heated
area;

4. Ascent of the inflowing currents above the heated area, or low;

5. Descent of the outflowing currents.

It is only the surface movement of this circulation that is called
winds; all other parts are currents.

Terrestrial Winds. If we consider the foregoing figure a verti-
cal section of the air along a meridian at the equator, we have an
explanation of the systematic winds of the earth.

The area C D represents the doldrum belt along or near the
equator, which, by reason of vertical insolation, is most heated.
The movement of the air above this belt being chiefly upward, this
is a belt of light winds, or calms.

The poleward overflow aloft leaves this region a belt of low
pressure, and at the same time produces on either side, somewhere
between the latitudes of 25 and 35, a belt of high pressure.
Above these belts of high pressure the movement of the air is
chiefly downward, and these, like the doldrum belt, are also belts

MOVEMENTS OF THE AIR

IOI

of light winds or calms. They are known as the Horse Latitudes.
Out from these high pressure belts the winds blow toward the
equator, and with less pronounced strength toward the poles.

Excessive heating along a belt near the equator results from:
(i) The globular shape of the earth; (2) Rotation about an axis
which remains parallel to
itself; (3) The source of heat
being a body distant from
the earth.

Since most of the planets
agree in these three par-
ticulars, it follows that the
circulation described for the
earth must be common to all
planets with atmospheres.
On this account the winds
produced by this circulation
are sometimes called planet-
ary winds.

Deflection of Winds. It
was long ago discovered that
winds do not follow a straight
course; but in the northern
hemisphere turn to the right,
and in the southern hemi-
sphere to the left of such a
course. The statement of
this systematic deflection is known as Ferrel's Law, and all
winds obey this law. It governs alike the constant winds that
blow out from the belts of high pressure calms, and the irreg-
ular winds that blow about high and low pressure areas. Because
of this deflection winds do not follow the barometric gradient.

Deflection of winds from a straight course results from the rota-
tion of the earth.

In explaining the relation between deflection of the winds and rotation,
two facts are important:

FIG. 48. TERRESTRIAL WIND BELTS
Owing to the inclination of the earth's axis, and
revolution of earth around the sun, the wind
belts shift. The figure shows their extreme
northern and southern positions. Where they
overlap we have the monsoons of the sub-tropical
and sub-equatorial belts. Note the bending of
the trades where they run across the equator, to
be explained later.

102 PHYSIOGRAPHY

1. The rotational velocity of places upon the earth decreases poleward;

2. The inertia of matter makes it impossible for a particle once set in
motion, of itself, to change its direction or motion.

In I in the figure below, if a marble be started from the center along
the radius C A, with sufficient velocity to carry it to the edge of the table
in one second, if the table be at rest the marble will leave the table at A.
If, however, at the instant the marble is set in motion the table be set
rotating in a counter-clockwise direction at such rate as to change the

I II

FIG. 49. ILLUSTRATING THE DEFLECTION or THE WINDS FROM A STRAIGHT COURSE, DUE TO
THE Rotation OF THE EARTH

position of radius C A to that of C A' in one second, the marble will fol-
low the curved path, leaving the table at B. The marble's inertia has
made it take the curved path, deflected continuously to the right of a
straight course as it moved into regions of greater rotational velocity.

If instead, the marble be started from A along A C at the instant the
table is set rotating, in one second it will have traversed the path A' B'.
The inertia of the marble has again made it take a curved path, deflected
to the right of a straight course, as it moved into regions of less rotational
velocity.

On any straight line that may be drawn upon the table, along which
the marble may be started, the marble will either approach the center or
recede from it; and since in both cases the counter-clockwise direction
of rotation causes a right-handed deflection, the marble will in all cases
be deflected to the right of the straight course along which it is started.

The rotation of the earth, as seen from above the north pole, is counter-
clockwise; and winds in the northern hemisphere behave in an analogous
way to that of the marble described above in I. ,

As seen from above the south pole the earth's rotation is clockwise,
and deflection is to the left of a straight course, as shown in II.

MOVEMENTS OF THE AIR 103

The deflective effect of rotation may be illustrated by pouring water
on a rotating globe. If the globe be held with its axis vertical, and
rotated in a direction corresponding to that of the earth, the minute
streams will be deflected to the right of the meridians along which they
start so long as they are in the northern hemisphere, and to the left after
they cross the equator. If started from the south pole of the globe they
will suffer first left-handed deflection, changing to right-handed upon
crossing the equator into the northern hemisphere.

The only winds which do not suffer deflection are those along the
equator. Set moving in any other direction or in any other latitude
they suffer deflection according to Ferrel's Law. The deflective effect
of rotation increases with the latitude, and in any latitude varies in-
versely as the velocity of the wind. It should be remembered that the
rotation of the earth has no power to set the air in motion, and its deflective
power exists only after motion is started.

Two important laws governing winds are:

1. Winds always blow from a region of higher pressure to a region
of lower pressure, with a velocity which varies with the pressure
gradient;

2. On account of the rotation of the earth winds turn to the right of
the pressure gradient in the northern hemisphere, and to the left of
the pressure gradient in the southern hemisphere.

Description of Wind Belts. The wind belts depend upon the
pressure belts, and these in turn are determined by the distribu-
tion of heat. Since the heat belts shift, in like manner the pres-
sure and wind belts shift.

The Doldrums are so named because of the light winds and
calms that characterize this belt. It is a belt of high tempera-
ture, and consequently low pressure. The winds move obliquely
m toward the doldrums from both sides. The low pressure is of
convectional origin, and the rapid ascent and cooling of the air
give rise to frequent and abundant rains.

The Trades are the winds that blow from either side obliquely
in toward the doldrum belt. They derive their name from their
constancy of duration and direction, qualities important to sailing
vessels engaged in trade. In the northern hemisphere they are
called northeast trades, and in the southern, southeast trades. They
have their origin in the high pressure belts of the horse latitudes,

104 PHYSIOGRAPHY

and move toward the low pressure belt of the doldrums. Instead
of following the steepest gradients, along the meridians, as they
should do if there was no rotation, they are deflected, in accord-
ance with Ferrel's Law, to the right in the northern hemisphere,
and to the left in the southern.

In order to reach the doldrum belt the trade-winds sometimes
have to cross the equator. When northeast trades cross to the
south of the equator they become northwest winds; whereas south-
east trades crossing the equator become southwest winds. These
"hooked" winds are due to the difference in the deflective influence
of the earth's rotation north and south of the equator.

The Horse Latitudes are belts of high pressure next to and pole-
ward from the trades. The air that is warmed, and by convection
rises in the doldrums, moves away poleward at high altitudes.
Its overflow aloft, and consequent piling up in the regions of the
horse latitudes, causes the high pressure and descending currents in
these belts. As a result of its descent the air is warmed by com-
pression, and its capacity for water vapor is increased. These
belts are therefore prevailingly dry. The trade-winds begin
here; likewise the less regular winds that move away toward the
poles.

The Prevailing Westerlies flow away from the horse latitude
belts toward the poles, being deflected to the east by the rotation
of the earth. They are named from their direction, and in their
upper parts are continuations of the overflow above the trades.
The prevailing westerlies are neither so constant in duration nor
in direction as the trades.

As the prevailing westerlies approach the poles obliquely and
along converging courses, in each hemisphere there is developed a
circumsolar whirl. These winds spiral about the north polar re-
gion in a left-handed or counter-clockwise direction, and about
the south polar region in a right-handed or clockwise direction.

As a result of the spiral inflow toward the poles it is believed the polar
regions are areas of low pressure. The centrifugal force resulting from
the whirl tends to heap the air out around the polar center, and to pro-
duce a circumpolar ring of high pressure. From this ring the winds

MOVEMENTS OF THE AIR 105

move away into lower latitudes. The frequent northeast winds ob-
served on the northern coast of Alaska are thus explained.

Both temperature gradient and pressure gradient, between the
equator and the Arctic regions are steeper in winter than in summer.
As a consequence the circumpolar winds are strongest in winter.
This may account for the frequency and violence of our winter
cyclones.

Because of the excessive cooling of the northern continents in
winter, the North Atlantic and North Pacific oceans are warmer
than the northern continents and are therefore centers of low
pressure. About these centers the winds spiral in a way com-
parable to the circumpolar whirl; and from these secondary centers
winter cyclones are projected. Those from the Pacific often move
southeastward into Canada and the United States.

Shifting of the Wind Belts. The pressure belts and wind belts
follow the shifting of the belt of greatest heat. As a result of this
shifting, places near the border of the various wind belts lie alter-
nately in two belts. Southern Florida, southern California and
northern Mexico lie alternately in the trades and the horse lati-
tudes; and the Amazon Valley, usually in the doldrums, is peri-
odically swept over by the trades.

This shifting of the wind belts has the effect of widening the
belts which at some time during the year lie under the low pressure
and the high pressure calms. To these widened belts the names
sub-equatorial and sub-tropical are respectively given.

Places so situated as to have seasonal change of wind are said
to have monsoon winds, or simply monsoons. While many places,
both in the sub-tropical and sub-equatorial belts have monsoon
winds, perhaps the most pronounced and best known monsoons
are those of the northern Indian Ocean, and the adjacent lands to
the north and east.

During the northern summer the heat equator migrates far into
the heated continent of Asia. Then the southeast trades, which
run to the doldrum belt, cross the geographical equator, and in the
northern hemisphere become, according to FerrePs Law, south-
west winds. In winter the heat equator migrates southward, and

io6

PHYSIOGRAPHY

over southern Asia and the northern Indian Ocean the strength-
ened northeast trades blow.

Continental Air Drifts. The land has a higher temperature
than the sea in the same latitude in summer, and a lower tem-
perature in winter. As a result the continents are areas of low

FIG. 50. WINDS OF NORTHERN INDIAN OCEAN FOR JULY

pressure in summer, and of high pressure in winter. Following
this adjustment of pressure there is a general outward drift of
the lower air from the continents in winter, and an inward drift
toward the continents in summer.

These movements, which in a strict sense are monsoons, are
never so called, but are sometimes called continental winds. They
are so easily obscured by other winds as to be scarcely noticeable
of themselves; and their chief office is to modify other winds.
Thus upon our western coast the westerlies are weakened in winter
and strengthened in summer; whereas upon our eastern coast they
are strengthened in winter and weakened in summer.

MOVEMENTS OF THE AIR

I0 7

Land and Sea Breezes. The winds that blow alternately from
and to the land, at and near the seashore, are called land and sea
breezes. When the land is colder than the sea the breeze blows

FIG. 51. WINDS or NORTHERN INDIAN OCEAN FOR JANUARY

from the land and is called the land breeze; and when the land is
warmer than the sea the breeze blows from the sea, and is called
sea breeze. The land breeze blows during the night, as the land
is then an area of relatively high pressure; and the sea breeze
blows during the day, the land being then a region of relatively
low pressure.

Advantage is taken of this twice daily change of breeze by fish-
ing and pleasure craft that depend upon sails to carry their fleets
away from and to bring them back to land. Similar land and lake
breezes are felt along the shores of our great lakes and inland seas.

If the land should remain throughout the twenty-four hours colder
than the sea, then there would be a continuous land breeze. This often
occurs in winter, especially when the land is covered with snow. On the

Io8 PHYSIOGRAPHY

other hand, in mid-summer it sometimes happens that the land does not
cool down during the night below the temperature of the adjacent sea;
then the sea breeze continues throughout the night.

Mountain and Valley Breezes. Another similar change of breeze
may be noted upon the slopes of mountains or of deep valleys. The top
of the mountain, or of the dividing ridge, being first warmed by the rays
of the rising sun, convectional ascent begins there. As the slopes and
the air in contact with them are warmed to lower and lower levels, the
warmed lighter air finds an easier escape obliquely up the slope and
through the convectional chimney above the summit than vertically
through the overlying stratum of inert air. This obliquely ascending
current is felt upon the slope as a valley breeze.

At night radiation is most rapid from the mountain summit or ridge
crest, and the cooled air from the upper slopes, being heavier, moves
down the slopes toward the valley. This is the mountain breeze. The
alternation of mountain and valley breezes may be interrupted in the
same way as described for land and sea breezes.

Avalanche winds occur in valleys between steep mountain slopes, and
are caused by the moving mass of snow and rock on the mountain side.
Being compressed in the narrow valleys, these winds sweep with almost
irresistible force down the valley.

Cyclonic Winds. In temperate latitudes by far the most im-
portant winds are those unperiodic winds, caused by the irregular
distribution of pressure in lows and highs, and known as cyclonic
winds. These lows and highs are best understood if considered as
local disturbances in the terrestrial winds. Thus considered, their
general drift with the winds of the belt in which they occur is
accounted for.

Cyclonic winds move obliquely in toward the center of a low,
spiraling about the center in a counter-clockwise direction in the
northern hemisphere, and in a clockwise direction in the southern
hemisphere. They move spirally out from a high, turning in a
clockwise direction in the northern hemisphere and in a counter-
clockwise direction in the southern.

Although their origin is obscurely understood, some lows are known
to be of convectional origin; others are probably not. This has given
rise to two classes of cyclones, convectional and non-convectional.

Land varies indefinitely in its power to absorb insolation, owing to its
almost infinite variety of composition, covering, and form. If any given

MOVEMENTS OF THE AIR 109

limited area is heated in excess of the surrounding region, the air above
this area expands, overflows aloft, and the area becomes a low, while the
surrounding region has increased pressure. The overflow from adjacent
lows may unite and produce a high. On the other hand, the air over a.
given area, as a snow-covered region, may be excessively cooled and con-
densed. Then the upper air from the surrounding region flows in upon
it, producing a high, surrounded by a ring of lower pressure. Such lows
and highs are of convectional origin.

It seems probable that non-convectional cyclones originate in two
ways. In high latitudes, about the margins of the circumpolar whirls,
and in the northern hemisphere about the margins of the North Atlantic
and North Pacific eddies, smaller eddies of air are developed; analogous
to the eddies that arise about the perimeter of a strong water whirl.
These are sometimes called driven cyclones, and are most frequent and
best developed in winter when these great eddies are strongest. In
lower latitudes, even within the tropics, cyclonic whirls may result from
the friction between great poleward moving and equatorward moving
masses of air. Such cyclones are called frictional cyclones. They may
originate in the higher currents, and sink to the bottom of the air fully
developed.

Movements of Lows. Four distinct movements with reference
to lows must be noted: the oblique movement of the winds toward
the low center, their ascent as the center is approached, spiral out-
flow above, and the forward movement of the low itself.

Wherever their place of origin, most lows finish their course in
the zone of westerlies, following the general direction of these
winds. In the United States three general paths are followed, as
the cyclone originates in the northwest, in the southwest, or in the
southeast within the tropics. Those originating in the northwest
move first southeasterly, until they reach the axis of the Missis-
sippi Valley, when they change to a northeasterly course for the
remainder of their journey across the continent. Those having
their origin in the southwest move systematically northeastward
across the continent. Tropical cyclones having their origin in the
region of the West Indies move first northwestward, until they
reach the high pressure calms, north of which they conform to
the course of the westerlies. These usually cross the high pressure
belt over or near the land, the belt being less well developed
there.

HO PHYSIOGRAPHY

All cyclones, sooner or later, conform to the course of the west-
erlies. The upper air currents in all latitudes move to the north-
east; and it is probable that the spirally ascending column of air
'at the cyclone center reaches to these upper currents.

Strength of Cyclonic Winds. The velocity of the wind increases
as it approaches the center of a low; and if a strong spiral move-
ment is developed the velocity is greatly increased. On the other
hand, the winds start from the center of the high, and are therefore
weakest there. Consequently, the strength of the wind increases
with the approach of a low, and decreases with the approach of a
high.

Because of their greater strength, cyclonic winds obscure all
other winds in regions where they occur. The winds poleward
from the horse latitudes are chiefly cyclonic.

When the low pressure area is very small the spiraling winds
may attain excessive velocity, and become destructive. Such
wind storms are known as tornadoes, and in their greatest strength
the winds are so strong as to carry away the instruments for their
measurement. Velocities of more than one hundred miles an hour
have been measured. Similar storms in the West Indies are called
hurricanes, and in the East Indies typhoons.

Cyclones within the tropics, of wide extent, sometimes have
within the whirl of destructive winds an area of clear skies. This
area, called the " eye of the storm," may be as much as one- tenth
of the diameter of the cyclonic area. Vessels passing through the
eye of the storm experience equally strong winds in the front and
in the rear of the cyclone, though from opposite directions.

Shifting of Winds. When a station lies near the path of a low
or high the winds at the station shift in a systematic way as the
barometric disturbance passes. In the westerlies of the northern
hemisphere, where the disturbances advance from west to east:

i. If a low passes north of the station, the first effect is to induce
easterly winds. These veer (change with the sun) through the
southeast, south, and southwest to some westerly quarter as the
disturbance advances easterly.

MOVEMENTS OF THE AIR

III

2. If a low passes south of the station, the winds, northeasterly at
first, back (change against the sun) through the north, northwest
and west to a southwesterly direction.

FIG. 52. A B is the path of the center of a low which passed north of the station S. The
successive winds are numbered in order. The curved arrow X Y is the composite of the
sheaf of arrows passing through S toward the center of the low.

3. If a high passes north of the station, the winds, at first westerly
or northwesterly, veer to northerly and northeasterly winds in
succession.

FlG. 53. The path of the center, A B, is here south of the station S. As above, the curved
arrow X Y is the composite of the sheaf of arrows through S.

112

PHYSIOGRAPHY

4. If a high passes south of the station, the wind backs from the
southwest through the south, southeast, and east in turn.

FIG. 54. In Fig. 54 a high passes north of the station S, and the succession of winds,
blowing out from the center of the high, is shown by the sheaf of arrows through S. The
curved arrow X Y shows their composite.

FlG. 55 The path of the center of the high is south of S, and the sheaf of arrows and their
composite illustrate the backing of the wind.

MOVEMENTS OF THE AIR 113

Should the disturbance pass centrally over the station the wind
will hold steadily as a northeasterly wind if the disturbance is a
low, or as a westerly wind if the disturbance is a high, as the dis-
turbance advances, changing suddenly through a calm to the
opposite point of the compass as the center of the disturbance
passes.

In any other wind belt the direction of shifting may be determined by
noting the direction in which the disturbance advances, and considering
the direction of movement of winds about highs and lows in that belt.
For any given station the direction of shifting is systematic and uniform
for a given set of conditions.

Special Winds. In every part of the world winds of special
character and of exceptional occurrence are known, to which local
names are given. Some of these are warm winds, others are cold;
some owe their character to change in latitude, others owe theirs
to a change in altitude.

The change in temperature due to change in latitude is, on an
average, about i Fahrenheit for each degree of latitude, whereas
the temperature change in ascending currents of air is about 1.8 F.
for 300 feet. Therefore, any transfer of air, either in latitude or
altitude, is accompanied by a change in temperature, both of the
transferred air and of the region to which it is transferred.

Generally speaking, winds blowing from lower to higher lati-
tudes are warm winds, while those blowing from higher to lower
latitudes are cool. Winds descending from higher to lower alti-
tudes are generally cool or cold winds, though their temperature
rapidly rises as they descend and makes them warm, while those
blowing up a slope are apt to be warm.

Among warm winds may be mentioned:

The Hot Wave, of western central United States. It blows in
summer from the west or southwest, sometimes continuing for
days, and withers all growing vegetation.

The Sirocco, a south wind from the Sahara desert, felt as far as the
north shore of the Mediterranean Sea. It is commonly dry and dust
laden.

The Simoom, an intensely hot, dry, and generally sand-laden wind of

H4 PHYSIOGRAPHY

the Arabian desert. It is probably a convectional whirlwind, similar to
the dust-laden whirlwinds of all dry, hot climates. It lasts usually less
than ten minutes, and often forms sand-spouts.

The Chinook, an American wind, which moves down the slopes
of mountains toward a low pressure area at their base. Though
starting upon its descent as a cold wind, it warms by compression
in its descent, and if the mountain is high it may reach the base
as a warm or even hot wind. In all cases, because of its dryness, it
evaporates or melts the snow fields over which it blows, and often
causes destructive avalanches, by melting the snow on the steep
slopes. It is of frequent occurrence along the eastern bases of the
Rocky and Sierra Nevada mountains. Many valleys here are
kept practically free from snow; and their temperatures are so
mild as to make shelter for stock in winter unnecessary, and to
permit grazing throughout the year. The Chinook is scarcely
noticeable in summer.

The Foehn is the European wind of the Chinook type, common on
the northern slopes of the Alps, where, in the north-south valleys it
hastens the ripening of grapes in the fall; and in winter rapidly melts
the snows in its path. This has earned for it the name of "snow-eater."

To the class of cold winds belong:

The Norther, of southwestern United States. It is the cold
inflow of winds from the north, at the rear of the winter cyclone.

A fall of temperature of fifty degrees
in two hours has been noted.

These winds often cause great suf-
fering, loss of live stock, and even loss
of human life.

The Blizzard, the American name
for a cold wind of high velocity, ac-
companied by snow. It is common
east of the Rocky Mountains, and
often causes great loss of life among
stock.

The Bora and Mistral are European

FIG. 56. ANEMOMETER cold winds.

MOVEMENTS OF THE AIR 115

Velocity of Winds. Winds are sometimes classified according
to their velocity. The velocity of the wind is measured by means
of the anemometer, and is expressed in miles per hour, or feet per
second.

When the wind reaches a velocity of 100 miles or more per hour
the instruments are usually carried away. Therefore, we have no
record of the velocity of the wind in our most violent tornadoes.

The accompanying table gives the ordinary names for winds,
their approximate velocities in miles per hour, and a common and
easy way of judging them:

Name Distinctive Characters Velocity

Calm Flags limp, leaves unmoved o

Light breeze .... Moves leaves of trees 1-5

Fresh wind Moves branches of trees, blows up dust 5-15

Brisk to strong. .Sways branches of trees, makes white caps 15-25

High wind Sways trees, moves twigs on ground 25-35

Gale Breaks branches of trees, dangerous for sailing.. 3 5-7 5

Hurricane wind.. Destroys houses, uproots trees 75-100

QUESTIONS

1. Why does the wind blow? Why is there not always wind?

2. What determines the direction and velocity of the wind?

3. Interpret "no wind" in terms of pressure gradient.

4. Why are summer days, as a rule, more apt to be windy than sum-
mer nights? Than winter days?

5. Why are the " trades," on the whole, stronger than the " wes-
terlies "?

6. Why are upper currents stronger than surface winds; and winter
winds stronger than summer?

7. Why are the " trades " more regular over the sea than over the
land?

8. Why are the upper currents in all latitudes westerly currents?

9. Why do winds spiral about " lows "? Why are not equally strong
whirls developed about "highs"?

10. Why are southern California and southern Florida more apt to
have westerly winds in winter than in summer?

n. Why are storms that come from the southwest often called "north-
easters"?

12. Why do cyclones in the eastern United States move so generally
toward the northeast?

Il6 PHYSIOGRAPHY

13. Why do thunderstorms occur so rarely at night?

14. What should be the direction of shifting of the wind with passing
"lows" and "highs" at Havana, Cuba? At Buenos Aires?

15. Why, in the natural ventilation of our rooms, do we admit the air
at the bottom of the room, whereas in forced ventilation the air is ad-
mitted at the top? Which is better?

1 6. Why is a room better ventilated when windows are both raised
from the bottom and lowered from the top than when merely raised or
lowered?

17. Why do fires burn better in cold than in warm weather?

18. Why is a flue less liable to smoke after the fire is well started than
when it is first lighted?

19. Why do snow-drifts and sand-dunes accumulate behind an ob-
stacle rather than in front of it?

20. Why do aviators avoid flights over cities and deep canyons?

CHAPTER XI
MOISTURE OF THE AIR

How Obtained. The moisture of the air is obtained by evapora-
tion from moist surfaces. Evaporation is the process by which
solids and liquids are changed to vapor. While evaporation is chiefly
from water surfaces, yet scarcely any land surface is so dry that it
does not supply some moisture to the air. The lower air is never
wholly free from moisture, the amount present depending chiefly
upon the temperature of the air. Evaporation takes place at all
temperatures. The moisture of the air is for the most part in its
invisible form, that of water vapor; but it becomes visible when,
by cooling, the vapor changes to the liquid or solid state as clouds.
Real steam issuing from a boiling kettle is invisible. So-called
steam, the visible cloud seen a few inches away from the spout,
is composed of minute particles of water, and not water vapor.

Humidity of the Air. The air at all temperatures has a certain
capacity for water vapor. Other conditions remaining the same,
the higher the temperature the greater the capacity. To satisfy
this " thirst " of the air evaporation goes on. If the temperature
is low, or the capacity of the air is nearly satisfied, evaporation is
slow; and when the capacity is fully satisfied, evaporation ceases.
The air is then said to be saturated.

The condition of the air as regards the amount of water vapor
present is called its humidity. The actual amount of water va-
por in a given volume of air, as the number of grains of water
vapor in a cubic foot of air, is its absolute humidity. The amount
of water vapor present, divided by the amount the air is capable of
holding at the time, is its relative humidity. Relative humidity is
expressed in per cents.

Ii8 PHYSIOGRAPHY

If the air at a given temperature has six grains of water to the
cubic foot of air present, and its capacity is ten grains to the cubic
foot, its absolute humidity is " six grains per cubic foot," and its
relative humidity is 60%. When the air is saturated its rela-
tive humidity is 100%. The temperature of saturation is called
the dew point.

Condensation. As evaporation is promoted by increase of tem-
perature, so it is retarded by any lowering of temperature; and
when the air is saturated evaporation stops, or if it continues con-
densation goes on at an equal rate.

Saturation may result from continued evaporation without
change of temperature, or from a lowering of the temperature;
and when the air is saturated, any lowering of the temperature
results in a change from the vapor to the liquid or solid state,
depending upon the temperature at which the change takes place.
This change is condensation. It is condensation, not of the air, but
of the water vapor of the air.

The cooling which causes condensation may result from:

1. Loss of heat by radiation, chiefly to the land or water from
the lower air, when fogs are apt to result;

2. Contact with cold surfaces, when dew or frost may form;

3. Horizontal mixing of cold and warm currents, when clouds or
precipitation may occur;

4. Mechanical cooling by expansion, incident to ascending cur-
rents, producing clouds or yielding precipitation.

The last is probably the most effective cause of condensation.
Air in rising expands and is mechanically cooled, at the rate of
1.8 F. for every 300 feet of ascent. This rate is the same whether
the air rises by convection, or is forced to ascend by reason of
winds blowing against a rising land surface.

When air descends, as at the center of a high, or on the leeward
slope of mountains, it warms up at the same rate.

The doldrum belt, the regions of lows, and the windward slopes
of mountains are, therefore, apt to be rainy, while the high pressure
calm belts, the regions of highs, and the leeward slopes of mountains
have prevailingly clear skies.

MOISTURE OF THE AIR 119

If saturation is reached at a temperature lower than freezing,
with further cooling the water vapor changes at once to the solid
state, without passing through the liquid state; but if saturation
occurs above the freezing point, the condensed product is liquid.
As water vapor may change immediately to the solid state, so ice
may evaporate without melting.

It is a familiar fact that snow gradually disappears from the
land during days when the temperature does not rise to the freez-
ing point; and a wet garment hung in the air may freeze dry.
Roads in winter become dusty though continuously frozen.

Effects Upon Temperature. In order to change ice or water to
vapor, heat is absorbed, or becomes latent. This heat is used in
the mechanical task of driving the molecules of ice or water apart,
and is lost, as far as affecting temperature is concerned. The
vapor formed is of the same temperature as the substance from
which it came. The heat necessary for evaporation is obtained
from surrounding objects, chiefly from the lower air, so that
evaporation has the effect of cooling the air. Evaporation is always
a cooling process with respect to surrounding objects.

The custom of fanning to cool one's self is an illustration of
cooling by evaporation, the constantly changing air in contact
with the moist skin taking up the moisture more rapidly.

A thermometer suspended in front of a rapidly rotating fan registers
a slightly increased temperature, there being no evaporation from its
surface, and the air being slightly compressed against the thermometer.
If, however, the thermometer bulb be wrapped in a thin piece of moist
muslin, the draft from the fan causes a marked lowering of the ther-
mometer.

Gasolene, bay rum, or any easily vaporized liquid, when rubbed upon
the hands cools them; and the high fever temperatures may be cooled
by alcohol baths. The flesh may be frozen by the application of ether.

The principle of cooling by evaporation is used in the construc-
tion of the psychrometer, an instrument for the determination of
the humidity of the air. It consists simply of a wet and a dry
bulb thermometer. By keeping the bulb of one thermometer con-
stantly moist, evaporation from the moist surface cools the mer-

120

PHYSIOGRAPHY

cury in the enclosed bulb. The wet bulb thermometer, therefore,
commonly shows a lower temperature than the dry bulb. The
more rapid the evaporation the greater the difference in the read-
ings of the two thermometers, and the drier the air. Little differ-
ence in the readings indicates slow evaporation and a humid air.

When evaporation ceases, as when the
air is saturated, the two thermometers
show the same readings.

When the reverse process takes place,
and vapor is changed to liquid or solid,
the latent 'heat of evaporation is released;
and the heat thus liberated becomes
available for affecting temperature.
Surrounding objects are then warmed.
Condensation of water vapor is, there-
fore, a warming process.

A burn from steam at 2 1 2 F. is much
more severe than a burn from water at
the same temperature, since a great
amount of heat is liberated in reducing
steam to a liquid without changing its
temperature.

Distribution of Water Vapor. Since
most evaporation takes place at the bot-
tom of the air, it follows that the lower
air is of higher absolute humidity than

the air at greater altitudes. Also the air over the oceans and
other water surfaces is more humid than the air over the land.
But by diffusion and by vertical currents the water vapor is
distributed generally through the lower air to a height of about
seven miles.

Although there is more water vapor in the lower air, the relative
humidity of the lower air is not usually so high as that at greater
altitudes, because of the lowering of temperature with increase of
altitude. Clouds and rain are usually the result of condensation in
the upper air, since the air cooJs in rising, due to expansion. This

FlG. 57. PSYCHROMETER

MOISTURE OF THE AIR 1 21

is perhaps the most important, though not the only cause of con-
densation of water vapor.

The relative humidity of the air varies, not only with change
of place or of altitude, but it also varies from hour to hour
at any place. It is highest at the coolest hour of the day,
usually in the early morning, and decreases as the day ad-
vances and the air warms up. It is lowest at the warmest hour
of the day, from one to three o'clock P. M., after which hour it
slowly rises.

Dew and Frost. When air at temperatures above the dew-
point is cooled to saturation by contact with objects below the
dew-point, condensation upon the cooling object occurs. If satu-
ration occurs above freezing, the condensation is liquid, and we call
it dew. If saturation occurs below freezing, the vapor changes at
once to ice, without visible liquefaction, and this is called frost.
Dew and frost form; they do not fall. Frost is not frozen dew, frozen
dew being transparent pellets of ice.

These forms of condensation are most common on or near the
surface of the ground. At a height of five feet there may be no
frost or dew formed on plants when the ground and objects near
it, as the grass, are white with frost or wet with dew.

Anything that checks the cooling of the ground and lower air
tends to prevent the formation of dew and frost. The ground
beneath trees and shrubs is often protected from dew and frost
when these form freely upon the unprotected ground. A cloudy
sky, by checking radiation, prevents excessive cooling and hinders
the formation of dew or frost. Likewise wind, by constantly
changing the layer of air next to cold surfaces, hinders cooling and
condensation. It is rare to have frost or dew on cloudy or windy
nights.

It is a commonly recognized fact among dwellers in the country
that clearing skies, and the " laying " of the wind toward night-
fall, may bring frost at those seasons when early planted or late
maturing crops would be injured by it.

Many orchardists protect their trees from frost by building
smudges among the trees. The smoke cloud thus produced,

122 PHYSIOGRAPHY

hanging above the orchard, checks radiation as a blanket would,
and thus often prevents frost.

When condensation begins, the liberation of latent heat tends
to check further cooling; so if saturation occurs much above
freezing, frost is unlikely because freezing temperatures are not
apt to be reached.

Housewives often protect their flowers from frost, on cold
nights, by exposing shallow pans of water in the warm room with

FIG. 58. CIRRUS CLOUD

the plants, and thus greatly increasing the humidity. When the
room cools saturation will be reached at a temperature well above
freezing; and the liberated latent heat, as condensation goes on,
may suffice to keep the temperature above freezing. In a few
cases this principle has been applied on a large scale for the pro-
tection of orchards.

Clouds. When the vapor in the air is condensed above the sur-
face of the ground, into particles of water or ice so small that they
remain suspended in the air, the product of condensation is called
cloud.

MOISTURE OF THE AIR 123

If the cloud is so low that it reaches the land or water it is called
fog-

Clouds are classified chiefly by their form. The thin, feathery,
white clouds, seen high in the air, and frequently on fair days, are
cirrus clouds. These filmy clouds are, for the most part, com-
posed of ice spicules, and are the highest clouds. Their average
summer altitude is about 6 miles; and they generally move east-

FIG. 59. CUMULUS CLOUD

ward at the rate of about 60 miles an hour. In winter their average
height is about 5 miles, and their eastward motion is about 100
miles an hour.

Cirrus clouds are often precursors of storms, being the high-
level overflow in front of the cyclone center.

Cumulus clouds are the massive piles of cloud, with an even
base, and resembling piled-up fleeces of wool, or volumes of con-
densed steam and smoke from a locomotive. They are the result
of rising currents of air, usually of convectional origin, and are,
therefore, storm clouds. They are sometimes called thunder-heads,

124 PHYSIOGRAPHY

and are land clouds rather than sea clouds, day rather than night
clouds, and are more commonly in motion than at rest.

The average summer height of cumulus clouds exceeds a mile,
whereas their winter height is somewhat less; and their summer
and winter velocities are about 6 and 9 miles an hour respectively.
Their even bases are usually from one-fourth to one-half mile high.

Nimbus is the name given to any cloud from which rain or snow
falls or may be expected to fall. These clouds are of very variable
height, being, on the whole, higher in summer than in winter.
Their height decreases toward the poles.

Nimbus clouds are of an even grayish tint, sometimes completely
overcasting the skies for hours and even for days continuously.

Stratus is any low-lying cloud spread out in parallel sheets or
bands. It is a night rather than a day cloud, and more common
over the sea and in valleys than over the higher lands. It includes
fogs. The bands of clouds seen at higher levels may be designated
by such compound names as cirro-stratus and cumulo-stratus.

Fog is the cloudy condensation near, or resting on, the land or
sea. It usually results from warm, humid air passing over cold
surfaces. In winter, winds blowing from the sea upon land are
apt to produce fogs. An east wind upon eastern coasts, and a
west wind upon western coasts are the chief sources of fogs.

Fogs are more frequent in valleys than on the slopes or tops
of hills and mountains. The cooler, heavier air accumulates in
the valley, where there is likewise more moisture; and these two
conditions combine to produce the fogs in valleys and lowlands.

The fogs of Newfoundland are known to all navigators of the
North Atlantic. The warm and cold ocean currents or drifts
which meet there are accountable for the fogs. When the winds
are from an easterly quarter their temperatures are lowered and
their moisture condensed as they pass over the cold Labrador cur-
rent to the west. Another cause of much o the Newfoundland
fog is the ice brought down from the Arctic regions by the Labra-
dor current. These icebergs are apt to be centers of dense fogs,
especially in summer.

The fogginess of great cities, as London, and particularly of

MOISTURE OF THE AIR 125

manufacturing cities, is greatly increased by reason of the particles
of soot issuing from chimneys. Each soot particle is a center of
cooling and condensation.

Rain and Snow. When condensation takes place in the air, and
the condensed particles become heavy enough to sink through the
air, the process is called precipitation. If saturation occurs above

Fig. 60. VALLEY FOG

freezing, the product is rain; if below freezing, snow. As in the
production of frost, so in snow, the water vapor passes at once
from the vapor condition to the solid. Rain and snow bear the
same relation to each other as do dew and frost. Snow is not
frozen rain.

Although clouds almost invariably precede rain and snow, precipitation
without clouds may occur. Such cloudless rain, usually in very fine
drops, and more resembling mist, is called serein.

Snowflakes are crystallized water vapor, and are built up on patterns
resembling beautiful six-rayed stars. The size of the flake as well as
the exact pattern seems to depend upon the temperature at which the
flake is formed.

In all latitudes an altitude may be reached at and above which

126 PHYSIOGRAPHY

the precipitation, even in summer, is chiefly in the form of snow.
At a slightly lower level more snow falls than melts during the
year, with the result that there is perennial snow. The lower
limit of perennial snow is known as the snow line. It is about
16,000 feet above the sea at the equator, and descends gradually
toward the poles, reaching sea level within the polar circles.

Sleet and Hail. If raindrops pass through increasingly colder
air in their fall, and are frozen into small pellets of ice, the product
is sleet. As this arrangement of temperature is most apt to occur
in winter, when the land is colder than the lower air above it, sleet
is a winter phenomenon. Sleet is frozen rain.

In summer, especially on hot afternoons, and near the center of
a cyclonic storm, large pellets of ice called hail often fall. Upon
examination hailstones prove to be made of concentric layers of
ice. This structure, together with their often great size, suggests
that they are frozen raindrops enlarged by successive condensa-
tions and freezings upon their surfaces. Their often spongy tex-
ture indicates that snow may also^ enter into their composition.
Hail is chiefly a summer phenomenon.

Hail storms are sometimes very destructive. Their paths are
usually only a few miles in width, and fortunately not of great
length; for often growing crops, orchards, and even forests are
destroyed. Leaves, bark, and branches are stripped from trees;
young animals are killed, and windows and roofs broken by the
hailstones.

It has been suggested that hail is rain frozen by being carried in strong
ascending currents to higher, freezing altitudes. The occurrence of
hail near the storm center, where the convectional ascent is strongest,
supports this claim.

It is also suggested that the meeting of cold and warm masses of air
may result in horizontal stratification, warm and freezing layers alternat-
ing; and that raindrops formed near the top of such a stratified cloud
are frozen and enlarged in passing through it. The addition of snow-
flakes, formed in the colder layers of the cloud, would explain both their
sponginess and size. Hailstones more than nine inches around have
been reported.

MOISTURE OF THE AIR

127

128 PHYSIOGRAPHY

Sheet-Ice. Sometimes, in winter, rain falls, but immediately
upon touching the ground or trees it is changed into ice. This
occurs when the lower air is just above the freezing point, while
the ground and all solid objects near it, being better radiators,
have cooled to a temperature below freezing. This is called
sheet-ice.

During such ice storms ice encases the twigs and boughs of
trees and shrubs; and sometimes the weight of ice is sufficient
to break the branches. Such storms are especially destructive if
strong winds occur before the ice disappears. Telephone and
telegraph lines are often broken down.

QUESTIONS

1. Precipitation increases for a time, then decreases with increase of
altitude upon the slopes of most mountains; give reasons.

2. Why is it that if we determine the humidity of the air when it is
raining we rarely find the air saturated?

3. The relative humidity decreases, usually, as the day advances,
until i or 2 o'clock; explain. Does the absolute humidity decrease in the
same way?

4. Why is the relative humidity of the air 10 feet above the ground
usually higher at night and lower during the day than that of the air 100
feet above the ground?

5. Why is the absolute humidity greater over, and near, the sea than
inland?

6. Why does one become so much more quickly chilled in a wet gar-
ment than in a dry one?

7. Why, in winter, does frost form on the window panes of an occupied
room, but does not, if the chamber is unoccupied? Why will thorough
ventilation of the occupied room prevent such formation?

8. Why do fogs form over lakes before they form over the surrounding
lands? How are such fogs dissipated as the day advances?

9. Rain may often be seen falling from clouds, yet not reaching the
ground; what becomes of it?

10. Why is there no distinctly wet or dry coast under the equator?

CHAPTER XII
LIGHT AND ELECTRICITY OF THE AIR

Introduction. We see the sun at rising as a golden sphere, at
mid-day a globe of dazzling white, and at setting, if the air is
dusty, it may disappear below the horizon as a ball of fiery red.
As we ascend the mountain slope the noon-day sun takes on a
bluish tinge; and we are told that balloonists, in their highest
ascents, see a distinctly blue sun.

The ever-changing color of the sun, as it mounts toward the
zenith, or shines through clear or cloudy air, is the result of the
irregular reflection of the light. White light is composed of many
colors, each having its distinct length of ether wave, and because
of these different wave lengths white light may be separated into
its component colors. The shortest waves are blue, and like rip-
ples upon water are easily turned aside by obstacles in their path;
the longest visible rays are red, and these, like the great waves of
the sea, pass by obstacles which send back shorter waves. Hence
it follows that an object may appear one color by reflected light,
and quite a different color by transmitted light.

A glass of soapy water, viewed from above, looks bluish white,
but when the sun is seen through the water its color is red or red-
dish yellow. The short blue waves are turned back, whereas the
longer reds pass by the minute solid particles in the water.

Color of the Sky. The dust and cloud particles in the air reflect
the shorter waves, and transmit the longer; and the freer the air
from dust and cloud, the more completely do light waves of all
lengths pass through it. To an observer looking toward the sun
the irregularly reflected or diffused light is lost, and only the longer
waves reach the eye. If the shortest blues alone are scattered, as
when the air is moderately clear, the combination of the remaining

I 3 o PHYSIOGRAPHY

colors gives the sun a yellowish tinge; but if the light passes through
a very dusty air, or through a considerable thickness of cloud, the
sun takes on a distinctly reddish tinge.

If the eye is turned away from the sun the diffused light is
received; and as the blue rays are in excess in diffused light, the
sky appears blue. The less the admixture of other colors with the
blue the deeper the blue; and for this reason the sky, as seen from
a balloon or mountain top, above the dust-laden stratum of air,
appears intensely blue.

If the air is very dusty many other colors are diffused with the
blue, and the sky assumes a whitish glare. The sky is bluer at
sea, and after a rain, because the air is freer from dust. It is
believed that if we were to rise above all the dust and cloud par-
ticles the sky would have the blackness of night; and the stars
would shine as brilliantly by day as by night.

Refraction in the Air. As a ray of light is bent in passing
obliquely from one medium to another of different density, so all
the sun's rays are bent in passing through the atmosphere, except-
ing only the vertical ray. This bending, called refraction, increases
with increasing obliquity of the ray, being greatest when the sun
is on? the horizon, and least when on the meridian of a place. At
sunrise and sunset it is sufficient to displace the sun the width of
its disc; and as its effect is always to increase the altitude of the
sun, the entire disc of the sun appears to be above the horizon
when in reality below it.

The effect of refraction is to increase the length of the day in
all latitudes. While this increase amounts to only a few minutes
at the equator, at the poles it amounts to about four days. At
the time of the equinoxes the sun is wholly above the horizon at
both poles. The day at each pole is thus lengthened by about
four days, the polar night being shortened by this amount. An-
other effect of refraction is to give the sun, at rising, an elliptical
appearance, flattened vertically.

Looming. Normally the air is densest at the bottom, becoming rarer
with increase of altitude. Sometimes the lower air, for a thickness of a
few feet, is abnormally cooled and denser than the air ten or twenty feet

LIGHT AND ELECTRICITY OF THE AIR 131

higher. This is apt to occur in the early morning in summer, particularly
over the land, owing to the rapid radiation of the land at night, and the
cooling of the air near it.

Rays of light coming from an object that rises above the cooled stratum
of air to an observer within it are bent downward; and the object is seen
as occupying a position higher than is real. Sometimes objects appear
suspended in air, in their normal upright position; but more often they
are merely elongated upward. This is looming.

FIG. 62. LOOMING

In Fig. 62 above the surface A B the air to the height of C D is
abnormally cooled; coldest at the bottom, and becoming warmer and
rarer above C D. From the object X Z which rises above C D rays
come to the observer at E, within the denser layer of air. These rays are
bent downward upon entering the denser layer, making the part of the
object above C D appear to occupy a higher position. If the object is
distant, only the upper part is seen; but as the object is approached it
descends, until finally seen as an elongation of X Z.

Looming is an early morning or winter phenomenon, and ships at sea
are often discovered while yet below the observer's horizon.

Total Reflection. When a ray of light passes obliquely from a denser
to a rarer medium, the ray is bent away from a perpendicular to the sur-
face of the media at the point of passage. There is an angle of obliquity,
varying with the densities, beyond which the ray will not pass from the
denser to the rarer medium, but will be totally reflected from this surface
back into the denser medium. This angle is called the critical angle,
and this phenomenon is known as total reflection.

Mirage. The interesting phenomenon of mirage depends upon the
principle of total reflection in the air. Often in deserts or upon arid
plains, during a hot summer day, the air resting upon the land becomes
greatly heated, and rarer than the air above. If the day is calm, con-
siderable difference in density may be developed before convectional
motion sets up. When this condition of things exists, travelers often
see distant objects reflected as from a water surface. In reality the
reflection is from the upper surface of the rare stratum of air.

In Fig. 63, C Z) is the upper surface of a thin, rare layer of air overly-

I 3 2 PHYSIOGRAPHY

ing A B, and distinctly rarer than the air above. A distant object, as
X Z, rises above C D. The angle at which the rays from the object
strike the surface C D is greater than the critical angle, and the rays
are totally reflected. The object is then seen as in the position X Z', as
reflected from a water surface. The object and its image are both seen.
This species of mirage is peculiarly a land phenomenon, and is seen
in hot weather, and oftenest in low latitudes.

Flo. 63. HOT WEATHER, MIDDAY MIRAGE

Another species of mirage is seen over the sea as well as upon land,
occurring in winter, and most frequently in high latitudes.

In Fig. 64 the air up to the height of C D is distinctly colder and denser
than the air above C D. This dense stratum is of such thickness that a
vessel or city may be wholly immersed in it. Rays from the distant
object strike the under surface of the rare overlying layer at such an
angle that they are totally reflected to the observer at E. He thus sees
the reflection of the object in the inverted position shown, above its
real position.

The city of Baton Rouge, Louisiana, was thus seen by observers thirty
miles away; and travelers in Arctic waters frequently report seeing vessels
thus inverted and suspended in air. This usually occurs near the land,
and is due to an outrush of cold air from the snow-covered land over the
adjacent sea. Being heavier, it underruns the less dense air.

Halos. Sometimes, in front of a cyclone, when the cirrus clouds that
outrun and foretell the coming storm stretch across the sky, light or
colored rings are seen encircling the sun. These rings, of varying diam-
eter, are called halos. They are believed to result from refraction and
reflection of light, by the ice crystals that make up the cirrus cloud; or
diffraction by these crystals, or by the dust and cloud-mist at lower
levels. If the cloud sheet is thick the rings are nearer the sun. Similar
rings and luminous areas are observed about the moon.

Sometimes a more complex system of intersecting rings occur, which,
though too dim to be seen throughout their whole extent, are visible at
their points of intersection or tangency, where they appear as bright or

LIGHT AND ELECTRICITY OF THE AIR

133

colored spots. These spots are systematically arranged with reference
to the sun, and are known as mock suns or sun dogs.

Halos and mock suns are commonly thought to portend coming
storms. They are particularly bright and their occurrence frequent in
high latitudes.

Icr

ffo

FIG. 64. COLD WEATHER, EARLY MORNING MIRAGE

The illuminated areas encircling street lamps on foggy or misty nights
are analogous to the halo, as are also the brilliant borders of dense cloud
masses lying between the observer and the sun.

The Rainbow. If an observer stands with his back to the sun
while rain js falling in front of him, there often appears the arc of a
circle, made up of parallel bands of colored light, called the rainbow.
It results from refraction and reflection of light by the raindrops.

By refraction and dispersion the white light is decomposed into
its constituent colors. Red, being the least refracted, lies upon
the outer side of the bow, while blue, being most refracted, occu-
pies the inner side. The angle formed by drawing lines from the
eye to the ends of the bow's diameter is usually about 82 degrees,
and the center of the bow lie? upon the straight line passing
through the sun's center and the observer's eye. On this account
the bow is usually less than a half circle, unless seen from some
high point, as a mountain peak.

Though commonly a daytime phenomenon, rainbows are some-
times produced by moonlight.

I 3 4 PHYSIOGRAPHY

Colored bows, and even complete circles, analogous to rainbows may
be observed in the spray of fountains and waterfalls.

If favorably situated, with a sheet of calm water at his back, one may
see two distinct and intersecting bows, the shorter produced by the direct
rays from the sun, the longer by rays reflected from the water surface.

A less distinct secondary bow, outside the more brilliant one, and with
the order of the colors reversed, is sometimes seen. This bow is usually
more than 100 degrees in diameter.

The primary bow is thought to result from rays of light entering near
the side of the drop, being refracted upon entering, totally reflected from
the back surface of the drop, and again refracted upon leaving the drop.
This disperses the colors, so that we receive but one color from any drop.
The red is given by drops farther away from the line passing through the
sun and the observer's eye than the drops that produce the blue; hence
the red is on the outside of the bow. Since all drops which send a given
color to the eye lie at the same angular distance from this line, it follows
that the bands of color and the entire rainbow are arcs of circles, whose
center lies upon this line.

The secondary bow is thought to be produced by raindrops which
receive the rays in such a way that they are twice reflected within the drop
before leaving it. This double reflection accounts at the same time for
the dimness of the bow, its greater diameter, and the reversal of the
order of the colors from that of the primary bow.

Lightning. Every year records a considerable loss of life and
property in the United States from lightning. Men and animals
are killed, trees and houses shattered and often set on fire, and
hay and grain stacks burned. Lightning storms, commonly called
" thunderstorms," are usually associated with an overheated con-
dition of the air, locally, and are most common in the afternoon
of hot summer days. They occur not infrequently at night, and
may even occur in winter; but their cause seems always to be
found in a rapid convectional overturning of the lower air.

The air is always electrified, but it is only when clouds are form-
ing rapidly, as in the ascending currents near some storm center,
that discharge takes place. This discharge, known as the lightning
flash, may be between clouds, or it may be from cloud to earth.
When downward those objects which rise highest, as buildings
and trees, are most in danger of being struck by lightning, these
being better conductors than air.

LIGHT AND ELECTRICITY OF THE AIR 135

FIG. 65. PHOTOGRAPH OF LIGHTNING FLASH

In those sections of the country where wire fences are exten-
sively used great numbers of stock are annually killed by light-
ning, from collecting near these fences during thunderstorms.

136 PHYSIOGRAPHY

The wire serves as a conductor of the lightning horizontally, often
for considerable distances, being finally led to the ground by the
fence posts, which are thereby shattered. Telegraph and telephone
poles are shattered in a similar fashion. The belief that " lightning
does not strike twice in the same place " is a dangerous error. The
fact that lightning strikes in any given place argues the existence
there of favorable conditions.

Protection from Lightning. The fear of lightning, and the de-
sire for immunity from it, have led to the adoption of many pro-
tective measures. Perhaps the most common artificial protection
is the lightning rod.

As all solids are better conductors of electricity than the air, buildings
and trees are more often struck by lightning than the open surface of
the ground near them. The greater the number of buildings or trees
among which the discharge may be divided, the less the individual liabil-
ity. On this account a house in the city has greater immunity from
lightning than the isolated farm house ; and any tree in the forest is safer
than the "lone tree" upon the prairie.

Where country houses are surrounded by shade trees these offer per-
haps the best protection from lightning. Each individual tree becomes
a means for silently discharging the passing clouds of their electricity,
thus preventing heavy and destructive discharges.

The lightning rod, as a protection against lightning, has been in use
almost ever since the discovery of the nature of lightning. Its use is
based on the theory that metal, being a better conductor of electricity
than the building, by affording an easier route, will prevent the discharge
from passing to the building itself. It is also thought that the numerous
points which rise above the building may quietly drain away the electric
charge from the clouds, and thus prevent a destructive discharge.

The lightning rod consists, usually, of a metal ribbon or flattened tube,
commonly of copper or galvanized iron, laid over the roof of the building,
with frequent branches rising from six to ten feet in air. These branches
end in one or more sharp points; and the rod should extend sufficiently
deep into the ground to reach moist earth. If it ends in a cistern of
water so much the better. The greater the number of branches rising
above the building the better, as the discharge is thereby more divided.

Perhaps the greatest security from lightning is obtained by encasing
the structure in a network of wire.

Kinds of Lightning. Zigzag lightning is a very common form. The
course of the flash is probably a sinuous one, and only appears angular

LIGHT AND ELECTRICITY OF THE AIR

137

when seen along the direction of its path. Ramified or branching light-
ning may begin and end in a multitude of branches, uniting in a trunk
flash between. This kind of flash takes place between clouds. Heat
lightning, and sheet lightning, probably the same, are believed to be the
illumination of cloud masses so far away that the accompanying thunder
is not heard. St. Elmo's Fire is the name given to the discharge of

FIG. 66. TREE DESTROYED BY LIGHTNING

atmospheric electricity as a brush of bluish flame, often observed at the
ends of masts and spars of ships, at tree-tops, or house-tops, or any
pointed object. A crackling sound accompanies it, similar to that of the
artificially produced electric spark.

Thunder. The production of thunder may be likened to the pro-
duction of the noise accompanying the explosion of gunpowder. Along
the path of the lightning flash the air is intensely heated and pushed
away. The collision of the returning air particles, causing but a crack-
ling, of high pitch, in the case of a few short sparks, becomes a crash of
lower pitch in the case of a lightning flash, which is but a great number
of longer sparks. The quick succession of crashes following along the
path of the flash unite to produce the roar; and the roar, when echoed
and re-echoed from cloud masses, gives the rolling so often observed to
follow brilliant flashes of lightning during thunderstorms.

Relation of Lightning to Rain. The condensation of the moisture
in the air forms cloud particles, and the clumping of these particles, by

138 PHYSIOGRAPHY

reason of their mutual attraction, forms raindrops. With increase in
size of the raindrops the electric charge of the drop is increased, and
lightning discharge is made possible.

It would seem that lightning, for the most part, precedes the rain.
When the raindrops begin to fall each carries down with it a minute
charge, and in this way the cloud mass is discharged. Further produc-
tion of lightning is then impossible. The heaviest fall of rain in a thunder
shower often follows closely the most brilliant lightning and heaviest
thunder. So while lightning mainly precedes the rain, it seems probable
that they are mutually cause and effect.

The Aurora. This is the beautiful electrical display, common in
high latitudes, though often seen in northern United States. In the
northern hemisphere it is called Aurora Borealis, and in the southern,
Aurora Australis. It is believed to be due to the discharge of elec-
tricity into the rare upper air, and seems to bear some relation to
sun spots. The Aurora lessens the gloom of the long polar night.

As seen in the United States the Aurora, also called Northern Lights,
usually cpnsists of a more or less distinct arch of light, extending east
and west, and crossed at right angles by streamers of colored light which
radiate from a point in the northern horizon. The arch is highest above
the magnetic meridian; and the streamers of red, yellow and green light
change their position and length, and are called "the merry dancers."

Brilliant aurora displays are often accompanied by severe electrical
and magnetic disturbances throughout the country. The telegraph and
telephone services are often interrupted for hours;. and the magnetic
compass sometimes becomes so variable as to be useless.

QUESTIONS

1. Why do we not ordinarily observe the phenomenon of "looming"
at midday?

2. Would the phenomenon of " looming " be dispelled by ascending
into the air?

3. In the mirage resulting from the lower air being cooler than that
above, need the object, the image of which is seen, be visible?

4. It is a common notion that the greater the number of stars visible
within the halo rings about the moon, the greater the number of days
before the storm, which these rings presage, will arrive; what scientific
grounds exist for this belief?

5. How did Franklin discover the identity of lightning with the arti-
ficial electric spark?

6. Why should we avoid the shelter of tall trees in a thunderstorm ?
Why do we disconnect our telephones during thunderstorms ?

CHAPTER XIII
WEATHER AND CLIMATE

Weather and Climate Denned. Weather is the condition of the
air at a given time and place with reference to temperature, mois-
ture, state of the sky, and winds. These conditions, called weather
elements, are constantly changing; and as a consequence, for most
places in temperate latitudes, the weather is proverbially fickle.

As the day advances, after sunrise, the temperature normally
rises, reaching its maximum between one and three o'clock
p. M., after which it falls till near sunrise the following day. With
these changes in temperature come changes in the relative hu-
midity, and usually changes in wind direction and strength.

Climate is the average condition of the air with reference to tem-
perature, moisture, state of the sky, and winds; or it is average
weather. While in weather we consider current temperatures, in
climate maximum and minimum temperatures are of more im-
portance.

We use the term weather in referring to atmospheric conditions
at any given instant; also for such short periods as a day, a week,
or a month. We even speak of " summer " or " winter " weather;
but when we apply the term to these longer periods we refer rather
to the average conditions during these periods.

Pressure, though exercising a controlling influence upon weather
elements, is not itself commonly counted among them.

Weather Changes. Although the variability of the weather is
proverbial, yet there are important controls, which, by reason of
their orderly sequence, give a certain degree of system to the suc-
cession of weather changes. The most important of these are:

1. The alternation of day and night, due to rotation;

2. The annual succession of winter and summer, due to revolu-
tion;

140 PHYSIOGRAPHY

3. The more or less systematic passage of lows and highs.

The first two are fairly regular in period and value at any place,
though widely differing for different places; whereas the third
varies in both period and intensity.

The daily and annual changes of the weather are more pro-
nounced near sea level than at higher altitudes, in the interior
of the continent than near the coast, and in high than in low
latitudes. As a rule the temperature rises and the absolute hu-
midity increases during the day and in summer, both being lower
at night and in winter.

Convectional cyclones are more frequent over the land than
over water, and more vigorous in summer than in winter and in
the daytime than at night; whereas non-con vectional cyclones are
most frequent and intense in winter. Both classes of cyclone are
probably more highly developed, and likewise of longer duration,
over the sea than over the land.

In the United States during March and April, when the land
is warming up most rapidly, it is a common occurrence to have
days of blustery winds succeeded by nights of calm. This is due
to the rapid warming and con vectional overturning of the lower
air during the day.

Weather in the Tropics. Night has been called the " winter of
the tropics." This is because the variation in weather conditions
from day to night is greater than from winter to summer.

In the doldrum belt the days are uniformly warm, owing to the
nearly vertical rays of the sun; and the rapid con vectional ascent
of the air in the morning is usually followed later in the afternoon
by torrential downpours of rain, followed in turn by cloudless
nights. The nearly equal day and night, combined with the low
percentage of cloudiness, accounts for the great daily range of
temperature. Cyclonic interruptions are of secondary importance.

In the trade-wind belts, over the sea, there is a constancy of
weather conditions not found elsewhere. The extreme range of
temperature scarcely exceeds ten degrees; and the wind blows
continuously from the same direction, and with about the same
strength day and night. On land the range of temperature in-

WEATHER AND CLIMATE 141

creases, and over both land and sea there is little rainfall, except
where the winds are compelled to rise over the ascending land.
This is due to the fact that the trades grow warmer as they ad-
vance.

Regions on the borders of the trades have monsoon changes of
weather. If next the doldrums, there is the alternation of the
light winds and abundant rains of the doldrums, and the constant
winds and light rains of the trades. If next the high pressure
calms, then the characteristic conditions of the trades alternate
with the prevailing calms of the horse latitudes.

Weather Outside the Tropics. In the zone of prevailing wester-
lies weather changes are irregular, and mainly of cyclonic control,
with marked differences in the two hemispheres. In the southern
hemisphere, where there is little land to interrupt them, the
westerlies attain a constancy approaching that of the trades; and
so high a velocity that they are called the " Roaring Forties."
In winter the cyclones are more frequent and succeed each other
with almost periodic regularity.

In the northern hemisphere, where the land is massed, there is a
strong contrast between the weather of the land and water areas
of the prevailing westerlies, the land areas having much greater
extremes of weather conditions, both daily and seasonal.

In the frigid regions, although temperature changes are deter-
mined chiefly by the appearance and disappearance of the sun,
the other weather elements are controlled mainly by the passage
of cyclones:

Weather Prediction. After a thorough understanding of the
relative values of the factors determining weather in any region
it is possible to predict, with a high degree of accuracy, the changes
of weather likely to occur. The degree of accuracy attainable
varies with the season and with geographic position. Under the
doldrums and trade winds, where the diurnal change is dominant,
weather prediction may be made with an assurance almost amount-
ing to a certainty that it will be fulfilled. Indeed, the weather
changes there are so regular and certain that the weather is not a
topic of conversation.

14.2 PHYSIOGRAPHY

In regions where the control of the weather is mainly cyclonic,
it is not possible to predict with nearly so high a degree of accu-
racy. Yet even here the relative values of the factors are so well
known, and the systematic movement of cyclonic disturbances so
well understood, that predictions may be made with the reason-
able expectation that a large per cent of them will be fulfilled.
These predictions for any station must take account of: (i) The
systematic movement of cyclonic disturbances, their strength of
development and place of origin, and direction and rate of move-
ment; (2) The season; and (3) Local topography.

Weather in the Cyclone. To understand the weather condi-
tions which prevail about lows and highs it is necessary to remem-
ber the directions of the winds about these disturbances, and the
effect upon the humidity of the air resulting from a change of
temperature.

In the United States cyclones, as we have seen, move eastward,
and the winds blow in toward the cyclonic center, in counter-
clockwise spirals. At any station the wind will not, as a rule, be
blowing directly toward the center, but a little to the right of it.
Therefore, in front of the cyclone the winds are blowing from a
warmer to a cooler latitude, and their relative humidity is increased.
As they approach the center of the low the air rises, and its humidity
is further increased by cooling from expansion. This may be suffi-
cient to bring the air to saturation. As a result of these conditions
a rising temperature, with cloudiness or precipitation, generally
characterizes the front of the low, and may be predicted as a well-
developed cyclone approaches.

In the rear of the cyclone the winds are moving from colder into
warmer regions, and as a result the relative humidity of the winds
is lowered. As they near the center of low and begin to rise, their
temperature falls as a result of expansion; but the cooling must
first counteract the warming due to their moving into warmer lati-
tudes before their relative humidity reaches that possessed by the
winds when they were inaugurated.

As a result of this difference in conditions in front of and be-
hind the cyclone the increase in humidity, due to ascent, may

WEATHER AND CLIMATE

143

bring the air in front of the cyclone's center to the saturation
point, and yet not saturate the less humid air in its rear.

Consequently falling temperatures and clearing skies may be
expected after the center of a well-developed cyclone passes.

The direction of the shifting of the wind depends, as we have
seen, upon the position of the path of the cyclone's center, whether

FIG. 67. AN IDEAL Low, SHOWING THE DISTRIBUTION OF THE WEATHER ELEMENTS ABOUT

ITS CENTER. (AFTER DAVIS)

D E F, ABC, and G H J are paths of observers through the storm area where the storm passes
north, centrally over or south of the observer respectively. Note the succession of winds
each will experience, also the probability of precipitation for each. Observe that the southerly
winds in front, and the northerly winds behind the storm center, give the isotherms a general
north-east-south-west trend, and the observer will not experience a very severe change of tem-
perature. If the storm passes centrally over him he will experience a lull in the wind near
the center, after which the wind springs up from the opposite direction and increases in
strength.

north or south of the station. Ordinarily the strength of the wind
increases as the cyclone approaches, and decreases as the cyclone
recedes.

144 PHYSIOGRAPHY

In winter the strong indraft of cold air in the rear of a cyclone,
if accompanied by snow, is known as a blizzard.^

Weather in the Anti-Cyclone. Since the movements of the air
about a high are the reverse of its movements about a low, it fol-
lows that the conditions as regards temperature and humidity
which prevail about a high are likewise the reverse of those which
prevail about a low. In front of a high the winds are northerly,
and behind a high the winds are from some southerly quarter,
while at the center of the high the air is sinking. Consequently
fair and cooler weather is predicted, usually, as the high ap-
proaches, and rising temperatures with possible cloudiness or even
precipitation as the high recedes.

Since the winds start from the center of the high, unlike the low,
the winds weaken as the high approaches, and strengthen as it
recedes. As with the low, the direction of the shifting of the
wind is determined by the position of the station with reference
to the path of the center of the high.

In winter, if a high follows closely in the wake of a well-developed
low, the fall in temperature may be abnormal. If it is as much as
20 degrees F. in 24 hours, reaching a temperature of zero or lower,
it is called a cold wave. In the southern part of the United States
the term is applied to changes that are somewhat less than 20
degrees, and that reach a somewhat higher minimum.

Thunderstorms and Tornadoes. In summer, after a day or so
of excessive heat, the rapid convectional ascent of the air about a
low may set up, locally, a more limited though more intense cy-
clonic whirl. The rapid condensation of vapor in the rising and
cooling air may give rise to, or be accompanied by, brilliant displays
of lightning and heavy thunder. Such storms are known as thunder-
storms. Torrential downpours of rain may follow quickly after
the most brilliant discharges of lightning; but it is a notable fact
that the lightning flashes become rapidly less frequent after the
rain begins to fall.

Thunderstorms are usually summer and day-time phenomena,
though they sometimes occur in winter and at night. They are
much more common in front of lows than behind them. In the

WEATHER AND CLIMATE 145

United States they occur most frequently in the southeastern
quadrant of the low pressure area.

If the local whirl thus developed is destructive in violence, it is
called a tornado. The destructive path of a tornado is rarely a
mile in width, and usually but a few miles in length; more com-
monly it is but a few hundred yards in width. Within that nar-
row path the violence of the wind is such that few structures above
ground are strong enough to withstand it. In those States in the
Middle West, where tornadoes are most frequent, underground
structures called " cyclone-cellars " are built. These seem to offer
the greatest security from danger.

Tornadoes progress normally in a northeastern direction, at a
rate of twenty or thirty miles an hour; whereas the spiraling
winds about the tornado center may attain a velocity of more
than one hundred miles an hour. Tornadoes are most fre-
quent in the afternoon of hot summer days, and seem to need for
their development a fairly level land surface; hence we do not have
them in mountain regions, nor do they occur upon the Pacific
coast. The broad, level Mississippi Valley seems best suited of all
places in the United States for their development.

Weather Service. For the purpose of a more thorough study of
the weather, and more accurate prediction of weather changes, the
United States Government has established a weather service ex-
tending to all settled parts of the country. This service, which is
the work of the Weather Bureau, a division of the Department of
Agriculture, has its central office in Washington, D. C. Its corps
of observers, paid and voluntary, to the number of more than
three thousand, are distributed throughout the country. Regular
observations of the weather are made at more than two hundred
stations, as nearly as possible at the same instant, 8 'o'clock A. M.
and P. M. 75th meridian time, and are reported by telegraph to
the central office at Washington, and to each other. The most
important observations are: pressure; temperatures, current, maxi-
mum and minimum; direction and strength of the wind; amount and
kind of precipitation during the past 24 hours, and percentage of
cloudiness.

146

PHYSIOGRAPHY

Weather Maps. When these data are collected and plotted on
a map of the United States the result is a weather map. The daily
weather map is published at the central office in Washington, and
also at one or more sub-stations in every State. It not only sets
forth the weather conditions existing at the time of observation,
but it also serves as a basis of prediction of the weather for the

FIG. 68. WEATHER MAP

Isotherms, dotted lines, drawn for every 10 degrees; and isobars, unbroken lines, drawn for every
tenth of an inch. Line of arrows indicates the ordinary path across the U. S. of this type of
low. Such lows usually advance at the rate of about 30 miles an hour.

24 or 36 hours following. Each local map supplements the general
prediction for the entire country with a forecast for the particular
locality.

To be of value for purposes of forecasting, weather maps must
be distributed the day issued, since weather conditions are con-
stantly changing.

Since our weather is mainly of cyclonic control, and since the cyclonic
disturbances move eastward across the country, the weather map as a
basis of weather prediction is of more value to the eastern than to the
western part of the country. On the Pacific coast it is of little value,

WEATHER AND CLIMATE 147

since there are few stations further west to report coming changes.
With further extension of wireless telegraphic service, the value of the
weather service to our western coast will be correspondingly enhanced.

Value of Weather Predictions. Every observer is familiar with
the daily and seasonal changes of temperature; also with the fact
that there are other almost equally important changes that are
irregular in their occurrence. More and more people are learning
to appreciate the relation of these unperiodic changes of the weather
to the eastward march of cyclonic disturbances, and to appreciate
the great value of our weather predictions. Each year brings a
wider use of these predictions, and a more general rejection of
the predictions of charlatans who make year-long forecasts.

Among the first to realize the benefits of our weather forecasts
were the shipping interests of our southern and eastern coasts and
of the Great Lakes. Not infrequently censuses have shown that
marine property to the amount of more than $25,000,000 has been
held in port because of storm warnings issued. Few masters of
vessels now leave port without knowing the latest forecast of the
weather.

Shippers of perishable goods are also interested in weather pre-
dictions. Estimates from shippers place the value of property
saved by the warning of the cold wave of January ist, 1898, at
nearly $5,000,000. Farmers, planters, truck growers and fruit
growers are interested in being forewarned of changes in the
weather, especially when these changes mean destructive winds,
floods or frosts. With the wide extension of the use of the tele-
phone and of rural mail delivery is coming a wider use and appre-
ciation in all fields of the great value of weather predictions.

It is our confident belief that with a more extended field of observation,
and a better knowledge of upper air conditions, the present practical
limit for safe predictions of 36 hours may be considerably increased.
By means of kites and balloons the upper air is being explored.

Weather Signs and Proverbs. There are two distinct classes of
weather signs. The first are based on century-long observations
by those whose occupations have led them to observe closely
weather changes; the second class includes a mass of superstitions

148 PHYSIOGRAPHY

that have been strangely preserved and transmitted. The signs of
the first class have usually found expression in trite sayings that
have come to be known as weather proverbs. As an aid to memory
these proverbs are commonly expressed in rhyme.

Weather proverbs are usually of only local application, though
many are world-wide. When local, in order to appreciate them,
one must be acquainted with the local conditions.

"Rainbow in the morning, sailors' warning; Rainbow at night, sailors'
delight," is a proverb that is true only in those regions where cyclonic
storms move eastward. If the rainbow is seen in the morning, the storm
center is apt to be westward, and its further progress will bring it nearer.

"Mackerel scales and mares' tails, Make lofty ships carry low sails,"
is applicable the world over. The long, wispy clouds called "mares'
tails," and the sky flecked with cirro-cumulus clouds, and known as a
"mackerel sky," are the result of the high-level overflow of air in front
of a cyclone. Consequently they presage a coming storm. "Mist ris-
ing o'er the hill, Brings more water to the mill" the world over.

Climatic Controls. Since climate is but average weather, those
conditions which control weather likewise control climate. The
most obvious, and perhaps the most important, climatic controls
are: latitude, height above sea level, distance from the sea, posi-
tion with reference to mountain ranges, and with reference to
prevailing cyclonic paths.

Although climate is defined as the average condition of the air
with reference to the various climatic elements, it does not follow
that where these averages are the same the climates are alike or even
similar.

New York City and San Francisco have about the same average
temperature for the year, but New York has hot summers and cold
winters, whereas San Francisco has equable temperatures through-
out the year. The central Mississippi Valley has about the same
annual rainfall as the coast of California; yet in the interior the
rains are distributed through the year, while on the coast they are
confined to the winter months.

Of vastly more importance than averages are the extremes of
climatic conditions, and the distribution of these conditions
through the year.

WEATHER AND CLIMATE

149

150 PHYSIOGRAPHY

Climatic Zones. Temperature being the most important cli-
matic element, and depending, as it does, mainly upon latitude,
the earth may be divided into east-west zones, each of which
furnishes a distinct type of climate. Within any zone there may
be considerable variation from the type, yet there is sufficient
similarity to justify the division into zones.

The customary division, whereby the zones are bounded by
parallels, gives us light zones rather than climatic zones; there-
fore, the Tropics and Polar Circles are not boundaries for torrid,
temperate, and frigid climates. A more reasonable boundary is
the isotherm. It has been suggested that the average annual iso-
therm of 68 F. be taken as the poleward boundary of the torrid
zone, and the summer isotherm of 50 F. as the poleward boundary
of the temperate zones.

The temperature of 68 F. is about the temperature we desire
for our houses in winter, and the temperature necessary for so-
called tropical plants; and a temperature of 50 F. is necessary
for trees and for the maturing of the hardier cereals. Warm tem-
peratures during the growing season are more important than low
temperatures during the dormant season.

The temperate zone is the widest zone, and wider in the northern
than in the southern hemisphere. This is due to the excess of land
north of the equator, land being a better absorber of insolation
than the sea. The frigid zones, or more accurately the polar cold
caps, have a temperature, even in the hottest month, below 50 F.

The Torrid Zone. As its name suggests, the most distinctive
character of the torrid zone is its high temperatures. In every
part, except where mountains rise into cold altitudes, the daily
maximum is from 75 to ico F., and often higher. But the other
climatic elements vary so widely in this zone as to justify its
division into three parts:

i. The belt of doldrums or equatorial calms is a belt of high tem-
perature, light and variable winds, and abundant rainfall. The
almost vertical rays of the sun heat up the lower air in the early
forenoon and cause rapid convectional currents. These rise by
noon to such altitudes that their cooling by expansion produces

WEATHER AND CLIMATE 151

condensation of their vapor, the formation of clouds, and in the
early afternoon rain. The rains are thus of almost daily occurrence
and abundant throughout the year. More abundant rainfall and
a higher percentage of cloudiness are to be found over the sea than
over the land, due to higher humidity over the sea.

The days vary little in length, and twilight and dawn are of
short duration, owing to the nearly perpendicular position of the
sun-path to the horizon.

In this belt occur the dense forests of South America, Africa,
and the East Indies.

2. The trades, like the doldrums, have a prevailingly high tem-
perature, but unlike the doldrums they have little rainfall. Their
most marked characteristic is the constancy of their winds. These
blow day and night, winter and summer, with a constancy of both
direction and strength equaled in no other zone. In the northern
hemisphere they are northeast winds, and in the southern hemi-
sphere southeast. They run to the doldrum belt, where the air
rises.

On land the climate of the trades depends upon the direction of
slope, the eastward and westward slopes having unlike climates.
The winds being forced up the eastward slopes may yield abun-
dant rainfall, as upon the eastern coasts of Central America,
Brazil, Africa and Australia; whereas the descending winds upon
the westward slopes yield little or no rainfall, as shown by the
dry western coasts of these countries.

The eastward and northeastward slopes of the mountainous
islands of the West Indies and the Hawaiian group have abundant
rainfall, and are heavily forested; whereas the southwestern slopes
have deficient rainfall, and in some cases are almost desert.

Any low-lying land area, island or continent, under the trade
winds, is apt to be desert because of its slight rainfall. The great
deserts of Africa and Australia are trade-wind deserts. The winds
moving toward the equator are warmed, and their capacity for
water vapor is increased, consequently they not only yield little
rainfall, but they also absorb the moisture of the regions over
which they blow.

PHYSIOGRAPHY

FIG. 70. WINDS OF THE ATLANTIC OCEAN FOR JANUARY

Note the well-developed low pressure area over the northern ocean, and the equally well-
developed high pressure area over the southern ocean. Account for this. Long arrows indicate
steady winds, and heavy or double arrows indicate strong winds.

WEATHER AND CLIMATE

153

FIG. 71. WINDS or THE ATLANTIC OCEAN FOR JULY

Observe that the low in the northern ocean has disappeared and a well-developed high appears
in the belt of horse latitudes. Find a reason for this. The high continues, though weakened,

over the southern ocean.

154 PHYSIOGRAPHY

3. With the shifting of the wind belts, regions near the border
line of the doldrums and trades lie alternately under these belts.
Such regions have seasonal changes of climate, and are known as
monsoon belts. If near the equator they have two distinct wet
seasons, when under the doldrums, and two dry seasons when under
the trades.

While these transitional or monsoon belts extend around the
earth, they are most pronounced where, as in India, the monsoon
winds are reenforced by the continental winds, owing to the great
continental mass lying poleward from them. When the south-
west monsoon blows over India the "hooked" southeast trades
are strengthened by the continental in-draft toward central Asia;
and when the northeast monsoon blows, the normal northeast
trades are strengthened by the outflow from the cold continent to
the north. The southwest monsoon is much stronger than the
northeast. Why ?

Similar monsoon winds are observed upon the coasts of South
America, Africa, and Australia.

The Temperate Zone. Poleward from the torrid zone in each
hemisphere lies the temperate zone. It comprises about one-half
of the earth's surface, and shows the greatest variety and range
of climatic conditions. Lying mainly under the westerlies, its
weather and climate are mainly cyclonic in character, thus ac-
counting for its variability of climate. To appreciate the climate
of any part of this belt it is more necessary to know extremes
than averages. In northeastern Siberia, for example, an extreme
range of temperature during the year of over 200 F. occurs, the
average temperature being about zero.

In general the variability of climate in the temperate zone is
greater over the land than over the sea, and greater in the north-
ern than in the southern hemisphere.

Divisions of the Temperate Zone. On this account it is desir-
able to divide the temperate zones into:

1. Ocean and land areas;

2. Northern and southern belts;

3. Eastern coasts, interiors, and western coasts.

WEATHER AND CLIMATE 155

1. The ocean areas of this zone have smaller range of temperature,
more rainfall and cloudiness, and stronger winds than the land areas.
The oceans, being low pressure areas in winter, have their greatest rain-
fall at that season; whereas the continents, on the whole, receive their
greatest amount of rain in summer.

2. Because of its large water area the climate of the south temperate
belt is most equable of all the regions in the westerlies. The winds,
toward its southern edge, have high velocity and blow with great regular-
ity; and westward-bound ships around Cape Horn are delayed by these
head winds.

The north temperate zone has a climate that has been likened to "A
crazy quilt of patches," so changeable is it in its various parts. The
excess of land with its varying altitudes, in this zone, breaks up whatever
tendency there might be toward uniformity in climatic conditions.

3. The eastern coasts, though tempered by their adjacence to the sea,
lying to leeward of the continents, partake of the variability of climate
characteristic of the land. The succession of lows and highs that march
eastward across the continents carry to the eastern coasts continental
conditions. Winds and rains are the result of passing lows and highs;
and rainfall is slightly more abundant in winter, when the winds from
the sea blow upon the colder land, than in summer.

The climates of the eastern coasts are to a slight extent modified by the
warm and cold ocean currents that follow them. The eastern coast of
the United States as far north as Cape Cod is to some extent warmed in
winter when the wind blows from the southeast over the warm Gulf Stream;
and the eastern coast of North America north of Cape Cod is cooled in
summer, in like manner, by winds from the cold Labrador Current. On
this account we find summer resorts on the coast of New England, and
winter resorts from Florida northward to New York.

In a similar manner the Japan Current affects the climate of south-
eastern Asia; while northward from Korea the coast is cooled by winds
from the cold Kamchatka Current.

In the southern hemisphere the eastern coasts of all continents are
warmed by winds from warm currents.

The interiors of the continents show greater extremes of temperature
and rainfall than do the coasts. The summers are warmer and the
winters colder than the latitude justifies. The winds and rains are
cyclonic, and the rainfall is most abundant in summer, when the lands
are great centers of low pressure, with inflowing winds. If the region is
plateau it is characterized by slightly lower temperature and less rainfall
than if it is plain.

The western coasts have the benefit of the tempered westerlies coming
from the ocean. This gives an evenness of temperature throughout the

156 PHYSIOGRAPHY

year not found upon the eastern coasts of the temperate zone. Rain is
both more abundant and more markedly concentrated in the winter
months than it is upon the eastern coasts, because in winter the colder
land chills the moist winds from the ocean. Winter fogs are most
frequent on this coast.

The northern and southern hemispheres differ widely in the climates
of their western coasts as a result of ocean currents. Alaska and north-
western Europe are warmed many degrees above the normal for their
latitude by winds from the great warm drifts in the Pacific and the At-
lantic oceans, while Chile, western Africa and western Australia are
cooled as a result of the branches along these coasts from the cold Ant-
arctic Drift.

The climatic influence of warm and cold currents is much greater on
windward than on leeward coasts. In the westerlies the windward
coasts are the western coasts, whereas in the trade belts the windward
coasts are the eastern coasts. Winds blowing over ocean currents acquire
the temperatures of those currents, which they carry to the lands.
Winds blowing across continents tend to carry continental conditions
to the eastern coasts in the westerlies, and to the western coasts in the
trades.

While the chief effect of the warm currents in the northern oceans is
to temper the cold of the western coasts in high latitudes, a less marked but
noticeable effect is to temper the heat of the western coasts in low latitudes,
upon the return of the currents toward the equator. Southern California,
western Mexico, and northwestern Africa are thus made cooler.

Between the temperate and torrid zones lies a transitional zone
of high pressure and descending air currents. This belt shifts
during the year with the northward and southward movement of
the sun. These are, therefore, monsoon belts, places in them having
alternately the climate of the trades and of the westerlies. Rain-
fall is scant, owing to the increased temperature, due to compres-
sion, of the descending currents; and the winds are never strong,
since this belt is the starting point of both the trade winds and the
westerlies.

In general, seasons in the temperate zone are based chiefly upon
temperature, whereas those of the torrid are based upon rainfall.
The transition seasons of spring and fall, though well marked in
the middle of the temperate zone, disappear toward the poleward
and equatorward edges. Mountain ranges with a north-south
trend induce rainfall upon their western slopes, while arid or des-

WEATHER AND CLIMATE 157

ert regions are apt to lie to eastward of them. This is the reverse
of conditions in the trade wind belts, where the eastward are the
rainy, and the westward the dry slopes.

The Polar Cold Caps. The polar areas, though called the frigid
zones, are not belts, but an area about each terrestrial pole in-
cluded within the summer isotherm of 50 F. The polar cold cap
is much more extensive in the southern than in the northern
hemisphere, possibly due to the great glacial ice-sheet covering
the Antarctic continent, or to aphelion winter. In the Antarctic
regions the boundary isotherm reaches in some places to latitude
55, while in the Arctic regions it crosses and re-crosses the Arctic
circle.

These regions are rightly named frigid, the chief characteristic
of their climate being freezing temperatures for most of the year.
At no time is the temperature high. The lands, for the most part
covered by ice, are frozen deserts. It is only where there are slopes
favorably inclined to the sun that the snow melts and the soil is
sufficiently warmed and drained to permit plants to grow. Even
here the temperature is too low for trees to thrive, and mosses and
lichens are the chief growth. Temperatures of 83 F. have been
reported.

The winds over the polar cold caps are cyclonic, and the cyclones
are probably driven cyclones. The winds are often burdened with
fine dry snow, which covers all land surfaces for most of the year.
Over the stretches of frozen plain these winds sweep with great
violence, and are comparable to the blizzards of winter climates
in temperate latitudes.

Precipitation, which on the whole decreases toward the poles, is
deficient here. It is mainly in the form of snow, there being some
regions where rain probably never falls. Though the precipitation
is light, yet it probably exceeds evaporation over the region, the
excess being imported by the westerlies from lower latitudes.

Since more snow falls than melts, the land areas of Greenland
and the Antarctic continent become covered with a glacial ice-
sheet. When the glaciers reach down to the sea, great blocks of
ice break off and float away to lower latitudes, as icebergs.

158 PHYSIOGRAPHY

Continental and Marine Climates. The interiors of all conti-
nents are marked by great variability of temperature. This
variability decreases toward the coasts, being least upon windward
coasts. In strong contrast with the climates of continent in-
teriors is the climate of islands, which is practically that of the
surrounding sea.

The sea is less heated and^less cooled than the land, under the
same conditions of insolation. It is a better reflector and trans-
mitter than the land, hence less insolation is absorbed to warm
the surface layers. The specific heat of water is higher than that
of the land, hence a given amount of heat will not raise the tempera-
ture of the water through as many degrees as it will the same mass of
land. Much of the heat absorbed by water is used up in the me-
chanical work of evaporation, leaving just so much less for raising
the temperature; while on land, the amount of evaporation is lim-
ited by the amount of moisture brought up to the surface by capillarity,
and nearly all of its absorbed heat is used to increase its tempera-
ture. Then there are currents in the ocean that distribute the
heat over great distances; and there is a greater percentage of
cloudiness over the sea than over the land.

All of these variables combine to give to places surrounded or
bordered by the sea peculiarly equable climates as compared with
continent interiors in the same latitude. The one climate we call
marine, the other continental.

Mountain Climates. As we ascend a mountain, in any lati-
tude, all the climatic elements change from those prevailing at
the mountain's base. The temperature, as we have seen, falls at
the rate of about i F. for every 300 feet of ascent; the absolute
humidity decreases, while the relative humidity generally increases
up to a certain altitude, depending upon the latitude, after which it
also decreases; precipitation increases for a time, as we ascend, then
gradually fails; and with increasing altitude the winds increase in
strength and constancy.

On the whole, all of the climatic elements become more constant
with increase of altitude, and this equability is the most marked
characteristic of mountain climates.

WEATHER AND CLIMATE 159

The windward and leeward sides of mountains, and particularly
of mountain ranges, are apt to present very different types oi
climate. The windward, owing to ascending and cooling currents,
has an excess of rainfall, while the leeward, with descending and
warming winds, is apt to be dry. If in the trades, the eastern
slopes receive the rainfall, whereas in the westerlies the eastern
slopes are dry.

The desert of southern California in the United States, and of
Atacama in northern Chile, are examples of deserts to leeward of
mountains in the westerlies, and the arid western coasts of Mex-
ico and Peru lie to leeward of mountains in the trade wind belts.
The southern slopes of the Himalayas receive most of their rain-
fall while the southwest monsoon blows; and the northern slopes
of the Atlas mountains in northern Africa are the windward and,
therefore, the rainy slopes.

QUESTIONS

1. Why are weather changes more pronounced near sea level than at
higher altitudes? Inland, than near the coast? In high, than in low
latitudes?

2. Why do cyclones endure longer at sea than on the land?

3. Why should rainfall be more abundant behind than in front of the
cyclone centers in the trade wind belts?

4. What is the direction of the wind in front of, and behind, tropical
cyclones, both in the northern and the southern part of the torrid zone?

5. Why are thunderstorms more frequent in summer than in winter?

6. Why do tornadoes generally occur in the afternoon? And why do
people in the United States not seriously fear a threatening cloud in the
north, but do, one in the southwest?

7. Classify the weather proverbs you know with respect to their scien-
tific basis.

8. Why do the continents in temperate latitudes have their greatest
rainfall in summer?

9. Why is the climate of the north temperate zone more variable
than that of the south temperate?

10. Why do the interiors of continents show the greatest extremes of
climate?

11. Why do the eastern coasts in the trade wind belts have more
equable climates than the western coasts of these belts, whereas the re-
verse is true in the belts of prevailing " westerlies "?

CHAPTER XIV
CLIMATE OF THE UNITED STATES

Climatic Regions. The United States is situated in the zone
of prevailing westerlies, hence its climate is chiefly of cyclonic
control. However, its wide range in latitude, its great variation
in distance from the sea, and the difference in altitude of the various
parts give to -different sections sufficiently characteristic climates
to justify their separate consideration.

Minnesota and Maine, because of their higher latitude, have
lower average temperatures than Louisiana and Florida; Kansas
and Nebraska, lying near the center of the continent, have greater
ranges of temperature and less rainfall than northern California
and Maryland; and Denver, in the foothills of the mountains, has
a more equable climate than St. Louis, in about the same latitude
but at a lower level.

Based upon these three conditions governing climate, latitude,
distance from the sea, and altitude, it has been suggested that the
United States be divided into north-south climatic regions. Some
of these regions vary considerably from north to south.

The Pacific Coast Region. This zone extends inland from the
Pacific coast about 200 miles to the backbone of the Sierra Nevada
and Cascade Mountains. Like all regions situated to leeward of
an ocean, it is characterized throughout by an equable climate. The
isotherms, instead of having a roughly east-west trend, as is their
usual habit, run almost parallel with the coast. The continuation
of the Japan Current, the North Pacific Drift, cooled during its
long journey through north Pacific waters, in its southward flow
washes the entire length of coast of this region. The influence of
the winds from over this current is perhaps to increase the temper-
ature slightly, of the State of Washington, above the average for

CLIMATE OF THE UNITED STATES 161

that latitude, but to lower the temperature decidedly, of southern
California. Frost seldom occurs here in the lowlands.

In strong contrast with this sameness of temperature through-
out its north-south extent is its wide difference in annual rainfall.
The westerly winds come from the Pacific, moisture laden at
all seasons. In summer they blow upon land surfaces warmer
than themselves and, therefore, yield no rain until they begin to
rise up the mountain slopes. In winter the cooler land induces
rainfall, even over the lowlands, thus making the winter rains
the most marked characteristic of the Pacific Coast climate.

Upon the mountain slopes the rainfall is abundant throughout
the year, though most abundant in winter. There we find the
giant trees and dense forests. On the coast of Washington, where
the high mountains lie near the sea, we find the greatest rainfall
of the United States, over 100 inches; whereas in southern Cali-
fornia, with its coastal plain, and its nearness to the high pressure
calms, it is less than 10 inches.

The cultivated lowlands at the south are parched during the
growing season, and but for their nearness to the mountains, which
makes irrigation possible, these lowlands would be of but little
value. As it is, they are among the most valuable cultivated lands
in the United States.

It is claimed for these lands that they are peculiarly adapted to
the production of fruits, inasmuch as the fruits grow and ripen in
sunshine, thus giving them higher color and superior flavor.

Along the coast fogs are common, especially in winter. Severe
storms are almost unknown, thunder being rarely heard upon the
coast. Upon the mountain slopes thunderstorms break, and the
lightning flashes are seen, though at distances from the coast too
great for the sound of the accompanying thunder to carry.

The Plateau Region. This region embraces the high plateau
lands lying between the Sierra Nevada and Cascade Mountains on
the west and the Rocky Mountains on the east. Its most marked
characteristic is its dryness. Lying as it does to the leeward of the
Sierras, the descending winds on the eastern slopes of these moun-
tains yield little rain. It is only after they have crossed the

162 PHYSIOGRAPHY

greater part of the region, and begin their ascent of the western
slopes of the Rockies, that rain is induced. Occasional cyclonic
storms yield some rain, but over most of the region the rainfall
is insufficient for agriculture, without irrigation. It varies from
20 inches in Washington to 3 inches in Arizona. The greater part
of this region is too remote from the mountains to permit of irri-
gation, and must, therefore, remain arid and unproductive.

The skies over the plateau region are prevailingly clear, conse-
quently the daily range of temperature is excessive. The winters
are cold, and the summer days extremely hot. Cold winter cy-
clones sweep down from the north; and it has been suggested that
the hot desert region about the head of the Gulf of California is
the birthplace of most of our southwest summer cyclones.

Rainfall is nowhere in the region sufficient to support heavy
forests. At the north, where most abundant, it falls mostly in
winter, the growing season being almost without rain. Owing to
the deep and retentive soil, so fine-grained and homogeneous that
it brings capillary water from unusual depths, this part of the
region yields abundant wheat harvests. Apples and other tem-
perate latitude fruits are grown where irrigation is possible.

The chinook winds, which sink down the mountain slopes,
warming as they advance, keep the narrow mountain valleys free
from snow. On this account these valleys are much sought by
both wild and domestic animals for winter grazing grounds.

The Great Plains. This name is applied to the region of east-
ward sloping lands from the Rocky Mountains to about the
meridian of 100 W. It grades imperceptibly eastward into the
next climatic region, and is characterized over most of the region
by the cold winters and hot summers, typical of continental inte-
riors in this latitude.

Rainfall, which increases eastward with increasing distance from
the mountains, is in the main insufficient for agriculture, unaided
by irrigation. Much of the region is capable of irrigation, from
streams or artesian wells, and by this means is becoming increas-
ingly valuable. The rainfall is insufficient for forests, but it suffices
for the growth of abundant and nutritious grasses. These are the

CLIMATE OF THE UNITED STATES 163

great natural grazing grounds of the United States. Before the
advent of the white man vast herds of buffalo roamed these plains,
but disappeared with the march of civilization westward. In their
stead came herds of cattle and flocks of sheep, and that typically
western product, the " cow-boy."

The seasons are variable in the extreme. Occasional abundant
harvests are gathered, only to be followed by one or more seasons
of disastrous failure. With wider adoption of the methods of "dry-
farming," much more of the Great Plains region will be devoted to
agriculture.

The rains and winds of the region are wholly determined by the
passage of highs and lows. The rainfall is distributed through the
year, though slightly in excess in summer, and nowhere exceeds
20 inches.

This region is the continuation of the great Arctic plain, which
extends unbroken southward past Hudson Bay. With no east-
west mountain range to interrupt, it is swept over by the winter
cyclones from the north, which sometimes reach even to the Gulf
of Mexico before turning to their final northeastward course.
Owing to the level and prairie character of the region, wind veloc-
ities are often excessive.

The Central Prairie Lands. As already stated, this region is a
continuation eastward of the Plains region, there being no natural
boundary between them. It is bounded eastward by the Missis-
sippi River.

The Central Prairie Lands differ from the Great Plains chiefly
in having a more abundant rainfall, 20 inches or more. This is
everywhere sufficient for agriculture, and increases southward.
On the coast of Louisiana it is 60 inches, due to the in-draft of
warm winds from the Gulf, toward lows crossing the region far-
ther north.

The climate, though typically cyclonic, is not so extreme as
farther west. At the south the influence of the Gulf in tempering
both the cold of winter and the heat of summer is marked. The
annual range of temperature is 160 in North Dakota, whereas
it is but half that on the Gulf coast.

1 64 PHYSIOGRAPHY

Over most of this region the rainfall, 30 inches, is sufficient to
support forests, and their absence over much of the region has not
been satisfactorily explained. Forests border practically every
stream of the region.

Various explanations of the absence of forests in this region have been
proposed. The one which perhaps has received widest acceptance is the
destruction of the forests by fires. Inasmuch as attempts to extend the
forests have not been successful it would seem that perhaps the explana-
tion of the absence of forests is to be found in the character of the soil,
and deeper deposits, which are often glacial clays.

The great body of the more productive agricultural lands lies h
this division, and here most of the staple food products are grown.

The winds are variable in direction, though northerly winds
prevail at the north and southerly winds prevail at the south.
Owing to the greater interruption by forests the winds do not here
attain the average strength of the winds upon *" u e Great Plains.

This climatic region is visited by a greater number of destructive
wind storms than any other region of the United States. Torna-
does begin to occur in the Gulf States in February, though most
frequent from April to September. Their time of earliest occur-
rence is later the farther north we go.

The Western Appalachian Slope. This region embraces the area
extending from the Mississippi eastward to the axis of the Appala-
chian Mountains, and from the Great Lakes southward to Ten-
nessee. At the north the Great Lakes temper both the heat of
summer and the cold of winter, so that the climate, though conti-
nental, is not so extreme as in the regions between the Mississippi
River and the Rocky Mountains.

The westward trend of the Appalachians, while protecting the
Gulf slopes to the southward from cyclones originating in the west,
also protects the climatic region to the northward from the frequent
tropical storms that come up from the West Indies. These tropical
cyclones rarely cross the mountain barrier of the Appalachians.

The rainfall of the region exceeds that of the northern division
of the Prairies for two reasons: there is a greater water surface
adjacent, to yield vapor, and the prevailing westerlies are moving

CLIMATE OF THE UNITED STATES 165

up the slopes. The rivers are, therefore, numerous and strong,
and more evenly and abundantly supplied than are those of the
Prairie region. Though well distributed through the year, the
rainfall is more abundant in summer than in winter. This is in
part due to the greater absolute humidity of the air in summer,
and in part to the more frequent passage of lows having their
origin in the southwestern part of the United States. Winter
cyclones more commonly originate in the northwest, and are not so
likely to be accompanied by precipitation. The precipitation in
winter is chiefly in the form of snow, especially in the lake region.

The winds are cyclonic, but less strong than in the Prairie re-
gion, because of the generally forested character and greater un-
evenness of the lands of this region.

The Atlantic-Gulf Slope. This slope, extending from Maine to
Louisiana, presents a great variety of climate. Being near the
sea, neither the extreme cold of winter nor the heat of summer of
regions farther inland is felt; but, being to leeward of the continent,
the equalizing influence of the sea is not nearly so great as upon the
Pacific slope. At the north the winters are cold, and the summers
cool; while at the south the winters are temperate, and the sum-
mers, owing to the excessive humidity, are oppressive, though not
so warm as farther north inland.

Ocean currents are important factors in determining the climate
of the Atlantic Coast. The cold Labrador Current, coming down
the New England coast, makes that coast colder as far south as
Cape Cod, while the Gulf Stream influences the climate of the
coast from Florida northward to Cape Cod.

Rainfall, abundant throughout this climatic region, increases
generally toward the south, where it is more than 60 inches. It is
well distributed throughout the year, though for the greater part
of the region it is most abundant in the fall. Toward the south
the maximum fall is later, in southern Florida being most abundant
in winter, when the westerlies prevail. However, the southern
Florida rains are not of the Pacific Coast type of winter rains,
being mainly due to passing lows, and not to forced ascent over
cold lands.

166

PHYSIOGRAPHY

CLIMATE OF THE UNITED STATES

167

1 68 PHYSIOGRAPHY

South of Cape Hatteras the coast is often swept by tropical
cyclones which reach our coast from the southeast, whereas north
of Hatteras the cyclones are from the west or southwest, and orig-
inate outside the tropics. Both types of storms move, northeast-
ward, their paths converging, thus giving to New York and Bos-
ton a greater number of cyclonic storms than to points either
farther north or farther south.

Exceptional Conditions. In many of his activities man is con-
trolled not so much by usual as by exceptional conditions of
climate. Our buildings are constructed to withstand the strongest
wind, and the levees along our rivers to restrain the highest flood.

As we have seen, profitable agriculture is not so much dependent
upon the annual rainfall as upon the occurrence of rain during the
growing season.

In order to obtain a clear understanding of the climates of the
several climatic regions of the United States, it is necessary to
examine maps showing averages for given periods, and maps show-
ing departures from these averages.

From the January chart of temperatures is seen the wide difference
in the winter temperature of 70 at Key West, and of 5 in North
Dakota. This difference, while in part due to difference of latitude, is
to a much greater extent the result of the difference between coast and
continent-interior conditions. Along the Atlantic coast the change in
temperature is from 70 at Key West to 10 in northern Maine, while
along the Pacific coast, for the same change in latitude, there is a change
if only 10 in temperature. In the contrast shown by these two coasts
is seen the difference due mainly to position to leeward of a continent
and position to leeward of an ocean.

The July chart of temperatures tells a very different story. ' No
longer is the highest temperature found. at Key West, but in Arizona,
some 8 farther north. This is largely due to the arid character of this
region, with its prevailingly clear skies. Along the Atlantic slope there
is a difference of temperature of about 25 between Florida and Maine,
as compared with 60 in January, while on the Pacific coast the difference
for July is about the same as for January, the isotherms, as we see, run-
ning almost parallel with the coast. The interior of the continent, which
is colder than similar latitudes upon the coast in January, is now seen to
be warmer. The isotherms for July bend northward in crossing the con-
tinent, whereas those for January bend southward.

CLIMATE OF THE UNITED STATES 169

While the lines of equal minimum temperature follow, in general, the
trend of the isotherms for January, the lines of equal maximum tempera-
ture are not so regular. We find the lowest minimum, 63, in the
interior of the continent, near its northern boundary, and the highest
minimum, 40, at Key West. Here frost never occurs. The minimum
temperatures of the Atlantic coast are from 20 to 30 lower than those
of the Pacific coast in the same latitudes. Here again is shown the
difference in windward and leeward coasts.

The tempering influence of the sea is well shown by a comparison of
the average maximum of the coasts, about 95 F., with that of the in-
terior, which is about 10 higher. The lowest maxima, about 90, are
found in the extreme northeast and northwest coastal regions, while
the highest, almost 125, occur in the interior desert region of southern
California and Arizona. Maxima exceeding 105 are common over the
Great Plains region, but the dry heat of this region is not so oppressive
as that of the Gulf and Atlantic coasts, where the maxima are 5 lower.

The range of temperature is the difference between the summer maxi-
mum and the winter minimum. The greatest range is found in the
northern interior, whereas the lowest range occurs at Key West. In
general, range of temperature increases with increase of latitude and
with distance from the sea.

The range of temperature along the Pacific coast varies little, being
only 15 greater at Puget Sound than in southern California, whereas
the Atlantic coast varies in range from 50 at the south to 110 in Maine.
The range of temperature for most of the Gulf coast is about 85. whereas
that for Montana is twice as great.

Freezing Temperatures. The number of days with average temper-
ature below freezing varies from none in the Pacific, Gulf and Atlantic
coast regions northward to Chesapeake Bay, to 165 days in Minnesota
and North Dakota. Of much greater practical interest to farmers and
fruit growers, however, are the dates of occurrence of earliest and latest
killing frosts.

In the fall, with the lengthening night and increasing slant of the sun's
rays, there comes a time when the daily minimum falls almost to freezing.
The passage of a low across the continent then is apt to be followed by
frosts. These are due to the cold in-draft of north winds at the rear of
the low, where the sky is clear and the winds light.

The date of occurrence of the first killing frost in the extreme north
central part of the Unfted States is about September first. As the
winter season marches southward, and toward the coasts, the first kill-
ing frosts occur later and later in these directions, being as late as
December i5th in central Florida.

In spring, when the noon altitude of the sun is increasing, and the days

PHYSIOGRAPHY

CLIMATE OF THE UNITED STATES

171

172

PHYSIOGRAPHY

CLIMATE OF THE UNITED STATES

173

174

PHYSIOGRAPHY

CLIMATE OF THE UNITED STATES 175

are lengthening, there comes a time when the ordinary daily minimum
ceases to fall below freezing. But for weeks after this condition is
reached, a passing low, with its cold in-draft of northern winds behind,
may bring freezing temperatures; and thus the time of latest killing
frost be made later. At such times falling temperature and clearing
skies forewarn of frost.

Since spring marches northward and landward from the coasts, the
average time of latest killing frosts is earliest at the south; and in any
latitude, at the coast. It occurs along most of the Gulf coast about
February first, and is delayed in the extreme northern part of central
United States until almost June first.

The absolute date of latest killing frosts is considerably later than the
average date in all sections, being much nearer March first on the Gulf
coast, and July first in Minnesota.

From the accompanying rainfall charts we are able to locate the
regions of greatest and of least rainfall during the year, as well as
the more important matter of its distribution in time. For the
farmer and planter this last is of the greatest importance.

The least rainfall, three inches, occurs in southwestern Arizona.
Most of this amount may fall in a single day, or indeed in a few
hours, during a single thunderstorm. The greatest annual rain-
fall in the United States, over 100 inches, occurs in northwest
Washington, and while most abundant in winter, is fairly well
distributed through the year. The annual rainfall on the Pacific
coast decreases southward, in central California being but half of
the maximum in Washington.

On the Atlantic coast the maximum rainfall is near Cape Hat-
teras, decreasing northward and southward.

A rainfall of two to four inches a month during the growing
season is desirable for agriculture. Occasionally many times this
amount falls, as much as ten inches being recorded in a single day.
Such torrential downpours are injurious alike to growing crops
and to cultivated lands. The soil is washed away, streams are
flooded and overflow their banks, causing destruction of property
and life. These heavy downpours are popularly known as cloud-
bursts.

The recorded rainfall includes snowfall, ten inches of snowfall
being estimated, when melted, as the equivalent of one inch of rain.

PHYSIOGRAPHY

CLIMATE OF THE UNITED STATES 177

Distribution of Snow. Every part of the United States, excepting
southern Florida and southern California, receives some snowfall. It is
least at the south, and increases with latitude and altitude. It is more
than 40 inches in the region of the Great Lakes and in the Rocky Moun-
tains, and occasional heavy snowfalls occur in the extreme south. A
fall of 13 inches occurred at Baton Rouge, Louisiana, in 1895, during a
single storm in February; but such snows usually melt within a day or
two after falling.

The greatest annual snowfall in the lowlands of the United States,
130 inches, occurs in the northern peninsula of Michigan, the moisture
being supplied from the adjacent lakes. The greatest average annual
snowfall of the entire country, not including Alaska, occurs in the Sierra
Nevada Mountains. The moist westerlies from the Pacific, compelled
to rise in passing over the mountains, precipitate, on an average, 378
inches of snow at Summit, California.

The Rocky Mountain region has a heavy annual snowfall, though less
than the Sierra Nevada and Coast ranges. It is mainly to the melting
of these snows in the Rockies that the great irrigation projects look for
their supply of water. The floods in the Missouri and other eastward
flowing streams with sources in these mountains occur in May and June,
when the normal rainfall is augmented by the melting snow.

The snowfall in the northern plains and prairie regions is variable;
some winters it is excessive, others, light. When abundant in the wheat-
growing sections a good crop is expected, since the snow serves as a pro-
tection from the cold, and also leaves the soil in good condition.

In the lumbering sections of the north, from Minnesota to Maine,
the profits of the season are directly related to the snowfall, which is
usually abundant. Little snowfall in these regions means smaller output.

Number of Days with Precipitation. The number of rainy or snowy
days during the year varies widely in different sections of the country.
In general, it is least in the interior, and increases toward the coasts;
and is greater in the north than in the south. The greatest number,
1 80, occurs in northwest Washington; then follows the Great Lakes
region, with 170 days. In the southwest desert region the number falls
to 13. For most of the agricultural sections the number varies from 100
to 140. Forty consecutive rainy days are reported in northwestern
United States, and 150 days of consecutive drouth in the arid region of the
southwest. The more equable the distribution of rainfall during the
year the less the liability to long-continued rains or drouths.

Humidity. The absolute humidity of the air is greater in southern
United States than in northern; is greater in summer than in winter,
and greater near the coast than in the interior.

The relative humidity is on an average lowest on the Colorado plateau,

178 PHYSIOGRAPHY

where it is about 40%, and highest on the eastern and western coasts,
about latitude 40 N., where it is about 80%. In the Gulf region it is
about 75%, and approximately the same in the region of the Great
Lakes, although, as a rule, continent interiors favor low relative humidities.
The percentage of cloudiness agrees well in winter with the relative
humidity, but in summer one of the areas of greatest cloudiness is over
the Colorado plateau, where the average relative humidity is low. This
is probably due to the strong convectional currents set up during the
summer season, the air rising to sufficient heights for saturation.

Winds. As before stated, the winds are stronger upon the
coasts, and over the prairie regions than over forests and moun-
tainous regions. For most of the country the season of strongest
winds is spring; and the month of weakest winds, August.

Aside from tornadoes and hurricanes, during which, for a few
seconds, the velocity of the wind may considerably exceed 100
miles an hour, the strongest winds are about 70 miles an hour
inland, and 90 miles an hour on the coasts.

Though the direction of the wind is variable in all parts of the
United States, in valleys there is a decided up or down the valley
tendency in wind direction. On the coasts, in winter, there is a
predominance of land winds. This is especially true of the Gulf
and Atlantic coasts. On the Pacific coast the meeting of the land
winds and the prevailing westerlies produces " along-shore " winds.
In summer the conditions upon the Atlantic and Pacific coasts are
reversed. The Pacific now has strong ocean winds, while the in-
blowing winds upon the Atlantic coast are met by the westerlies,
producing "along-shore" winds from the southwest.

The winds which bring cloudy weather and precipitation vary
with the section. They are generally winds blowing from the
nearest great water body. On the Atlantic and Gulf coasts, and
over most of the interior of the United States, they are east and
southeast winds, while on the Pacific coast they are generally
southwest winds. In winter, in the northern section of the United
States, snow often accompanies winds from a northerly direction.
Winds from southerly directions, in front of the low, bring higher
temperatures, and yield rain, while the colder winds in the rear
yield snow.

CLIMATE OF THE UNITED STATES 179

QUESTIONS

1. Why is the rainfall of the Pacific coast so much greater in Wash-
ington than in southern California? And why are the rains at the north
less distinctly winter rains than those at the south?

2. Why should thunderstorms be practically unknown upon the
Pacific coast?

3 . Are the Sierra Nevada or the Rocky Mountain ranges more respon-
sible for the arid climate of the Great Basin? Why?

4. Why does the Atlantic coast have so much greater variation in
temperature than the Pacific?

5. From what direction do storms in your section usually come?

6. What direction of wind is usually coldest?

7. What direction of wind is most apt to bring snow in winter and rain
in summer?

8. How does knowledge of your climate concern your daily life and
occupation?

PART HI

THE SEA

CHAPTER XV
GENERAL CHARACTERISTICS OF THE SEA

The Relation of the Sea to the Land. Most of the phenomena
connected with the wearing away of the land, with moderating the
climate, and even with the existence of life itself, depend in large
measure upon the sea. The source of the water supply for the
land is the sea; and the streams with their sediments from the land
return to it.

The sea is a great international highway, and plays an impor-
tant part in the commerce of the world. It is no longer a barrier
between countries. The great steamships are little affected by
storms at sea. Being equipped with wireless telegraph instru-
ments, ships communicate with each other at sea and with land
stations, thus removing the isolation that was formerly experienced
in crossing the great oceans. Countries are connected by sub-
marine cables so that news is sent and business transacted be-
tween nations separated by oceans, almost as easily as between
different parts of the same country. The digging of canals across
isthmuses tends to change routes of travel and commerce at sea.
The Suez Canal has had a far-reaching effect on trade in the Old
World, and the Panama Canal will influence trade routes in the
New.

The surface of the sea is commonly regarded as having a very
nearly uniform level, known as the " level of the sea," from which
land elevations and sea depressions are measured. The sea is
drawn toward and upon the continents that surround it, especially
when large mountain masses are situated near the coast, so that
sea level cannot be of uniform curvature. The actual deformation
of the ocean level in different parts of the earth due to this cause
has been estimated to amount to several hundred feet. On the

1 84 PHYSIOGRAPHY

coast of India, owing to the attraction of the great Himalaya
Mountains, the water stands much higher than water in mid-
ocean or water along a lowland coast, such as western Europe or
that of the eastern United States.

The extent of the sea has not been constant in ages past and is
not now a fixed area. Much of the land furnishes evidence that
it has at some time been covered by the sea, and regions now sea-
bottom have been land. The great central valley of the United
States was once sea floor, there being an unbroken stretch of sea
from the Gulf of Mexico to the Arctic Ocean. On the other hand,
land along the eastern coast of North America has suffered drown-
ing.

Scientific explorations of the sea, made by different governments,
by societies, and by individuals, from time to time, have given
us most of our knowledge of the depth of the ocean, its tempera-
ture, its movements, its deposits, and its life.

Divisions of the Sea. The continuous body of salt water called
the sea, covering about three-fourths of the earth's surface, has five
divisions, called oceans. The polar circles, the continents, and the
meridians from their southern points form the boundaries.

The Pacific is the largest ocean, comprising three-eighths of the
entire sea area. Its greatest width is about 10,000 miles, in a
direction east and west along the equator. It is characterized on
its Asiatic shores by numerous border seas, festoons of islands, and
many rivers; and on its American shores by high mountain ranges
parallel to the shore, and few rivers.

The Atlantic is the second in size, with an area about one-
quarter of the whole sea surface. It has an average width of
3,600 miles. The equator divides both the Atlantic and the
Pacific Ocean into a northern and southern part. The North
Atlantic, both on the American and the European sides, has
many seas and bays which give it an irregular shore line. It has
a wide continental shelf and many rivers. The South Atlantic
has a more even shore line and few good harbors.

The Indian Ocean has an outline that is roughly circular. It
has one-eighth of the total sea area and a diameter of about

GENERAL CHARACTERISTICS OF THE SEA 185

6,000 miles. The Indian Ocean is bordered by large seas and bays,
and a northern and western boundary consisting of very high pla-
teaus and mountains.

The Arctic Ocean is an extension of the Atlantic. It has a
width of about 2,500 miles and about one-thirtieth of the sea area.
A considerable area of the Arctic is covered most of the year with
drifting ice.

The Antarctic Ocean lies within the Antarctic Circle. Within
this region there is a continent covered with an ice cap thousands
of feet thick- The relative area of land and water in this frozen
region is at present unknown.

Distribution of the Ocean Waters. By holding a globe so
that the greatest expanse of water is seen, the island of New Zealand
will be found to be near the center of the water hemisphere, or
what might be called the water pole of the earth. London,
England, will be found to be nearly opposite, and the center or
pole of the land hemisphere.

Depth. The greatest known depth of the ocean is 31,614 feet, in
the Pacific, near the Ladrone Islands. This depth is a little greater
than the height of the highest mountain above the sea level.
Many places in the sea are more than four miles deep, and the
area of surfaces of the sea floor in deep water greatly exceeds the
area of high land. The average depth of the ocean is about 2%
miles, and the average height of land about half a mile. It may
be inferred from this that the continental land masses would
make a small beginning in filling up the deep sea.

Composition of Sea Water. The water of the sea is so salt
and bitter as to be undrinkable. If 100 pounds of sea water are
evaporated, about 3^ pounds of a whitish powder will remain.
About three-fourths of this powder is common salt. The bitter-
ness is due to chloride of magnesia, Epsom salts, gypsum, and
small quantities of almost every soluble substance known. Sea
water contains in addition to mineral matter dissolved atmospheric
gases. Oxygen is more abundant in the water near the surface,
and the proportion of carbon dioxide increases toward the bottom.

i86 PHYSIOGRAPHY

The oxygen dissolved in the water is being consumed by marine life
and its supply is furnished by the atmosphere. The amount of
saltness of the sea varies slightly in different parts of the earth.
Where evaporation is more rapid, as in the trade wind belts, the
saltness of the water is greater, since salts are left behind when
sea water evaporates. When rainfall is abundant, as in the dol-
drum belt, the sea water becomes less salt and of less density.
Rivers bring to the sea fresh water which mixes with the salt
water and makes it of less density.

Temperature. The surface waters of the sea are warmest, as
the water is heated by the sun's rays; and the warmer water being
lighter than the colder water, remains at or near the surface. The
temperature varies from about 80 degrees near the equator to about
29 degrees near the poles. The decrease of temperature with
increase of latitude is far from being regular, the irregularity
being largely due to ocean currents which vary in temperature
from that of the surrounding water.

The surface waters of the sea are alternately warmed and
cooled in both hemispheres, depending upon the season of the
year. At the equator and the poles the seasonal change is slight,
but in middle latitudes it amounts to several degrees. In the
latitude of New York the winter temperatures are usually be-
tween 50 and 60 degrees, and the summer between 60 and 70
degrees.

The temperature of water below the surface falls rapidly with
increase of depth. Even near the equator the temperature at a
depth of less than half a mile is usually below 40 degrees. At the
bottom of the deep sea the temperature is generally below 35
degrees.

The decrease of temperature with increase of depth is not
uniform because of the deep circulation of the ocean water. Be-
cause of currents beneath the surface sometimes warmer and
sometimes colder, slight irregularities in temperature occur. Sea
water, when cooled either by cold air or by melting ice, tends to
sink. The great supply of cold water from the polar regions
creeps along the bottom of the sea and is the cause of the low tern-

GENERAL CHARACTERISTICS OF THE SEA 187

perature in the equatorial as well as in the temperate and polar
regions. The temperature of the deep water in enclosed por-
tions of the sea, such as the Mediterranean, in low latitudes,
never falls to the low temperature of the deep open sea because of
the raised sea bottom in the straits, which acts as a barrier and
keeps out the creep of cold water.

Sounding and Dredging. The depth of the ocean water and
the nature of its bottom are studied both for economic and scientific
reasons. Before submarine cables are laid, suitable routes must
be determined.

Soundings of the deep sea are made by means of a weighted
wire. The weight, called the sounding lead, surrounds a metal tube
and is attached in such a way that when the tube strikes bottom
the weight is released and remains on the bottom. The tube
has a device for bringing up specimens of material found on the
sea bottom. At intervals along the sounding wire specially de-
vised minimum thermometers are attached, which record the
temperature at the various depths reached. It will be seen that
by a single sounding, not only are depths measured, but tempera-
tures at different depths and a sample of deep sea deposit are
obtained.

By dredging, specimens of deep sea life are obtained. A basket
of large dimensions and with a flaring opening is dragged along
the ocean bottom, and various remains and forms of animal life
brought to the surface.

The ocean floor has its mountain ranges, its plateaus and its
plains. There are great volcanic peaks in many places, some of
which rise higher above the sea bottom than any mountain of
the land rises above the platform on which it rests. Dolphin
Ridge is a broad area in mid-Atlantic over which the depth varies
from 5,000 to 12,000 feet, and is bordered on either side by the
relatively steep slopes of great troughs in which the water is from
15,000 to 25,000 feet deep. Chains of islands like Cuba and its
neighbors are believed to be the peaks of submerged mountain
ranges. In these major features the ocean floor resembles the land.

1 88 PHYSIOGRAPHY

The most striking characteristics of the ocean bottom are the
smoothness and the absence of the steep slopes so familiar on land.
Below sea level the slopes of volcanoes and the "abrupt" slope
at the outer margin of the continental masses are rarely steeper
than a rise of one foot in twenty.

There are very few slopes on the ocean floor that would be
considered difficult for an automobile to climb, or that are steeper
than some of the grades on our trunk line railways.

The smoothness of the ocean floor is due largely to the absence
of those agents of erosion, which sculpture the land into hills and
valleys, and also to the accumulation of deposits in depressions.
Between the shore line and the seaward limit of wave action,
waves and shore currents are spreading out land sediments, form-
ing a smooth and nearly level area. Beyond this area deposits of
several kinds are constantly accumulating, and as the deep water
here is practically at rest, the sediments settle, filling depressions
and maintaining a nearly level surface.

It is interesting to study the way chalk settles from a mixture
of prepared chalk and water. This mixture is somewhat similar
to some of the oozes which settle on the ocean floor. We notice
that the surface of the sediments is more nearly horizontal and
more regular than that of the bottom of the vessel. This sort of
action is continually, though slowly, in progress on the ocean floor,
which is gradually approaching a level surface.

The Continental Shelf. Near the borders of the continents the
sediments brought down by streams, and materials worn from the
land by the waves, are spread out by the waves and currents,
forming a gently sloping smooth floor which is called the Conti-
nental Shelf. The continental shelf is, strictly speaking, a portion
of the continental mass rather than a portion of the ocean basin.
It extends seaward to the loo-fathom line, where the slope,
becoming steeper, descends to the bottom of the ocean basin
proper.

The continental shelf is well developed along the eastern coast of
North and of South America, and in places is more than 100 miles
wide. On the western coast it is in most places much narrower.

GENERAL CHARACTERISTICS OF THE SEA 189

FIG. 80. THE CONTINENTAL SHELF OF NORTH AMERICA
After model by Howell.

The British Isles are on the continental shelf that borders Northern
Europe.

There is evidence that much of the area of the continental
shelf has been above sea level. Several of the valleys of large
rivers flowing into the Atlantic may be traced seaward across
the continental shelf by valleys or canyons which were cor-
raded by the river when the continental shelf was a part of the
dry land.

IQO PHYSIOGRAPHY

Materials of the Ocean Floor. The ocean is the great settling
basin of the world. The rivers are constantly bringing in vast
quantities of sediment and lesser quantities of dissolved mineral.
Waves cut into the land and add much to the contribution of the
streams, and a considerable quantity is added by the winds.
The solid matter thus received is assorted, transported, and de-
posited in beds, which may ultimately become sedimentary rocks.
A large part of the dissolved carbonates is taken up by plants
and animals, which change it to some such solid form as coral or
shell, which is eventually added to the deposits of the ocean floor.

Deposits of the Continental Shelf. These consist of sand and
gravel beds, and mud beds. Gravel beds are usually found near
the mouths of rivers or in localities where the wave action is par-
ticularly violent. Sand beds sometimes extend many miles from
the shore. The mud beds are made up of the finest particles and
are located beyond the sand in the open sea or in the quiet water
of bays. Pure limestones are formed in clear water beyond the
mud beds. The deposits on the continental shelf grade into each
other.

Deposits of the Deeper Ocean. Beyond the mud deposits the
only material derived directly from the land, which accumulates on
the ocean bed, is the dust from the air, and this is so small in amount
that it is overshadowed by the organic remains. The waste mate-
rials of the land extend some distance beyond a depth of 100
fathoms, but they gradually disappear and are replaced by oozes
which cover the bottom of the deeper ocean where the depth is less
than two and one-half or three miles. The oozes consist of mi-
croscopic shells of animals that live in the surface waters even in
mid-ocean. When the animals die their shells sink to the bottom,
forming the soft and grayish deposit, known as ooze.

Deposits of the Deepest Ocean. As the depth of the ocean
increases, the percentage of calcareous matter in the deposits
decreases, and at a depth of about three miles the deposit is
chiefly red clay. It seems that at these great depths the minute
shells and other matter of similar composition which form the

GENERAL CHARACTERISTICS OF THE SEA 191

oozes are dissolved before they reach the bottom. The red clay
consists of the less soluble matter which settles from the air as
volcanic ash, and dust from meteors, several millions of which enter
our atmosphere every day. Fragments of pumice and particles
of meteoric iron occur in the red clay, and the insoluble parts of
the bodies of animals living on the surface are relatively abundant.
More than 100 shark teeth and between 30 and 40 ear bones of the
whale have been brought to the surface at a single haul of the
dredge. Since there are but two ear bones in a whale, this proves
that the deposit must accumulate very slowly indeed.

Life of the Ocean. All of the great classes of animal life are
represented in the ocean. Several of the mammals, an order
whose natural habitat is on land, live in the sea, though it is
necessary for them to come to the surface to breathe. Among
them are the whale, porpoise, walrus, seal, and sea lion. No birds
make their permanent home on the sea, but many aquatic species
spend much of their time there. Fish of great variety in size and
form are abundant. Thousands of species of invertebrates of
nearly every order, from the microscopic protozoan to the gigantic
squid, are found in great abundance. Among these are the lobster,
crab, shrimp, oyster, clam, star-fish, and the coral.

Various species of plants occur almost everywhere along the
shore. A few of them, like the mangrove and certain grasses, are
land plants which have adapted themselves to conditions of life
on the beach; but the majority of the plants are unlike those on
the land. Some species of seaweed reach great size, larger than
our tallest trees; but their structure is unlike that of the trees,
and the weight of the solid matter which they contain is only a
small fraction of that of our common trees.

Distribution of Plant and Animal Life. The distribution of the
life of the sea is controlled just as is that of the land, largely by
the climatic conditions of the various parts. The walrus, fur seal,
and narwhal are found in cold, and the corals only in warm waters.
The corals and certain allied species are also limited to the regions
where the water is clear and normally salt; other species, like the

I 9 2 PHYSIOGRAPHY

oyster, prefer brackish water and do not require absolute clear-
ness.

The depth of water controls the distribution of life as effectively
as any other varying condition. Light does not penetrate to depths
much greater than 100 fathoms, and animals and plants requiring
light must develop above this depth.

The temperature of the deep ocean is near the freezing point, hence
some forms of life are excluded. The pressure in the deeps is
so great that other forms are excluded. And finally the motion
of the water is so slight that fixed forms of life, whose food must
be brought to them, are excluded.

For these reasons the great depths of the sea are like the desert
regions of the land in the comparative sparseness of both animal
and plant life. Such animals as there are have strange forms;
some of them have eyes, but others are blind. Some of the forms
probably emit phosphorescent light which enables them to see
and to be seen. There are no plants in the very deep sea.

It has been claimed that the life of the sea, as a whole, exceeds
that of the land, equal areas being compared. It is doubtful,
however, if life is as abundant in any portion of the sea as it is
on the more fertile portions of the land. The surface waters every-
where abound in life. Many species and many individuals of each
species occur; but both the number of species and the number of
individuals is greater between the 100 fathom line and the shore
line than elsewhere.

Ice in the Sea. Sea water ordinarily freezes at a temperature between
26 and 28 F., depending upon the saltness of the water. In the higher
latitudes ice forms along the shores and also on the deep sea, often to a
thickness of eight or ten feet.

The ice formed in winter is usually broken in pieces in the summer.
These floating pieces, called field or floe-ice, are often crowded and jammed
together into an ice-pack, which, because of the lateral pressure, is raised
considerably above the water. The sea ice may be driven upon the land
by waves and tides and become twenty feet or more thick by accumula-
tions of snow. Rock fragments from overhanging cliffs and from the
imbedding of rocks along the shore, gather upon and in this ice of the
shore known as an icefoot. In winter the grinding of the ice foot up and
down the shores smooths and rounds the rocks of these coasts, In the

GENERAL CHARACTERISTICS OF THE SEA

193

summer it breaks up and scatters the rocky material, often long
distances.

Glaciers entering the sea from the land in both polar regions break at
the shore and send off larger masses of ice, known as icebergs. Some
icebergs are a mile or more in length, and have been known to rise 500

FIG. 81. ICE-BOUND SHORES (SHADED), AND LIMITS OF DRIFTING ICE IN NORTHERN WINTER
(BLACK LINES). DOTTED LINES, LIMITS or DRIFTING ICE, NORTHERN SUMMER

feet above the water. As ice is nearly as heavy as water, the greater
part of the floating iceberg is below the surface of the water. The relative
heights above and below are on the average about i to 8. The chief
work of an iceberg is to transport material in the form of bowlders and
glacial pebbles, dropping them on the sea bottom in the warmer and more
open seas.

QUESTIONS

1. Where is the great water supply for watering the land? What
other advantages does the land receive from the sea?

2. Name the boundaries of the different oceans. Compare the Arctic
and Antarctic oceans in respect to area.

3. Calculate roughly the number of cubic miles of water in the At-
lantic ocean. How does this compare with the volume of the land mass
of North America?

4. What mineral substances and gases are dissolved in sea water?
How much common salt in a hundred pounds of sea water? What
causes the sea water to change in density in different localities?

I 9 4 PHYSIOGRAPHY

5. Describe the distribution of the surface temperature of the sea in
different latitudes. Compare the temperature at the surface with that
at the bottom of the sea, both in the higher and lower latitudes. Ac-
count for the striking difference in the equatorial regions.

6. How are temperatures of the deep sea determined? How are
soundings made? What is the object of dredging?

7. Compare the ocean floor with that of the land. Account for dif-
ferences. What is a continental shelf? About how wide are continental
shelves, how deep is the water upon them, and what purpose do they
serve? What causes tend to change the area of continental shelves?

8. What is the character and source of ocean bottom materials?
How do the deposits differ in different localities? What conditions
determine the distribution of animal and plant life in the sea. Point
out specific examples.

9. Locate the two ice caps of the earth. Under what conditions and
how is the ice formed? What is the difference between floe ice and ice-
bergs? What effect does ice in the polar region have upon the land?

CHAPTER XVI
MOVEMENTS OF THE SEA

The most important movements of the ocean are: (i) waves;
(2) tides; and (3) currents.

WAVES

A gentle breeze causes ripples to form on the surface of water
over which it blows; a strong wind changes these ripples into great
waves. During the passage of a wave each particle of water af-
fected rises and falls and moves forward and backward, describing
a curved path in a vertical plane. The forward motion of the

3ea Level

FIG. 82. DIAGRAM OF WAVE, SHOWING MOVEMENT OF WATER PARTICLES

Particle 3 is going backward, 7 forward, 5 upward, and 9 and i downward.

From i to 5 the particles of water are going backward in the trough, from 5 to 9 forward on

the crest, from 3 to 7 upward on the front, from 7 to 9 and from i to 3 downward on the

back.

What two motions combined has each of the following: 2, 4, 6, and 8?
How long and how high is this wave ?
In what direction is the wave form advancing ?
If this wave should run ashore, would the water at the shore advance first or recede first ?

water is most rapid in the ridge or crest of the wave, and the back-
ward motion is most rapid in the furrow or trough. The forward
motion is slightly in excess of the backward motion. Because of
the excess of forward over backward motion of the water particles,
when the winds are long continued in the same direction, currents
are produced which flow in the same direction in which the wind
blows. On the front of the wave the water rises, and on the back
of the wave the water falls. As waves move new water enters in
front and leaves on the back of the wave.

196 PHYSIOGRAPHY

Height and Length of Waves. The horizontal distance from
the crest of one wave to the crest of the next is the length, and
the vertical distance between the crest and the bottom of the
trough is the height of the wave. The height and force of the
waves depend upon the force of the wind, the length of time the
wind continues to blow, the depth and breadth of the water, and
the form and direction of the coast line.

Ground-Swell. In the open sea during a gale waves are often
30 to 40 feet high, and have a length of a thousand feet or more.
High waves often pass out from an area of storm-winds into a
region of gentle winds many hundreds of miles away. They be-
come of less height, but keep their velocity and length. These
waves that have outrun the storm which started them, and per-
sist after the storm, are known as the ground-swell.

Breakers. When a wave approaches a gently sloping shore
the wave length is diminished, and the wave height is increased.
The front of the wave, because of a lack of water, becomes steeper
than the back; and as the wave continues to move into water of
less depth the crest curls and falls forward, forming a line of
breakers. At the line of breakers on a sandy shore a sand bar
is formed. Rocks or bars near the surface of the water may
also be located by breakers. Thus breakers are a warning of
danger.

Surf and Undertow. When the waves run into shallow water
and break near the shore, surf is formed. The water that is then
thrown forward in the crest of the waves returns as a current along
the bottom. This backward under-current along the bottom of
a shallow sea, due to waves and surface currents produced by the
wind, is called the undertow. When the waves reach the shore
obliquely, a current along the shore is produced.

THE WOPK OF THE WAVES

Pounding of the Waves. Waves are agents of erosion; that is,
they break and grind the material along the shore and transport
it varying distances from the shore.

MOVEMENTS OF THE SEA

197

The work of breaking and grinding is done by the fall of the
breakers upon the shores. In summer, in the Atlantic the average
blow of breaker is about six hundred pounds on every square
foot of surface. In winter the force of the breakers may be as
high as 3,000 pounds per square foot. The impact or pounding
of the waves on the shores is made effective by the sand, the
pebbles and such rock fragments as the waves are able to move.
Driven by the force of the waves, they serve as tools for cutting
and grinding, and become rounded by acting upon each other.

FIG. 83. A SEA CAVE

Weak rocks exposed along the shores are broken down and re-
moved. The more resistant rocks are loosened by undercutting,
and because of the joints and seams in the rocks fall as angular
blocks. These angular blocks in course of time become reduced
in size and rounded. Large masses of rock, too large at first to be
moved by the waves, are reduced by smaller fragments driven
against them until the waves are finally able to use them also as

198 PHYSIOGRAPHY

weapons of attack. Thus huge masses of rock are reduced in
turn to cobbles, pebbles and sand, and finally to the finest mud
particles, which may be carried away by the undertow.

Sea-Cliffs and Sea-Caves. The cutting of the waves at the
water level may be compared to a horizontal saw. As the waves
cut into the shore the unsupported material often falls, leaving a

FIG. 84. THE ACTION OF WAVES, SHOWING TENDENCY TO FOLLOW JOINTS IN THE ROCKS

steep face known as a sea-cliff. If the sea-cliff is a wall of rock,
and the waves continue undercutting at the base, a sea-cave may
be formed.

Sea Arches and Chimney Rocks. If the wearing away of the
roof continues, the remaining portion may form an arch or bridge.
Sometimes the waves remove block after block of rock along cer-
tain joints, so that a column or pillar of rock may be isolated from
the shore. These are then known as chimney or pulpit rocks.
The " Old Man of Hoy," on the coast of the Orkney Islands, is
an example.

Small irregularities in the shore line develop because of differences
in the resistance of the rocks, and in their exposure to the attack of
the waves; but as a rule the action of waves and shore current

MOVEMENTS OF THE SEA 199

tends to make the shore line more regular; the projecting head-
lands are worn away and bay heads are filled.

In certain places waves wear away the land and deposit the
material in the sea at a lower level. The rock fragments, pebbles,
and sand formed at the shore are ground finer and carried away
by the combined action of waves, undertow, and along-shore cur-
rents.

Deposition by Waves, Undertow and Shore Currents. In other
localities material is brought in from the sea by the waves and
deposited on the shore within the zone of wave action, and forms
the beach. When the material carried out by the undertow
meets that brought in by the waves, an accumulation begins at
the place of meeting. A low ridge called a barrier is formed, and
its position is shown by the line of breakers. Such barriers are
often built up to and above the surface of the water, making a
sand reef.

The free end of a beach or a barrier is called a spit. The de-
posits along the shore depend largely upon the shore currents.
The growth westward of Rockaway Beach, on the southern shore
of Long Island, is due partly to along-shore currents in that
direction.

The growth of shore deposits tends to fill up bay entrances and
interfere with navigation. At the entrance to New York harbor
dredging is necessary in order to deepen the channels through
;vhich the largest boats pass.

TIDES

Tides Defined. Along the shores of the ocean and its gulfs and
bays the water rises slowly for about 6 hours and 13 minutes, and
then falls slowly for about the same time, making on an average
12 hours and 26 minutes from high water to next high water, or
from low water to next low water. This periodic rise and fall of
the level of the sea twice in every 24 hours and 52 minutes constitutes
the tides.

This makes the hour of high water at any particular place vary
from day to day. If it is high water at the ocean shore this after-

200 PHYSIOGRAPHY

noon at 4 o'clock, the next high water will occur again 26 minutes
past 4 to-morrow morning, and high water again 52 minutes past
4 to-morrow afternoon, and so on.

Variation in Tidal Range. The amount of rise and fall is
greater along most continental coasts than in mid-ocean, and
greatest in bays with broad openings to the sea and narrow toward
their heads. The tidal range at Key West, Florida, is usually
not more than two feet, while in the Bay of Fundy it is often
more than 50 feet.

The amount of the rise and fall of the sea at any particular place
also varies. The tidal range may increase from day to day for
about a week and then decrease for the same period, making a
maximum and minimum range twice a month. At Governor's
Island in New York Harbor the tidal range may be as small as
3.4 feet, and as great as 5.3 feet during a single week.

Flood and Ebb Tides. The change of level of the sea is accom-
panied by tidal currents called the running of the tides. When
the tide is running from the open ocean into bays, it is flood or
incoming tide; and when the tide runs to the open ocean again,
it is the ebb or outgoing tide. During the few minutes when the
flood tide changes to ebb tide or ebb to flood slack water occurs.

Tidal Races. When the tidal currents pass through a strait,
such as a narrow inlet into a bay or between an island and the
mainland, the currents often run many miles an hour. Such
currents are called tidal races, and are often so strong as to inter-
fere with navigation. The tidal currents " race " through Hell
Gate, the narrow passage from the East River into Long Island
Sound, at the rate of five or six miles an hour.

Tides in Rivers. The tidal wave often runs up rivers to a point
many feet above sea level. The tide runs 1 50 miles up the Hud-
son River to Troy, five feet above sea level, where the tidal range
is more than two feet. The tide is felt 70 miles up the St. John
River in New Brunswick, where the elevation is fourteen feet above
sea level; and at Montreal, 280 miles up the St. Lawrence River.

MOVEMENTS OF THE SEA 201

The action of tidal currents in narrow rivers is very different
from the action of tidal currents on open seacoasts. In rivers,
when the water stands above the average level, the tidal current
flows up-stream along with the tidal wave, and when the water
stands below the average level the tidal current flows down-
stream, opposite to the direction of the tidal wave. Since the rate
of flow depends upon the difference in level, the flow is most rapid
at high and low water instead of being slack water at these times,
as on open coasts. Hence the tidal current flows up-stream for
some time after high water has passed and the water level is
falling; and the tidal current flows down-stream for some time
after low water is reached and the water level is rising. In broad,
deep mouths of rivers, slack water does not occur at high and
low water as on open coasts, nor at average level as in narrow shal-
low rivers, but at some intermediate level.

Tidal Bore. In the estuaries of many rivers broad flats of mud
or sand are nearly exposed at low water. The tidal wave when
entering these rivers often rises so rapidly that it assumes the
form of a wall of water. Such a wave is called a bore. Tidal
bores occur in some of the rivers of China, where in one case the
bore travels up the river at every high tide, often reaching a height
of twelve feet. After the bore has passed, an after-rush often
carries the water up several feet higher.

Bores have been observed on the Severn in England, on the
Seine in France, on the Amazon in South America, and on a few
other rivers of the world.

Causes of the Tides. Since Newton announced the law of uni-
versal gravitation it has been generally recognized that the tides
result from the attraction of the sun and moon. The tide-pro-
ducing forces of sun and moon can be computed with reasonable
certainty, but because of the modified effects due to local condi-
tions an agreement between theoretical and the actually observed
tides is not easily secured. Although the moon's mass is only a
small fraction of the sun's mass, the moon's nearness to the earth
makes it, rather than the sun, the principal cause of the tides.

202 PHYSIOGRAPHY

First Law of Motion. A body in motion will move in a straight
line unless deflected from its straight path by some external force.
This law of motion may be illustrated by whirling a stone around
the hand by means of a string. The natural tendency of the stone,
at each instant, is to move in a straight path. It is deflected and
moves in a curved path because of a pull or force, called centrip-
etal force, exerted by the string acting inward upon the stone.
The stone resists being pulled inward and so tends to move out-
ward, and exerts a pull or force upon the hand called centrifugal
force. The string being under tension when the stone is whirled,
is subject to equal and opposite forces, one acting toward
(centripetal), and the other away from (centrifugal), the center
of revolution.

Balance between Centripetal and Centrifugal Forces. The

revolution of the moon about the earth is illustrated by this simple ex-
periment. The invisible force called the gravitation which acts between

FIG. 85

the moon and the earth replaces the centripetal force exerted by the
string that holds the stone to the hand. The moon whirls about the
earth with sufficient velocity and at such a distance that her resistance
to curved motion, or centrifugal force, just equals and balances the at-
traction between the earth and moon.

Center of Gravity. The moon does not revolve about the center
of the earth, but about a point 3,000 miles from the center, or 1,000
miles below the surface. This is because the earth is eighty times as
heavy as the moon and the centers of the two bodies are 240,000 miles
apart.

MOVEMENTS OF THE SEA 203

This may be easily illustrated by balancing two balls, one eighty times
the weight of the other and connected by a slender rod. The place where
they balance, called the common center of gravity, will be one-eightieth
of the distance from the center of the larger ball to the center of the
smaller.

Revolution About Common Center of Gravity. The common cen-
ter of gravity of the earth and moon is at C. The big and little balls cor-
respond to the earth and the moon, and the stress in the rod represents the

^Common center of gravity
of earth and moon

FIG. 86

attraction that holds the earth and moon together. Both the earth and
the moon revolve about this common center of gravity, C, in about 28
days, and in so doing the earth's center describes a circle with a radius
of 3,000 miles.

The daily rotation of the earth, which is not now being considered,
must not be confused with the revolution of the earth, without angular
turning, about a point 1,000 miles below the earth's surface. Only the
earth-moon revolution about C without rotation of either body is here
considered. When a body revolves about another without rotation, a
given side always faces the same direction in space.

Revolution without Rotation. It may be stated as a general prop-
osition that whenever an object revolves without rotation, every particle
of the object describes a path the size and shape of that described by a
particle at the center of the object. The motion of the different particles
of a connecting rod attached to the driving wheels of a locomotive
illustrates this action.

All parts of the earth then must be subject to equal and parallel
centrifugal forces, due to the monthly revolution of the earth and moon
about their common center of gravity. These forces act in a direction
away from the moon. The tota.1 of centrifugal forces acting on the earth
is just balanced by the total centripetal force due to the moon's attrac-
tion, although it is evident that the two opposite forces acting on any
single particle are only equal at the center of the earth.

204 PHYSIOGRAPHY

Unequal Attraction of the Moon in Different Parts of the Earth.

The moon's attraction for the earth is always toward the moon, but
is not equally distributed, for the attraction on the side of the earth
nearest the moon is stronger than at the center, and on the side of the
earth farthest from the moon weaker than at the center.

Resultant of Two Opposite Forces. In the figure, A B C D, repre-
senting the equator of the earth, A is a particle farthest from the moon;
C a particle nearest to the moon, and E a particle at the center of the
earth. The arrows of equal length, extending to the left away from the
moon, represent the equal centrifugal forces; and the arrows of unequal

FIG. 87. OPPOSITE FORCES

lengths, extending to the right toward the moon, represent the unequal
value of the moon's attraction at these points.

When two forces act in opposite directions at the same point, the
effectiveness or resultant of the two forces is found in a force equal to
the difference between the two and acting in the direction of the greater
force.

At C the moon's attraction is greater than the centrifugal force at that
point, so that the tide-producing force, which is the difference or resultant
between these forces, acts toward the moon and causes the water on the
side of the earth toward the moon to bulge out toward the moon.

At A the moon's attraction is less than the centrifugal force, and the
tide-producing force consequently acts away from the moon, and causes
the water on the opposite side of the earth to bulge out away from the
moon.

At E the moon's attraction and the centrifugal force are equal and
opposite. If they were not, the earth and moon would either approach
or recede from each other.

These two bulges of the ocean are the two high tides, and midway
between them is the low tide zone.

The magnitude and direction of the resultant or tide-producing forces
acting at different points on the earth's equator are shown in Figure 87.

MOVEMENTS OF THE SEA 205

Effect of Rotation of the Earth. The daily rotation of the earth
from west to east constantly carries the high and low tide west-
ward around the earth, and brings places alternately to high tide
and low tide positions.

The tidal movements are interfered with by the continents
which tend to stop or change the direction of the tidal wave.
The tidal wave travels faster in the deep ocean than in the shallow
water near the continents. The tidal waves are also interfered
with by the strong winds and changes of atmospheric pressure.
Their advance in different parts of the ocean becomes so irregular
that they often interfere with one another. This explains in some
measure why the actual local tides in so many places fail to agree
with the general theory.

The Establishment of the Port. The rotation of the earth tends
to carry the tidal waves forward in the direction of the rotation.
The moon tends to hold the tidal waves back. The result is that
the tides are said to lag. The interval of time between the pas-
sage of the moon across the meridian and the next high tide,
mariners call " the establishment of the port." The establish-
ments of different ports have various values. The port of New
York has a value of 8 hours and 13 minutes.

Cause of Solar Tides. The explanation of solar tides is analo-
gous to that of lunar tides. Since the cause of lunar tides is the
difference between the moon's attraction and centrifugal force
in different parts of the earth, in like manner solar tides are
due to the difference between the sun's attraction and cen-
trifugal force in different parts of the earth, caused by the earth
moving about the common center of gravity of the earth and
the sun.

Effect of Solar upon Lunar Tides. The intensity of the tide-
producing force due to the sun is about half of that due to the
moon. Since the lunar tides are stronger than the solar tides,
the solar tides may be said to modify them, that is, to strengthen
the tides when sun and moon act together, and to weaken them
when they oppose each other.

206 PHYSIOGRAPHY

Twice a month, at times of new and full moon, the lunar and
solar tides fall together, producing a higher tide than usual. This
condition of greatest range is called spring tide. At first and last
quarters of the moon the solar high tide falls at lunar low tide,
and solar low tide falls at lunar high tide. The effect of this is
to lessen the tidal range, that is, the high tides are not so high
and the low tides are not so low as usual. This condition of least
range is called neap tide.

The relative ranges of spring and neap tides may be shown
graphically by the construction of tide curves for any station.
The data for these tide curves may be found in tide tables pub-
lished by the Government.

The tides in any latitude vary with the changing angular dis-
tance of the moon and sun north or south of the equator, as well
as with their changing distances from the earth.

Inequality of Tides. The two successive high tides of a given
place are usually of unequal height. They are of equal height only
when the moon is over the equator, and as this occurs on only two
days of the month two weeks apart, the two successive high tides
are usually unequal. 'The maximum inequality of successive high
tides occurs when the moon is farthest north or south of the equa-
tor. This variation at some places amounts to several feet.

Maximum Yearly Tide. The conditions that favor the greatest
tidal range in any particular harbor are: (i) new or full moon; (2)
moon and sun nearest to the earth; (3) moon and sun's zenith
distances approximating the latitude of the place affected; (4) wind
direction favorable to direction of tidal movement.

Effect of Tides. The erosion caused by tidal currents is known
as tidal scour. The tidal scour of the flow and ebb of the tide
maintains inlets in barrier reefs along many shores. An example
of this may be seen in the sand reefs along the shore of New
Jersey. Tidal scour also often maintains deep waterways in some
bays to the advantage of navigation; whereas at the entrance to
other bays the tidal currents tend to fill, making the water shal-
low, and because of shifting of deposits are dangerous to naviga-

MOVEMENTS OF THE SEA

207

208 PHYSIOGRAPHY

tion. Strong tides hinder the formation of beaches across the
entrance of some bays.

The tidal currents cause a circulation of water in bays and har-
bors which prevents stagnation and helps to remove the sewage that
is drained into them near cities. This circulation of water aids or
hinders boats, according to their direction, and sometimes drifts
vessels out of their course and subjects them to danger of rocks and
shoals, especially in times of dense fogs.

Tidal currents transport material along shore from more ex-
posed positions, such as headlands, to the less exposed position
at the heads of bays. This filling of bay heads tends to straighten
the shore line.

CURRENTS

Every continent is washed by ocean currents, and every ocean
has its distinct circulation. Currents from equatorial regions
carry warm water into polar regions, and other currents carry the
cold polar waters into lower latitudes.

While each ocean has its separate circulation, yet the separate
schemes of circulation fit into the general scheme as cog-wheels
in a vast machine.

The Pacific Ocean, which for most purposes is considered as one
ocean, is by reason of its circulation divided into two distinct
parts, the North Pacific and the South Pacific. The Atlantic and
Indian oceans lying, like the Pacific, on both sides of the equator,
are also divided into northern and southern oceans by reason of
their distinct circulation.

Systematic Movement. Ocean currents, like air currents, obey
Ferrel's Law, in that they turn to the right of a straight course in
the northern hemisphere, and to the left in the southern. This
results in a distinct eastward drift about the margin of the south
polar ocean, and a less distinct eastward movement about the
Arctic Ocean. In other oceans the northern divisions have a
clockwise circulation, whereas the southern divisions have their
circulation counter-clockwise.

The movement of the waters in all oceans is chiefly about the

120

OCEAN CURRENTS

Warm Currents F^ O>/d Currents |~~|
Zft (firertiow o/tA Current* i thoum by the

150

120 Longitude 90 West from 60

South

' x -
9X7 E A N

^""Tropic of Capr

B WORLD

SHOWING

AN CURRENTS

NOTE

In the Indian Ocean, the China Sea and the Wett Coatt of Mexico
and Central America, the Currento eJumgemth the Momount

30 Longftudo East from 90 Qreenwlod 120

MOVEMENTS OF THE SEA 209

margins, leaving the great central areas undisturbed. In these
areas of quiet water seaweed and other floating matter accumu-
lates, thus producing what are known as Sargasso Seas. These
seas are avoided by masters of sailing vessels, who find it difficult
to get out of these drift-covered waters when driven into them
by storms. Columbus thought, when he came to the Sargasso
Sea in the Atlantic, that he had come upon land.

Causes of Currents. Anything that produces a disturb-
ance of the level of the ocean surface will, at that place, cause
currents.

In the trade wind belts the hot dry winds cause a slight lower-
ing of the surface by evaporation, and there is a natural tendency
for the waters to flow in both from the north and the south.

In the doldrum belt, the excessive rainfall slightly raises the
surface of the sea, and the water flows out to north and to south.

Great storms, as at Galveston, pile the water up against the
land, often with great destruction, and the return of the sea to
its normal level produces local currents.

Differences of temperature, while effective in producing vertical
currents when the heavier water is at the top, can produce hori-
zontal movement only when by reason of these differences of
temperature the surface of the sea is raised or lowered. This may
cause a slight raising of the surface at the equator or a slight
lowering of the surface at the poles.

While these various causes may produce local currents, they
do not account for the systematic circulation of the oceans.
There remains to be considered the all-sufficient cause, the winds.

Origin of Ocean Currents in the Trades. All winds, however
fitful, brush the surface water along with them. If they constantly
vary in direction, no systematic or continuous currents can result.
When the same direction is held for several days, a distinct drift
with the wind is observed.

Continued east winds over Lake Erie have at times so heaped
up the water toward the west end that Niagara Falls have prac-
tically run dry. We are told, too, that strong east winds some-

210 PHYSIOGRAPHY

times drive the waters back from one of the northern arms of the
Red Sea, and make it possible to cross this basin " dry-shod."

It is only in the trade wind belts that we find winds blowing
continuously from the same direction; and we are disposed to
look upon these belts as the birthplace of ocean currents. Here
the direction of the ocean currents agrees with that of the trades,
and neither difference of density nor difference of temperature
can have any part in producing this westward movement of the
ocean waters. These are the north and south equatorial currents.

Poleward Currents. The equatorial currents are barred in
their westward movement by islands and continents across their
paths. They are thus forced to turn poleward along the western
shores of the oceans. Whether they turn northward or southward
is determined by the outline of the coast.

While the currents are moving along and near the equator, the
earth's rotation has but slight deflecting influence; and it is prob-
able that, if not interrupted by land barriers, the equatorial cur-
rents would continue their westward course around the earth.

As soon, however, as they begin to flow into other latitudes, the
rotation of the earth is effective in turning them from a straight
course to the right in the northern hemisphere and the left in the
southern.

These poleward currents are warm currents, and carry the warm
water from the equatorial regions into colder latitudes. At the
same time they spread out, lose their velocity, and are then known
as drifts, which move to the margins of the polar oceans, then
eastward to the eastern shore of the ocean in which they have
their origin.

Equatorward Currents. By continued deflection, these east-
ward moving currents, now cooled from loitering in high latitudes,
are turned toward the equator along the western coasts of the
continents. Returning thus to the trade wind belts, in which
they assume their westward direction, the circulation about the
ocean is complete.

The equatorward currents are cold or cool currents, and bring

MOVEMENTS OF THE SEA 211

lower temperatures toward or even to the equator, causing the
eastern sides of equatorial oceans to be cooler than the western
sides.

The movement of currents about the Arctic Ocean is less syste-
matic than about the Antarctic, because of the numerous islands
in the north that interrupt. Branches from the circumpolar move-
ment in the north are sent off southward into the Pacific and the
Atlantic. These cold current^ deflected to the right, follow closely
the eastern coasts of Asia and North America, until they sink
beneath the warm currents between the parallels of 40 and
50 N.

Creep. Their further journeying toward the equator is known
as creep. In this way the cold polar waters are carried even to
the equator, and the low temperatures of deep equatorial seas
are accounted for. We cannot observe the creep, but as more
surface water is carried into polar regions than returns as surface
currents, the excess must be equalized by under-surface return
currents.

Monsoon Currents. If any doubt existed as to the sufficiency
of the winds to produce ocean currents, that doubt would be
removed by a study of those currents which change their direc-
tion with the change of direction of the monsoons.

While there are monsoons at the Horse Latitudes, the winds
there are of neither sufficient strength nor constancy to be effective
in producing ocean currents. It is in the monsoon belt over
which the heat equator migrates that we find conditions favorable
for the production of ocean currents.

About the northern Indian Ocean, when the southwest monsoon
blows, the water is set drifting in a clockwise direction. As these
winds weaken, this drift slackens; and soon after the northeast
monsoon begins, the direction of the drift is reversed. It con-
tinues as a counter-clockwise circulation while the northeast
monsoon continues, changing again to the clockwise direction with
the return of the southwest monsoon. These changes of direction
of the ocean currents can be accounted for only by the reversal
of the winds.

212 PHYSIOGRAPHY

In the Pacific Ocean, where the heat equator lies prevailingly
north of the terrestrial equator, the southeast trades, changed to
southwest winds north of the equator, set up an ocean drift to
eastward. This is the Equatorial Counter Current. It is fairly
distinct throughout the year, though better developed during the
northern summer. Its explanation is the same as that of the
clockwise movement about the northern Indian Ocean during the
southwest monsoon.

Because of the narrowness of the Atlantic Ocean at the equator,
the counter-current is not so well developed as in the Pacific.

Currents and Navigation. Sailing vessels lay their courses to
suit the winds and ocean currents; and even steamships do not
scorn to take advantage of the great ocean circulation.

Sailing vessels from New York to English ports take advantage
of the northeast Atlantic drift; on their return they use the
trades. Those bound from New York to Rio Janeiro must lay
their courses far to eastward of the eastern cape of South America,
lest the equatorial currents carry them northward again while in
the doldrums, where winds are apt to fail.

Ships sailing from Atlantic ports for Australia sail eastward
around the Cape of Good Hope, to take advantage of the Antarc-
tic drift; while those returning also sail eastward past Cape Horn,
to have the advantage of the same drift.

Vessels bound from Honolulu to San Francisco sail northward
beyond the trades and equatorial current, then east; returning,
they take a more southerly route.

Currents and Life. The distribution of many marine forms is
determined by the temperature of the water, which in turn is in
part determined by ocean currents. Corals serve well to illustrate.
The waters about the Galapagos Islands are too cold for corals,
although these islands are situated upon the equator. The cold
Peruvian current makes these waters cold. Contrasted with these
are the Bermudas, in latitude about 35 N., which are largely
composed of coral rock and bordered by coral reefs. The warm
waters are brought to these islands by the Gulf Stream.

MOVEMENTS OF THE SEA 213

The seeds of many plants are distributed by means of ocean
currents; and insects and the smaller animals are carried upon
drifting materials in these currents.

Currents and Climate. The direct climatic influence of ocean
currents is confined to the ocean and immediately bordering
lands. Indirectly their influence may be felt hundreds of miles
inland. This is markedly true of lands lying to leeward of cur-
rents that are abnormally cold or warm.

The North Atlantic Drift, the continuation of the Gulf Stream,
is perhaps the most pronounced and far-reaching of all ocean
currents in its climatic influence. The winds from over this broad
sheet of warm water not only bring abundant rainfall to the
British Isles and Norway, but so temper the cold of these high
latitudes as to make them comparable in temperature to our
own eastern coasts, twenty degrees farther south.

The North Pacific Drift, the continuation of the Japan Current,
tempers the climate of Alaska and British Columbia in like
fashion.

These great drifts, in both oceans, continue or send branches
southward along the western coasts of the continent; and when
they reach the latitude of northern Mexico and Africa, their
effect is to temper the heat of these coasts.

The cold currents that follow closely the eastern coasts of
North America and Asia, being to leeward of those continents,
do not affect the climate so far inland. However, the bleakness
of Labrador and Kamchatka is in some degree traceable to these
currents.

In the southern hemisphere the western coasts are cooled and
the eastern coasts warmed by the ocean currents; but their influ-
ence is less pronounced than in the northern hemisphere.

Currents and Harbors. The harbor of Hammerfest, at the
north of Norway and well within the Arctic Circle, is about as
free from ice as that of Boston, 30 farther south. In the one
case we see the effect of the warm North Atlantic Drift; in the
other, of the cold Labrador Current.

214 PHYSIOGRAPHY

In the Pacific Ocean the barrier of the Aleutian Islands, to-
gether with the narrowness of Bering Strait, prevents the North
Pacific Drift from entering the Arctic Ocean. As a result, the bays
on the north coast of Alaska, in the same latitude as Hammerfest,
are practically closed by ice throughout the year.

The Russian- Japanese War had for one of its objects the secur-
ing for Russia of the open harbor of Port Arthur. The harbor of
Vladivostock, Russia's chief port on the Pacific, in about the
latitude of New York, is for a long time every year closed by
ice, owing to the cold current coming down through Bering Strait.

The Gulf Stream. This greatest and most important of all
ocean currents derives its name from the Gulf of Mexico, from
which it issues. It is in fact a continuation of the combined
equatorial currents.

The North Equatorial Current in the Atlantic is turned by the
land masses in its path wholly into the northern division of this
ocean. Much of its waters pass among the islands of the West
Indian group, while the remainder passes to the eastward.

The eastern cape of South America is so situated that it divides
the South Equatorial Current in two, part of it turning southwest
along the coast of Brazil as the Brazilian Current, while the other
part enters the Gulf of Mexico between the West Indies and the
mainland of South America. This water issues through the Strait
of Florida as the Gulf Stream. It is truly a stream, flowing between
banks of water. It is there deep and narrow, scouring the bottom
of the strait, and flows with a velocity greater than that of the
lower Mississippi River.

Joined by the waters that come through the West Indian group
of islands, and that which passes outside, the Gulf Stream is
greatly increased in volume. It passes parallel to and near enough
to the Carolina coasts to send off return eddies, which build the
Carolina capes. Spreading and decreasing in velocity, the Gulf
Stream becomes the North Atlantic Drift.

The frequent and dense fogs off Newfoundland are produced by
warm winds from the North Atlantic Drift, blowing over the cold

MOVEMENTS OF THE SEA 215

Labrador Current. The line of meeting of the cold and warm
waters is known as the cold wall.

QUESTIONS

1. Tides resemble waves in many respects. High and low tides
correspond to what parts of the wind wave? Tidal currents correspond
to what phenomenon of the wind wave? The change in tidal range,
the velocity and form of the tidal wave as it advances in shallow water
on the continental shelf and into bays may be compared to what changes
in the wind wave as it moves toward the shore? Compare the height
and length of wind waves with that of tidal waves.

2. Is sea-sickness more likely to occur on large or small boats? Why?
What is the difference between surf and a breaker? What work is done
by breakers and the undertow? Why are breakers a warning of danger?

3. Explain how the waves act as a horizontal saw cutting into the
land. What are some of the shore features resulting from wave action?
What effect has these features upon the value of harbors and shore
property?

4. How would a thoughtful person living at the shore for any
length of time naturally connect the cause of the rise and fall of the sea
with the moon?

5. Explain how navigation is affected by (a) Tidal range; (b) Flood
tide; (c) Ebb tide; (d) Tidal races; (e) Tidal bores. How do you
think the state of the tide affects fishing?

6. How can one moon cause two daily tides, or in other words, what
is the cause of a high tide on the side of the earth opposite to that of
the moon?

7. Which has a lower low water, a spring or a neap tide? Explain.
How often does the moon cross the equator? What effect has this on
the height of the two daily tides?

8. What effect has tidal scour upon waterways, inlets, and tidal
streams? What is the general effect of tides upon the water and shores
in and about bays and harbors?

9. What is an ocean current? How fast do they flow? How deep
are they? Describe a particular current in detail.

10. What is meant by the cog-wheel scheme of circulation? What is
a Sargossa Sea?

n. What is the general cause of ocean currents? Point out definite
evidence. Name and locate several ocean currents. What is a "creep"?

12. What is the effect of ocean currents upon climate? Point out
specific examples. What is the effect of ocean currents upon navigation?
Point out specific examples.

PART IV

THE LAND

CHAPTER XVII
THE MANTLE ROCK

Structure of the Solid Earth. Everyone is familiar with the fact
that s<^id rock appears on the surface of the land in but few places,
and that this surface nearly everywhere consists of loose or uncon-
solidated earthy matter. This is the mantle rock. In some places
it reaches a thickness of several hundred feet, but as a rule, the full
thickness is revealed in stream valleys, and one can find such sec-
tions as that shown in Fig. 89 in nearly all ravines.

The solid rock which underlies the mantle rock is called the bed
rock. In the ordinary sense the term rock does not include loose,
fragmental deposits, but natural formations of the same origin
show all degrees of consolidation from that of sand to the hardest
sandstone. We therefore define rock as a natural deposit of earthy
matter, whether consolidated or not.

Economic Importance of the Mantle Rock. The mantle rock
is of the greatest economic importance. Without it the surface
of the land would be solid rock, and agriculture would be impossible.

All the mantle rock, except the layers of pure clay, permits water
to pass readily through it, thus acting as a distributer of water. A
portion of the rainfall sinks into it, and through the action of grav-
ity is slowly distributed to all parts below the water table (p. 270).
Above the water table, water is diffused by capillary action.

The mantle rock acts as a great reservoir which receives and
temporarily stores a large portion of the rainfall, thus tending
to prevent floods which would otherwise occur after every heavy
rainstorm. The quantity thus conserved is much greater than
that conserved by the forests, important as is this latter amount.
The water in this reservoir supplies wells and springs, keeps
plants alive in dry weather, and much of it gradually makes its

22O

PHYSIOGRAPHY

way into the streams, furnishing a supply of water even in dry
seasons, thus making the larger streams permanent and fairly uni-
form in size. A large portion of the water supply of Brooklyn,
N. Y., is obtained from wells that do not reach the bed rock and
from which several million gallons per day are pumped.

The mantle rock is a natural filter. Rain washes the air, beating
down dust particles and removing disease germs. On the surface

FIG. 89. NATURAL SECTION SHOWING MANTLE ROCK AND BED ROCK .
Lockport, New York. Geological Survey of New York.

of the earth it becomes muddy and is contaminated in many ways,
making the surface water unsafe for household use. The water of
wells and springs is clear because the mantle rock has filtered it,
and if wells are not too shallow the water is generally pure and
safe to use.

The mantle rock is a great storehouse of plant food. As it is a
poorer conductor of heat than the solid rock it acts as a blanket,
diminishing the earth's loss of heat by radiation.

Origin of the Mantle Rock. The mantle rock consists of frag-
ments of bed rock in various stages of disintegration and decay,
that have been loosened and changed through the action of a num-
ber of natural agents which accomplish the result in different ways.

The quiet action of the atmosphere, with its moisture and its
changes in temperature, slowly disintegrates solid rock, and in this
manner has formed much of the mantle rock and is of great im-

THE MANTLE ROCK 221

portance. This process is known as weathering. Glaciers and
running water wear away the surface of the rock over which they
move and add the loosened particles to the mantle rock.

An appreciable addition to the mantle rock results from the
action of wind-blown sand and the waves on solid rock. Figs.

FIG. 90. OVOIDAL BLOCK OF GRANITE

Produced by weathering. Redstone Quarry, Westerly, R. I.

From U. S. Geological Survey.

94 and 97 show rock that has been much worn by wind-blown
sand and Fig. 83 by wave action.
The most important source of mantle rock is weathering.

Weathering. Every boy has learned that the stones found in
tke fields differ greatly in hardness and strength. Sometimes one
finds a stone that will crumble in one's hands or that will scale off
on the outside and is well preserved and hard in' the center. Such
specimens illustrate weathering.

The difference in the appearance and the solidity of freshly
quarried rock, and that of the same rock which has been exposed
long to the action of the elements, is due to weathering. The

222

PHYSIOGRAPHY

stones of many buildings less than a quarter of a century old show
the effect of weathering, and some of the stones that are used
extensively for building in the United States, weather to such an

FIG. 91. GRANITE BROKEN BY INTERNAL STRESS AND AFTERWARD WEATHERED
The rounded forms and apparent stratification were caused by rapid weathering along the

lines of fracture.

extent in a few years that it is necessary to protect them in some
manner to prevent their entire destruction.

Weathering is the term applied to the various natural processes of
softening and disintegrating the surface layers of rock exposed to the
atmosphere.

Chemical Weathering. Certain agents of weathering attack
rock in practically the same way that articles made of iron are
attacked when they rust. These agents produce chemical changes
ill the rock and the products of their action are new substances

THE MANTLE ROCK

223

FIG. 02. HOODOO BASIN, AHOARAKA RANGE, YELLOWSTONE PARK
Showing fantastic lorms carved from igneous rock by rain and weathering.

entirely unlike the original, just as iron rust is unlike the iron from
which it was formed. These are the chemical agents of weathering.
The most important chemical agents concerned in weathering are
oxygen, carbon dioxide, and water.

224 PHYSIOGRAPHY

Oxygen. This is the most active of the elements in the air.
In the presence of moisture it not only combines with iron and a
number of other metals but it also attacks many compounds found
in the rocks, uniting with them and forming new compounds. A
ledge of rock is often easily crumbled and of a brownish or yel-
lowish color on the outside, and firmer within. Such changes are
due to the action of oxygen, and the changed substance is said to
be oxidized.

Carbon Dioxide. This is another constituent of the air which
corrodes rock. It is most active when dissolved in water. The
igneous rocks are largely composed of complex minerals and are
decomposed by water containing carbon dioxide. When the con-
stituent minerals contain calcium, one of the products of this
action is calcium carbonate. Being soluble the calcium carbonate
thus formed is carried to the sea by streams, where much of it re-
appears in solid form as limestone.

Water. Water often combines with some of the constituents of rocks,
with an increase in volume which causes the remainder of the rock to
crumble. Certain micas illustrate this action, and this probably accounts
for the rapid weathering of micaceous sandstones.

Other Chemicals. Nitric acid formed in the air by lightning, certain
sulphurous gases erupted by volcanoes, and acids formed by decaying
vegetation also produce chemical changes in rocks which result in their
disintegration.

Mechanical Weathering. Certain agents abrade rocks in the
same way that a file wears away iron. This is a mechanical process
and the products remain the same material as the original sub-
stance, just as iron filings are the same material as the piece of iron
from which they were separated. Other agents disintegrate rocks
by blows like those of particles of flying sand. All of the agents
which disintegrate without changing the identity of the material
are mechanical agents.

Changes in Temperature. When stone is heated or cooled it
expands or contracts. If the heating or cooling is slow enough to
change the temperature uniformly throughout the mass the effect

THE MANTLE ROCK 225

is slight. If, however, the rock is unequally heated or cooled it
produces the same sort of stress in the rock that is produced in a
glass jar when hot fruit is poured into a cold jar. This stress
caused by unequal expansion of different parts frequently breaks
the rock just as it does the fruit jar. Both glass and rock are poor
conductors of heat, and, therefore, when the surface of either sub-
stance is heated the temperature of the surface rises more rapidly
than that of the interior, thus establishing the condition of stress
which tends to disrupt the substance. Every ledge of rock upon
which the sun shines is subjected to this action to a greater or less
degree, and when the daily range of temperature of the rocks is
large, as it is in high altitudes, expansion and contraction is
sometimes the most effective agent concerned in local weathering.

When a layer of rock has been uncovered so as to receive the
sun's rays, as at the bottom of a stone quarry, the resulting rise
in temperature expands the rock, producing tremendous lateral
pressure which sometimes causes the rock to buckle and break.
This pressure is increased in the daytime and diminished at night.
These daily fluctuations in stress are effective in weakening the
cohesion of the rock, thus assisting in weathering it, and the vary-
ing lateral pressure may materially aid in displacing the adjoining
rock.

In New York City a cement sidewalk 700 feet long and 15 feet
wide was completed in February. One warm day the following
June the lateral pressure due to the high temperature caused the
sidewalk to buckle in three places, raising three miniature moun-
tain ranges nearly a foot high across the walk. The stone was
much broken at these places. It was repaired in July and has not
since repeated the phenomenon. Why?

When the Chicago and Northwestern Railroad was in process of
construction a portion of its line along the shore of Devil's Lake,
Wisconsin, passed over a large mass of very hard rock, quartz-
ite, occupying a narrow space between a nearly vertical cliff of the
same substance and the shore. After expending large sums of
money experimenting with various kinds of drills, including the
diamond drill, in an effort to remove the rock by blasting, they

226

PHYSIOGRAPHY

FIG. 93. TOP OF PIKE'S PEAK, SHOWING ROCK BROKEN BY FREEZING AND THAWING

were about to abandon the work when someone suggested that
wood fires be built upon the rock and that when the rock was well
heated a stream of cold water be thrown upon it. The plan was a

FIG 94. EFFECTS OF WIND-BLOWN SAND (ARIZONA)
By permission of Oliver Lippincott.

THE MANTLE ROCK 227

success and the quartzite was removed in this way. Farmers
sometimes remove bowlders by this process.

Frost (Freezing and Thawing). Water is usually found in
crevices and the minute spaces between the particles which com-
pose the rock. When this water freezes it expands and breaks the
rock just as water freezing in water pipes breaks the pipes. The

FIG. 95. OVAL CONCRETIONS

Exposed by weathering of the weaker sandstone surrounding them. Near New Castle,

Wyoming.

effect upon the rock is the same as would be produced by driving
minute wedges into each space containing water. This action is
sometimes called the "wedge work" of ice.

The process of freezing and thawing is more effective in weather-
ing porous rocks, particularly those composed of large crystals,
than compact rocks. In a dry though cold climate this action is
of much less importance than in a moist, cold climate.

The obelisk now in Central Park, New York City, stood for

228

PHYSIOGRAPHY

FIG. 96. RIPPLE MARKS, FORMED BY WIND (VOLTS TRADING POST, NEW MEXICO)
By permission of Oliver Lippincott.

3,000 years near the mouth of the Nile in Egypt, yet when it ar-
rived in the city the inscriptions on it were finely preserved. In
a short time freezing and thawing had weathered it to such an
extent that it became necessary to treat the surface of the obelisk
with paraffine to fill the pores and keep the water out. Fig. 93
shows the extent to which the rock forming the top of Pike's Peak
has been broken by this action.

Wind. The sand blast, a device which blows a stream of sand
against objects, is widely used as a means of cleaning the outside of
stone buildings, removing rust from metals, etching glass, and similar
processes. Wind-blown sand is a natural sand blast; it loosens par-
ticles from exposed surfaces of rock, and adds them to the mantle rock.

Window panes in houses, in certain localities on Cape Cod, are
abraded by wind-blown sand and their transparency destroyed; and
in regions of strong winds pebbles are worn into triangular shapes and
even perforated.

Plants and Animals. The roots of plants find their way into cracks
in rocks and as they grow larger exert great pressure on the rock, often

THE MANTLE ROCK

229

breaking off large pieces. Roots of trees growing near a city sidewalk
frequently illustrate this action by raising or breaking the walk. The
decay of vegetable matter supplies acids which act vigorously on cer-
tain minerals.

' Earth worms, moles, ants, and other animals living in the ground
bring much soil to the surface, exposing it to the air, and thus play an
important part in changing insoluble minerals into the soluble form suit-
able for plant food. They also aid in the distribution of air and ground
water through the tunnels and holes which they make.

Gravity assists in weathering rock by removing loosened fragments
from steep rock walls, thus exposing fresh surfaces to the air.

Weathering Below the Surface. Certain kinds of weathering
take place below the surface, but it is in general much less rapid
than on the surface; indeed, one foot of impervious soil has fre-
quently been found to have quite perfectly preserved the polish
and the scratches given the bed rock by continental glaciers. In
porous mantle rock weathering certainly takes place at consider-
able depths. This is proved by the thick deposit of residual man-
tle rock which overlies some deposits of granite and other durable
rocks.

FIG. 97. SANDSTONE UNDERCUT BY WIND-BLOWN SAND (BANNER COUNTY, NEBRASKA)

230 PHYSIOGRAPHY

Residual Mantle Rock. Some portions of the mantle rock re-
main in the position in which they were formed and such deposits
are called residual mantle rock. All residual mantle rock is a
product of the weathering of the bed rock below it, and consists
only of such materials as can be formed from the bed rock by
the processes of weathering. The gravel and stones scattered
through the deposit are all like the bed rock except as they show
various stages of decomposition. The upper layers of residual

FIG. 98. DIAGRAM OF RESIDUAL MANTLE ROCK

mantle rock consist of smaller and more perfectly decomposed
particles than the layers below them, because these upper layers
protect to some extent those below them. There is usually a
gradual increase in size and angularity of the fragments as we de-
scend, as indicated in Fig. 98.

A change in the character of the bed rock is at once indicated
by a change in the nature of the mantle rock, and it is not usual
to find large areas having the same kind of residual mantle rock.

Deposits of Vegetable Matter. During the last stages of the
destruction of a pond or a lake vegetable matter accumulates
more rapidly than the other materials which fill them, and the
swamp thus formed is often a bed of plant fibre that is quite free
from earthy matter and that burns well when dried. Peat is
formed in this way. It consists chiefly of the remains of mosses

THE MANTLE ROCK 231

and marsh grasses which are but slightly decomposed. Many
ponds and marshes illustrate a stage in the formation of peat,
and many peat bogs are found in the New England States, in
New York, and in many other parts of the United States. The
peat bogs of Ireland are well known and very extensive, one of
them having an area of more than 600 square miles.

FIG. 99. TRANSPORTED MANTLE ROCK
Bluff Point, N. Y.

The Dismal Swamp of Virginia, and the million acre swamp of
the Kissimmee Valley of Florida, are examples of large deposits
of a similar nature in the United States. These deposits of partly
decomposed vegetable matter are a part of the mantle rock and
are like the residual mantle rock in that they have not been re-
moved from the locality where they were formed.

When exposed to the air vegetable matter decays and its con-
stituents pass into the air, but when under water it loses its volatile
constituents and gradually approaches more and more nearly a
pure form of carbon.

The mosses that form these deposits grow on the surface and die

232 PHYSIOGRAPHY

beneath, thus raising the surface so that it sometimes rises above
the surrounding land or even climbs an adjoining hillside as in
the "climbing peat bogs."

Transported Mantle Rock is that which has been carried to the
location where it is found by some natural agency. Its composi-
tion, as a rule, bears no relation to that of the underlying bed rock,
and is a mixture of fragments of many kinds of bed rock. Since
certain agents which transport rock-waste act over large areas, we
sometimes find deposits of transported mantle rock of quite uni-
form composition and structure extending over thousands of square
miles. The deposits of all large rivers illustrate this fact.

Transportation of Mantle Rock. Five agents are chiefly respon-
sible for the transported mantle rock.

1. Rivers. Every muddy stream is actively engaged in the work
of transporting mantle rock, and each stream has a burden in prog-
ress toward its mouth that is measured by the extent of the bottom
lands along its valley and the depth of the transported mantle rock
that forms the bottom lands. Mantle rock that has been trans-
ported by streams is called alluvial mantle rock.

2. Glaciers. The glaciers carry mantle rock slowly, but the size
of the particle carried is not limited by the velocity, as it is in the
case of rivers, and the total load that a glacier can carry is limited
only by the amount that it can get. The greater part of the trans-
ported mantle rock in the northern United States and in north-
ern Europe is glacial mantle rock.

3. Wind. The presence of dust in the air is a familiar fact in
every household; it settles on everything that air reaches. No
building is so tall that the upper story rooms never need dusting,
and no mountain is so high that its snows are free from dust.

Dust and sand grains are supported in the air by irregular cur-
rents, both convectional and forced.

In the Missouri Valley during low water great clouds of sand
and dust are picked up by the winds and carried many miles.
In the arid regions of the Southwest it is claimed that the dust
storms are as dangerous as the blizzards of the Northwest.

THE MANTLE ROCK 233

Volcanic eruptions sometimes project great quantities of ash or
volcanic dust (finely divided lava) into the air. The finer par-
ticles of this dust are carried great distances; indeed, it is be-
lieved that the dust projected into the air during the great erup-
tion of Krakatoa in 1883 was carried several times around the
earth and that some of it remained in the air for three years. This
is probably the only way in which material from the land adds to
the deposits forming in mid-ocean.

4. Gravity. Avalanches and landslides are well-known illustrations
of transportation through the action of gravity which occasionally moves
great masses of mantle rock. A recent landslide in British Columbia
removed a large section of a mountain, buried a town located in an ad-
joining valley, and portions of the mountain were carried some distance
up the opposite side of the valley.

A disastrous avalanche occurred February 2yth, 1910, in northern
Idaho. It buried the mining towns of Mace and Burke, with great loss
of life and destruction of property. On March ist, 1910, a train on the
Great Northern Railroad was swept from the tracks by an avalanche
which buried the track beneath a mixture of snow and earth. The ac-
cident occurred at Wellington, Wash., near the summit of the Cascade
Mountains.

In connection with the ground water gravity moves the mantle rock
slowly down slopes, sometimes breaking off pieces of inclined strata
over which it passes, and opening the layers so that air and water may
circulate more freely. This action is known as creep.

Mantle rock that has been transported by gravity is called collumal
mantle rock.

5. Waves. Between the breakers and the shore line water
dashes up the beach from every incoming wave and carries so much
of the beach sand with it that the water usually looks muddy. If
the sand is white and free from clay, the water becomes clear at
the instant that the shoreward motion ceases, to become muddy
again as it gains velocity during its return. This latter motion
follows the laws which govern motion down an inclined plane; it
moves in the direction of the slope of the plane and increases its
velocity at a rate which depends upon the slope of the beach.
This return motion, the undertow, carries the finer particles of the
beach deposit with it.

234 PHYSIOGRAPHY

When the wave is oblique to the shore line the to and fro motion
oi the water between the breakers and the shore is not along the
same line as it is when the waves are parallel to the shore. This
backward and forward motion transports the beach materials
slowly along the shore. The amount of material transported in
this way increases as the waves become more oblique and reaches
a maximum when the wind is parallel to the shore.

Deposition. The agents that transport mantle rock deposit
it as they lose carrying power and form physical features that differ
so widely in shape and structure that, in most cases, the agent that
transported a given deposit may be readily determined.

i. Alluvial Deposits. The sediments carried by streams are
quite perfectly assorted, giving us layers of mud, silt, sand or gravel
in the various deposits, but the stratification is very irregular. A
layer of clay may be found a given distance below the surface at
one point, and 100 feet away gravel may take its place. Such
changes are due to the fluctuations in volume which vary the trans-
porting power of the stream and which often wear away portions
of a deposit, afterward filling the depression thus formed with
material that differs from that removed, and breaking the continu-
ity of the layers. The gravel of the deposits consists largely of
rounded pebbles of the more durable rocks. The physical features
thus formed have a nearly level surface.

Flood plains, as the valley flats which border many streams are
called, have the characteristic irregular stratification mentioned.
They are usually somewhat higher along the margin of the stream
than farther away and often slope gently down stream, following
the river profile.

Deltas. The upper and lower beds of a delta consist of nearly
horizontal layers of fine material and the middle portion of diagonal
layers of coarser particles. The middle layers are formed by the
material rolled along the bottom of the stream.

Fans and Cones. These are ordinarily semi-circular deposits
with very imperfect stratification. They occur where a stream
leaves a gorge or ravine having a steep slope and flows over a low-
land of more gentle slope. The coarsest material is found where

THE MANTLE ROCK 235

the most abrupt change in slope occurs, that is, at the mouth of
the gorge.

2. Glacial Deposits. The deposits formed by a glacier are always
unassorted and unstratified, and they consist of many kinds of
rock. Fragments of weak rocks, like shale, are found in them.
The pebbles are angular instead of rounded and their surfaces are
rough like freshly broken stone, except where one has been smoothed
and flattened by contact with the rock over which the glacier
passed. Unlike the river sediments, glacial deposits contain little
decomposed rock; even the smallest particles are ground rock
rather than decomposed rock.

Among the more important features formed by glaciers is the
terminal moraine described on page 337. Its surface is irregular,
with mounds and hummocks associated with irregular depressions.
Level sky lines are conspicuous by their absence.

The drumlin described on page 344 is an oval hill of bowlder
clay and was deposited under the ice. Deposits formed under the
ice are sometimes composed of rock fragments and bowlders im-
bedded in a tough clay. This is called bowlder clay.

3. Aeolian Deposits. Inasmuch as the wind holds the fine
particles in suspension longer than the coarse, moving air deposits
the coarse and fine particles in different places. This results in
layers made up of particles which within certain limits are uniform
in size and weight. The assorting is much less perfect than that
of water, and the conditions causing deposition of a stratum of a
given character are generally less permanent. The velocity
of the wind is proverbially inconstant and every change alters the
size of particle deposited; but deposits formed by wind show dis-
tinct and characteristic stratification.

Obstructions are effective in determining the location of the
coarser particles to a somewhat greater extent even than they are
in determining the location of snowdrifts, because the greater part
of the sand is carried in the lower layers of the air. A rather larger
proportion of such deposits, therefore, will be found about ob-
structions. The deposit itself becomes an obstruction of increasing
^importance. Such hills of wind-deposited sand are called sand dunes.

PHYSIOGRAPHY

FIG. loo. SAND DUNE ADVANCING OVER TREES (DUNE PARK, INDIANA)
Note the steep slope of the lee side.

Sand Dunes. The typical sand dune has a much more gentle
slope on the windward side than on the leeward. This is true of
even the smallest deposit, such as that formed about a chip; the
sand grains carried by the air strike the chip, lose velocity and
drop or bound back, piling up on the windward side until the pile
forms an inclined plane up which the wind can roll grains of sand.

Standing beside a small dune when a strong wind is blowing one
sees the sand moving up the windward slope, streaming over the
crest, and falling upon the leeward slope, which from time to time
adjusts itself to the proper angle by miniature landslides formed
where it has become too steep. The angle at which such a slope
will come to rest is called the " angle of repose," and varies with

the size and shape of the par-
ticles.
FIG. XOI.-DIAGRAMO* SAND D E Dunes are numerous along

Arrow shows wind direction. COastS, because Sand IS COm-

THE MANTLE ROCK

237

monly found there. They are more likely to be formed by on-shore
than by off-shore winds (why?); and they are more common on the
east side of bodies of water in the prevailing westerlies and on the
west side of similar bodies in the trade wind belt, than on the oppo-
site sides. For example, dunes of great height occur on the east

FIG. 102. TREE STUMPS UNCOVERED AS A SAND DUNE MIGRATED (DUNE PARK, IND.>
Note the gentle slope of the windward side.

side of Lake Michigan, as at Grand Haven, Mich., and very
few are found on the west side of the lake.

Dunes also abound in deserts and in the semi-arid regions of the
United States, sometimes reaching the height of several hundred
feet. In regions subject to nearly constant winds the removal
of sand from the windward side and its deposition on the leeward
side causes the dunes to migrate slowly in the direction of the
prevailing wind, sometimes burying buildings and forests.

The damage caused by the migration of sand dunes may be post-
poned for a time by building fences across the path of the dune. The

PHYSIOGRAPHY

FIG. 103. SOIL ABOVE MANTLE ROCK (PORTLAND, OREGON)
The mantle rock consists of sand above and gravel below.

best way to prevent the progress is to plant such grasses and shrubs as
will grow in sand on the windward slope of the dune. If once started
vegetation will check the migration quite satisfactorily.

The Loess. In Kansas and other western States, in Europe, and
notably in China, there are deposits called loess, consisting of particles
larger than those of clay but smaller than those of sand. Their origin
is in dispute, but there seems to be good evidence that a part of it, at
least, is a wind deposit. It is without the distinct horizontal stratifica-

THE MANTLE ROCK 239

tion of aqueous deposits and approaches consolidated rock in its ability
to stand with a nearly vertical face. Some deposits of loess are 1,000
feet in thickness.

Volcanic Dust. In Kansas and Nebraska there are beds of vol-
canic dust three feet thick which cover large areas and which are
hundreds of miles from either active or extinct volcanoes. Pompeii
was buried to a depth of about 20 feet by such a deposit.

FIG. 104. STRATIFIED CLAY (HAVERSTRAW, N. Y.)
Used chiefly for bricks.

4. Collumal Deposits. The most numerous of these deposits is
the talus slope that forms at the foot of ledges of bare rock and
that eventually covers the ledge with mantle rock.

5. Shore Deposits. The assorting action of the waves deposits
layers of clay composed of particles of remarkable uniformity in
size. Only the harder and more durable minerals remain on the
beach, and as these grow smaller they are carried out to deeper
water. This is why beach sand is chiefly quartz fragments. Quite
sizable pebbles may be mixed with the sand grains, but vigorous
wave action completely removes the fine particles and often leaves
the sand white. Beach pebbles are generally quartz pebbles and
are smoothed and rounded.

240

PHYSIOGRAPHY

Useful Materials from the Mantle Rock. In addition to the
economic importance of the mantle rock as a whole, it is of much
importance as a source of supply of clay, sand, gravel, marl, peat,
and many materials used in the arts.

Clay occurs in very large quantities, is widely distributed, is of
various degrees of purity, and is suitable for many uses. The
purest clay, kaolin, is used in manufacturing the better class of

FIG. 105. CLAY PIT (NEAR VANCOUVER, WASH.)
B, gray brick loam. P, blue clay used for terra cotta.

porcelain. The less pure varieties are used in making chinaware,
pottery, terra cotta, tiles, drain tiles, and bricks. Fire clay, used
in the manufacture of furnace and stove linings, owes its ability
to withstand high temperatures to the absence of lime and such
alkaline substances as act as a flux.

The clay products manufactured in the United States are valued
at about $160,000,000 a year.

Sand is used in making glass, mortar, and cement. It is also
used in molding metals and as an abrasive. The sand used for
these purposes yearly is valued at about $15,000,000.

THE MANTLE ROCK 241

Gravel is used in roofing, in concrete, and in road building.

Marl is used as a fertilizer, in making certain kinds of bricks,
and in making Portland cement.

We obtain from the mantle rock of the United States more than
half a million dollars worth of these necessary materials every
working day of the year.

THE SOIL

Economic Importance. The upper and fertile portion of the
mantle rock is called soil. It differs from that below it, which is
called sub-soil, chiefly in the greater quantity of decaying animal
and vegetable matter called humus and in the large number of
bacteria which it contains.

Agriculture has been the most important means of support from
the earliest times, and the progress of the early nations depended
in a more marked degree even than that of modern nations upon
the fertility of their soil and their skill in cultivating it. In the
United States the yearly value of the direct and indirect products
of the soil exceeds $7,000,000,000, or more than three times the
total value of all the mineral products.

Fertility. Soils differ greatly in fertility from place to place,
because of unlike composition and unlike texture.

Composition. All plants require nitrogen, potash, and phos-
phorus, and these elements of plant food must be natural constit-
uents of the soil or must be supplied artificially to make the soil
fertile. The soils of residual mantle rock contain only such of these
elements as were in the rock from which they were formed. Granite
and kindred rocks are usually rich in potash and deficient in phos-
phorus, though some of them contain the latter. A pure lime-
stone usually contains an abundance of phosphorus, derived from
shells, but is deficient in potash, and soil formed by its decomposi-
tion would be similarly deficient. A shaly limestone, like that at
Trenton, N. Y., contains both phosphorus and potash. The fa-
mous " Blue Grass Region " of Kentucky has a soil formed by the
decay of such a limestone. A pure sandstone contains neither
phosphorus nor potash, and would form an unproductive soil ; but

242 PHYSIOGRAPHY

sandstones containing many fossils produce a soil containing phos-
phorus. The unproductiveness of the sandstone soils in Kentucky
is in marked contrast with the fertility of the " Blue Grass Region."
Transported soils are likely to be more fertile than residual soils
because the processes of transportation tend to grind them finer,
to mix the soils of different localities, and to increase the amount
of organic matter in them. Such soils necessarily differ among
themselves as the agents by which they were transported and
deposited differ.

Texture. The physical condition of the soil is fully as impor-
tant to its fertility as is the chemical composition. If the particles
composing it are very small the amount of water retained in the
fine capillary passages between them will be large, and because
of the high specific heat of water the soil will warm slowly. Such
soils are "cold "and "late."

Fine grained soils do not absorb so much of the rainfall as coarse
grained soils, and the run-off on the former is greater in propor-
tion than on the latter type. The size of the particles composing
the soil also determines the nature of the plant's water supply, and
hence the ability of the crop to withstand drought. If they are
too large, water is not lifted a great distance by capillary action,
and plants die when the water table is too far below the surface.
If they are too small water rises too slowly, with the same
result.

The situation of soil controls the accumulation or loss of humus
and the finer particles of the soil, as well as the available plant
food. On steep slopes the swift flow of the run-off above the sur-
face and of the ground water below causes them to wash away the
smaller and more perfectly decomposed particles, and to dissolve
and remove the soluble parts of the soil from which plants derive
their food. Soils on such slopes are always less fertile than soils
of the same origin on more gentle grades.

Fertility of soil requires something more than plant food. It
requires water in the right amount and at the right time; it requires
heat; it requires air, which must be distributed through the soil
and must be renewed as it is exhausted; and finally, it requires

THE MANTLE ROCK 243

tillage, which contributes mellowness, facilitates the renewal of the
air supply, and conserves the supply of moisture.

Origin. The flood plain of the Mississippi and the valley of the
Sacramento in California have alluvial soils of great fertility.
The valley of the Red River of the North in North Dakota has
lacustrine soil, deposited on the bottom of a former lake, and is
one of our great wheat growing regions. The region covered by
the continental glacier has a glacial soil. It is less uniform in its
character than either alluvial or lacustrine soils. On Long Island
and Cape Cod it is sandy, whereas the New England States
have many clay soils which are parts of the ground moraine. The
large deposit of till in northwestern Ohio provides a soil that
is more fertile than the residual soil of the southeastern part of
the State, but is less fertile than the alluvial soils bordering the
Ohio River.

Types of Soil. The common classification of soils as sands,
loams, and clays is based upon the physical structure or texture
of the soil rather than upon its chemical composition. It is true
that coarse sands are usually composed chiefly of quartz grains,
and that clays contain a larger percentage of kaolin than either
sand or loam; but the distinguishing characteristics such as plas-
ticity and ability to hold moisture depend chiefly upon the size of
the particles composing the soil.

Sands are composed of particles between i mm. and .05 mm. in
size. Their distinguishing characteristic is their want of coherence
when dry, and this characteristic is possessed equally by the sand
composed of quartz fragments, with which we are all familiar, and
by the sand found about coral islands, which is made up of frag-
ments of coral and shells.

Sandy soils are porous and well drained; they permit free circu-
lation of the air, but are likely to suffer from drought. They are
classed as " early " and " warm " soils, and if they are not too
coarse yield excellent crops of garden truck and potatoes.

Clay is composed of particles less than .005 mm. in size. It is
plastic when wet, shrinks on drying, but retains the form given it

244

PHYSIOGRAPHY

when plastic. It becomes impervious to water when puddled
(worked with water to a thick paste).

Clay soils permit very little circulation of the air. They are
usually poorly drained and are therefore likely to be " drowned "
in a wet season. They are less likely to suffer from drought than
gravel or sand, but do not stand dry weather as well as loam.
They are "cold" and "late" soils, but make good meadows.

Loam. This is a mixture of sand and clay containing enough
coarse particles to make the soil mellow and to permit free circu^
lation of air. It also contains enough fine particles to facilitate cap-
illary circulation, but not enough to make the soil sticky in wet
weather. Loams are well drained and therefore stand wet weather
well. They also stand dry weather well.

Silt is the term applied to deposits of river-borne sediments com-
posed of particles between .05 and .005 mm. in size.

Muck is a black soil formed in swamps and contains a large
quantity of humus; hence it is rich in nitrogen.

The following table shows the percentage of particles of various
sizes to be found in some of the types of soil:

Size Particles

Barren Sand

Coarse Sandy
Loam

Clay Loam

Clay

Sand i Os mm

8^ 6

71 6

48 i

7 6

Silt o^~oo^ mm

5 A

7 2

24. 3

92 2

Clay, .005 mm

1.8

ii . 7

18.5

42.2

The sandy loam described in the table is an early and warm soil
that is well drained and stands drought well. The clay contains
so large a percentage of the finest particles that it is very wet
during a rainy season, and supplies water to plants so slowly that
they would be parched during a drought.

The Department of Agriculture at Washington publishes the
following table of the percentage of each size of particles in typical
soils for certain crops:

THE MANTLE ROCK

245

Truck

Corn

Wheat

Grass

Bright
Tobacco

Heavy
Tobacco

Barren
Clay

Gravel, 2-1 mm. ..

3-09

I . 12

Sand, I-.25 mm...

6-34

2.80

i-95

.21

28.90

3-iQ

.29

Fine Sand,

.25-.O5 mm

81.92

43.06

42.90

11.47

49-68

5-73

IO.2O

Silt, .05-.005 mm. .

8.17

40.90

32-13

23.69

21.41

44.98

36.98

Clay,

.oo5~.oooi mm..

2.80

10. 10

23-78

51-75

4.80

35-24

50.02

The typical soil for vegetables or garden truck seems to be warm,
sandy, and well drained; that for corn a sandy loam, and that for
wheat a clay loam.

QUESTIONS

1. Is all rock properly called stone? Why?

2. What two forces distribute water through the mantle rock?

3. Show that the mantle rock tends to keep the flow of streams uni-
form.

4. Why is spring water better for drinking purposes than surface
water?

5. How does the action of the chemical agents of weathering differ
from that of the mechanical agents?

6. Under what climatic conditions is the action of freezing and thaw-
ing most effective in disintegrating rock?

7. What kind of soil retards weathering below the surface?

8. Dust is always present in the air, yet it is always settling. How is
it supported in the air?

9. Compare residual soils formed from granite with those formed from
a pure limestone.

10. What should result when rain falls on heated rock? In what
two ways may igneous rocks become surface rocks?

CHAPTER XVIII
THE BED ROCK

Rock-making Minerals. The consolidated rock of the litho-
sphere was formed in various ways and is of many kinds, but with
the exception of coal and a few similar deposits of animal or of
vegetable origin it is all composed of mineral matter. Much of it
consists of minerals in crystalline form, and the rest, with the ex-
ception of coal, consists either of fragments or decomposition-prod-
ucts of minerals, or is a fused mass of mineral matter.

The term mineral was originally used to designate a substance
found in a mine, hence something found in the rocks as distin-
guished from animal and vegetable products.

A mineral is a natural substance not of obvious organic origin and
having definite chemical and physical properties.

The durability and economic value of building stones depends
to a large extent upon the physical properties of the minerals
from which they were formed. The following are the rock-making
minerals of most frequent occurrence:

Quartz. This is the hardest of the common minerals. It is
harder than glass, is almost infusible, and is not affected by com-
mon acids. It is quite brittle and the broken surface is curved
like the surface of a shell. It has no cleavage, is of glassy luster,
and occurs in many colors. When in crystals it forms six-sided
prisms, terminated at one or both ends by six-sided pyramids.

The gems, amethyst, carnelian, opal, onyx, and bloodstone, be-
long to the group of minerals of which quartz is the type and have
almost identical properties. Flint, another similar mineral, was
of great importance to prehistoric man because of the sharp cutting
edge of broken pieces. From this substance he fashioned his cut-
ting implements such as knives, awls, spearheads, and arrow points.

THE BED ROCK 247

Later the flint-lock musket was used in the American Revolution,
and it is reported that many of the guns used during the Civil
War were altered to "flint locks," and sold to the savage tribes in
Africa.

Feldspar is first in importance as a rock-making mineral. It
occurs in a variety of colors, commonly pale pink, yellow, or white,
but sometimes gray, blue or iridescent. It is nearly as hard as
quartz but cleaves easily in two directions, giving flat reflecting
surfaces. When exposed to moist air containing carbon dioxide,
or to infiltrating water containing carbon dioxide or other acids, its
luster is quickly lost and it soon crumbles into a soft clay, called
" kaolin." Because of its ready cleavage and its lack of permanence
under natural conditions, feldspar is not a durable mineral, and
most of the clay and the mud rocks of the earth are chiefly products
of its decomposition. Feldspar and kaolin are used in making
porcelain and china, and feldspar is valuable as a fertilizer.

Mica is familiar to everyone in the misnamed isinglass used
in stoves. Its most important properties are its perfect cleavage
into very thin, elastic leaves which have a pearly luster, its ability
to withstand high temperature, and the resistance it offers to the
passage of currents of electricity. It usually occurs in rocks in
rather small sheets or scales, but sometimes large masses are found
which furnish large sheets. White and black are the common
colors. Mica is very soft, and rocks containing an excess of it
are easily broken. It is used as an insulator in electrical appara-
tus, also in stove doors, lamp chimneys, and wall papers.

Calcite. When pure and crystallized, calcite is a transparent,
colorless crystal which cleaves in three directions, making oblique
angles with each other. It is much softer than quartz and is easily
scratched with a knife. Its effervescence with dilute acid and its
double refraction distinguish it from other common minerals.

Calcite is one of the most abundant minerals, for it forms the
basis of limestone, one of the commonest rocks. It is dissolved
by water containing carbon dioxide in solution; therefore, lime-
stone and other rocks containing much calcite are worn away by
rain water.

248

PHYSIOGRAPHY

Structure of the Bed Rock. When one visits a stone quarry or
a rocky ledge he often finds that the rocks are arranged in parallel
layers like those shown in Fig. 106. The layers may differ in
color and in kind of rock, or there may be many layers of the same
kind; some of the layers may be less than an eighth of an inch in

FIG. 106. STRATIFIED ROCK

Near Engineer Mountain, Cal. Beds of hard sandstone or limestone alternate with shale.
The pass of Coal-Bank Hill is shown on the right.

thickness and others many feet thick. The layers are commonly
horizontal, though sometimes upturned like those shown in Fig.
114. These are the "bedded" or stratified rocks. They are chiefly
sandstones, limestones, and shales, but sometimes layers of coal,
conglomerate, or iron ore are found.

In exceptional localities, particularly in mountainous regions,
we sometimes find massive rocks, as shown in Fig. 108. These are
the "crystalline" or unstratified rocks. They are more apt to ap-
pear on the surface in mountains, but are found everywhere below
the stratified rocks when we dig deep enough. The bed rock con-
>, we must conclude, of a great mass of unstratified rock which

THE BED ROCK

249

is covered in most places by the beds of stratified rock. As a rule,
the unstratified rock is reached by borings less than a mile deep,
but in some places the stratified rocks are much thicker than that.
In the Colorado Canon, more than 8,000 feet of consecutive
stratified rocks are exposed at one point, but at the bottom of the
canon unstratified rock is found.

250

PHYSIOGRAPHY

FIG. 108. UNSTRATIFIED ROCK. YOSEMITE VALLEY
Copyright by Underwood and Underwood.

THE BED ROCK 251

Origin of the Bed Rock. Portions of the bed rock show that
they assumed their present form on cooling from a molten state and
are therefore called igneous rocks. Modern lavas belong to this
class, but form a very small part of it. Much of the unstratified
rock which underlies the stratified rock is of igneous origin.

Other portions of the bed rock accumulated as sediments in some
body of water. These are called sedimentary rocks. They are always

FIG. 109. A LAVA FLOW WITH UNBROKEN SURFACES (HAWAII)

stratified, and so large a portion of all the stratified rock was formed
in this way that it is customary to treat those that accumulated
on land with the sedimentary rocks. Sedimentary rocks are made
up of products of the disintegration and decay of former rocks; in
other words, they consist of mantle rock which has been assorted,
accumulated in layers and consolidated.

A third portion of the bed rock has been so changed through the
action of natural agencies as to give the rocks new properties.
These are the metamorphic rocks. They are usually composed
wholly or partly of crystals.

The Igneous Rocks. Every volcanic eruption contributes to
the igneous rock of the surface. Some lavas come to the surface

252

PHYSIOGRAPHY

in a very fluid state, and cooling quickly, form a volcanic glass
called obsidian. Some lavas are mixed with steam so thor-
oughly as to form a very porous glassy rock called pumice. In
some cases the explosion of steam and other gases separates the

* Areas in which the top of the bed rock is metamorphic are shaded; those in which it is
igneous are marked vv. The white area includes certain minor areas in which the origin of
the bed rock is unknown, but with these exceptions it is sedimentary.

THE BED ROCK

lava into fine particles, which fall as volcanic ash or dust. These
different forms are due to the conditions of the eruption rather
than to differences in composition of the lava.

Classes. Certain lavas, particularly at the bottom of a thick lava
flow, show the beginnings of a crystalline structure. Many extinct vol-
canoes have been so worn away that lava, which was covered by thick
beds of rock while it cooled, is now exposed, and these lavas are found

FIG. in. A GRANITE QUARRY

to be perfectly crystallized. They differ from the lavas which cooled
at the surface to such an extent that igneous rocks are divided into two
classes: the eruptive class, or those which cooled rapidly at the surface, and
the plutonic, or those which cooled slowly beneath a thick rock blanket,

Granite is chiefly composed of quartz and feldspar, though mica
is usually present. The minerals are often in such coarse crystals
that they may be readily recognized without the aid of a lens, and
they are irregularly distributed through the mass. Granite cooled
very slowly and crystallized beneath a thick blanket of rock, which
in many cases has been worn away. Granite is extensively used

254 PHYSIOGRAPHY

in buildings, in monuments, and in pavements. It is one of the
more durable rocks.

Sedimentary Rocks. The beds of assorted mantle rock such
as clay, sand, and gravel, when consolidated, form sedimentary
rocks known as shale, sandstone, and conglomerate respectively.
They were deposited in horizontal or nearly horizontal layers in
some body of water, generally the ocean. Other sedimentary
rocks were formed from material dissolved from the mantle or
bed rock, carried to the ocean in solution, and recovered from the
solution by plants and animals which absorbed the material from
the sea water to form shells and skeletons. The most important
rock formed in this way is limestone.

Sandstone. Quartz is the most durable of the common rock-
making minerals, and fragments of quartz should therefore pre-
ponderate in the coarser deposits of rock-waste formed in the sea.
The sands of most shores are chiefly quartz fragments. Sandstone
is a sand bed held together by some natural cement and may be
recognized by its hardness, its rough feel, and the fact that it is
composed of quartz grains. It is usually quite porous.*

Conglomerate is a consolidated gravel bed composed of rounded
pebbles, usually embedded in finer material. The mass is bound
together by some mineral substance which forms the cement.

Shale. Decomposed fragments of feldspar and other minerals
less durable than quartz reach the ocean in very small particles
and settle to the bottom in quiet water, farther from the shore than
the sand deposits. When consolidated, the beds thus formed are
called mud stones or shales.

Shale is so soft that it may be scratched with the finger nail.
It splits readily into thin layers parallel to the planes of stratifi-
cation, and it weathers quickly. It is not affected by acid, and when
moistened has an odor of wet clay. Shale is useless for building
purposes, but is used in manufacturing cement, terra cotta, and

* Sandstone has a variety of colors. Perhaps the various shades of red and yellow are
most frequently seen, but some specimens are nearly white, and impure sandstones often
are blue or gray. It is used for grindstones, scythe stones, and in building. Shaly sandstones
make excellent sidewalks.

THE BED ROCK

255

bricks. Some of the black shales contain valuable oils like petro-
leum which are recovered by distillation.

Limestone. The deposits of the calcareous parts of animals and
plants which gather in the sea form limestone when consolidated.
Some of these deposits are shell beds, like those that form
wquina off the coast of Florida; others are beds of coral sand such

FIG. 112. A LIMESTONE QUARRY

as form the beaches of coral islands ; and still others are made up of
the harder parts of minute animals that inhabit the upper part of
the sea even in mid-ocean. The latter deposits form chalk.

Limestones formed near the continents are usually rendered
impure by sand or mud brought by waves and shore currents.
Pure limestone can only be formed in a region which does not
receive such deposits. It may be formed in the deep sea, beyond
the muds; or in shallow water, about coral islands; and in excep-
tional localities about continents where sediment from the land is
not being deposited.

256

PHYSIOGRAPHY

FIG. 113. DIAGRAM SHOWING RELATIVE LOCATION OF SEDI-
MENTARY DEPOSITS

Some small deposits of limestone are formed on land by direct deposit
from spring water which has lost its dissolved carbon dioxide, and can,
therefore, no longer hold the limestone in solution. Calcareous tuff is
thus formed. In some other cases limestone has been deposited in the
beds of salt lakes through the evaporation of the concentrated sea water,
but it is not believed that any of the important limestone deposits of
the world have been formed in this way.

^^ Limestone effer-

vesces when treated
with a weak acid,
and has about the
same hardness as
calcke. It is slowly
dissolved by water
containing carbon
dioxide or acids de-
rived from the decomposition of vegetable matter, and is there-
fore one of the less durable rocks.

About one quarter of the limestone quarried is used in making
quick-lime and cement. The remainder is used in buildings and
as a flux in smelting ores.

Location. Wave action assorts the sediments deposited in the
sea and we find the coarsest material, the gravels which form
conglomerates, nearest the shore; next beyond them come the
sands which form sandstones; and next come the muds and clays
which form shales. Beyond the muds are the calcareous deposits
which form limestone. The diagram (Fig. 113) indicates the
order in which these deposits would occur in a region where all
of them were forming at the same time.

Bituminous Coal When peat is buried beneath deposits which

exclude the air it becomes more compact and gradually loses some
of the constituents of woody fibre, approaching more and more
nearly to a form of pure carbon. This is the way in which bitu-
minous coal was formed, but the deposits from which it was formed
contained the remains of ferns which grew to be large trees as well
as palms and other forms of tropical plant life, in addition to the
mosses and grasses which commonly form peat.

THE BED ROCK 237

Bituminous coal burns with much smoke and flame. This is
due to the large amount of volatile matter which it contains and
which makes it valuable in the manufacture of artificial gas. It
has a dull black color and usually breaks along bedding planes
parallel to the coal seam.

Chemical Deposits: Rock Salt and Gypsum Beds. When a
salt lake dries up, or a body of water that has been isolated
from the sea evaporates, all of the dissolved mineral matter is
deposited, and an interesting assorting action of great importance
to mankind accompanies the deposition. This assorting is due
to the varying solubilities of minerals. In the case of sea
water, gypsum, a very difficultly soluble mineral, begins to
be deposited when 37% of the water is evaporated; common
salt, a very soluble mineral, is not deposited until 93% is
evaporated. About 10,000 square miles of New York state, in the
southwestern part, are underlaid by a deposit of rock salt formed
as described. It is mined at Livonia, Piffard, Warsaw, and at
several other points. In some of the mines the deposit is 80 feet
thick. Other important deposits are found in Michigan, Kansas,
and Utah. The most important European deposit is at Stassfurt
in Saxony.

Metamorphic Rocks. Some of the metamorphic rocks are known
to have been slowly formed from sedimentary rocks, others have
been formed from igneous or other metamorphic locks. The chief
agents by which rocks are metamorphosed are heat, moisture, and
pressure.

Rocks have sometimes been metamorphosed by heat from a
lava flow or a dike, and we may find layers of sedimentary rocks
passing gradually into metamorphic rocks as we approach the
lava, thus showing what kind of rock was metamorphosed and
the cause of the change. Sandstones have been changed into
quartzite under such conditions; shales and clays into slate and
then into mica schist. Pure limestone has been changed into
white marble, and shaly limestone into a marble containing such
minerals as mica and garnet; and in several States along the

258 PHYSIOGRAPHY

Appalachian Mountains bituminous coal has been changed into
a natural coke or into an anthracite. When large areas have
been metamorphosed it is not always possible to determine the
original form of the metamorphic rock. The changes produced by
pressure are of fully as much importance as are those due to heat.
Pressure has caused the characteristic cleavage of slate, Fig. 1 14,
and the foliated structure of gneisses and schists.

Quartzite is a metamorphosed sandstone. The separate grains
can easily be distinguished by the aid of a lens, but they are much
more firmly cemented together than are those of sandstone, and
the rock is less porous. The cement is silica, like the grains, and
it has a shell-like fracture. Metamorphism seems to be destroy-
ing its granular structure and restoring the properties of the
quartz crystal.

Marble is a metamorphosed limestone which in its typical form is
quite perfectly crystallized. Pure limestone becomes white mar-
ble. Colored marbles are due to impurities, such as iron. Marble
is used for statuary, in the ornamentation of buildings, and was
formerly much used for tombstones.

Slate is metamorphosed shale. It is somewhat- harder and more
durable than shale, and it cleaves into thin layers having smoother
surfaces than those of shale and quite independent of the original
bedding of the mud deposit. Fig. 114 shows the cleavage lines of
the slate across the bedding. The principal use of slate is for roof-
ing. Small quantities are used for school slates and blackboards.
It is also used in making imitation marble and as a support for
electrical fixtures.

Gneiss is composed of crystals of quartz, feldspar, and mica ar-
ranged in parallel layers. Mica schist is composed chiefly of quartz
and mica. Gneiss and mica schist may be of either igneous or sedi-
mentary origin. They are widely distributed and of great extent
in northern North America and in the central portion of mountain
ranges.

Anthracite Coal, seen in thin sections under the microscope,
shows cellular structure, indicating that it is of vegetable origin;
but it differs from bituminous coal in that it contains very little

THE BED ROCK

FIG. 114. A SLATE QUARRY
Slatington, Pa.

volatile matter and burns without the large amount of smoke and
flame characteristic of bituminous coal. It is a hard, lustrous,
dense substance which does not break along bedding planes as
does soft coal, but has a shell-like fracture.

Anthracite is found only in regions of disturbed and folded strata.

Table. The relation between the deposits of the mantle rock and the
sedimentary and metamorphic rocks which they form when consolidated
and metamorphosed is shown in the following table:

MANTLE ROCK

SEDIMENTARY ROCK

METAMORPHIC ROCK

Clay.

Shale

Slate schist

Sand

Sandstone

Quartzite

Gravel

Conglomerate

Marl )

Limestone

Marble

Shell Beds j '
Peat

Bituminous coal

Anthracite coal

(Graphite)

260 PHYSIOGRAPHY

ECONOMIC IMPORTANCE OF THE BED ROCK

Metals. Several metals occur in their pure state in the rocks
and are called native metals. They are occasionally found in large
masses but more frequently are scattered through the rock in small
particles. Gold, silver, platinum, mercury, and copper are the
principal native metals.

Much of the metal of commerce is obtained from minerals in
which the metal is combined with other elements. Such minerals
are called ores if they yield a metal in profitable quantities. The
yearly output of our iron mines exceeds the total value of all the
other metals together, and the value of our output of copper ex-
ceeds that of gold and silver together.

The principal ores of iron are compounds of iron and oxygen,
from which the iron is obtained in a pure state by heating the ore
with coke and limestone in a blast furnace. Most of the iron ore
used in the United States comes from the Lake Superior region,
but important mines are located in Alabama, New York, and
Pennsylvania.

The principal ores of zinc, lead, copper, and silver are compounds
in which sulphur is combined with the given metal. Each of them
must be treated by a more or less complicated chemical process
to secure the pure metal.

Coal. About 300,000 square miles of land in the United States
are underlaid with coalbeds. Not all of this is workable, because
of its impurity or of the thinness of the layers, and the area at
present producing coal is but a small fraction of the total. Very
large areas in Alaska are also coal lands, but they are at present
undeveloped. All varieties of coal are found in the United States,
from the graphitic anthracite of Rhode Island, which burns with
great difficulty, to the lignite of Texas, which retains much of its
woody structure.

The amount of coal mined in the United States last year ex-
ceeded that of any other nation, reaching the total of more than
500,000,000 tons.

The map, Fig. 115, shows the regions in this country in which

THE BED ROCK

261

coal is found. The coal is not all high grade, but recent progress
in the construction of furnaces for low-grade coals and in the
development of the " producer gas " process has made it possible
to use low grade coals for heating purposes and also to produce a
gas which is suitable for use in a gas engine and which works well
under a Welsbach mantle. Now that we have learned to handle

FIG. 115. COAL BEDS OF THE UNITED STATES

these coals, they have become immensely valuable. They usually
have at least 60% of the fuel value of the high grade coals, and
occur in regions where other grades of coal are expensive because
of heavy freight charges.

Petroleum and Natural Gas. For the past fifty years we have
been taking from the bed rock great quantities of petroleum and
natural gas, which have accumulated during the ages.

The value of these products secured during 1907 was about
$173,000,000.

In all of the oil fields old wells have ceased to produce or have
diminished their output, showing that petroleum is an accumula-
tion and that the supply is not renewed as fast as it is removed.
We are rapidly using up the great store, and unless new fields are

262

PHYSIOGRAPHY

FIG. 1 1 6. A GROUP OF OIL WELLS

discovered men will have to learn before many generations to
do without petroleum and natural gas. The most important oil
fields are in western Pennsylvania and in Ohio. The Oklahoma
field has recently come into prominence, and Kansas, Illinois,
Texas, California, Colorado, and West Virginia each produces oil.

FIG. 117. DISTRIBUTION OF PETROLEUM AND NATURAL GAS FIELDS IN THE UNITED STATES

THE BED ROCK 263

Cement. The manufacture of Portland cement has become an
important industry in the United States. In 1885 the yearly out-
put was valued at about $3,500,000; at the present time it exceeds
$50,000,000. It is made by burning certain proportions of lime-
stone or marl with clay or pulverized shale. A hard and durable
artificial stone is made by adding water to a mixture of cement
and sand. It is now being used extensively in sidewalks, buildings,
and other structures.

Stone. Building stones are so widely distributed that almost
every locality in the United States supplies its own demands. It
is only when some unusual requirement, such as a particularly
large-sized piece or a certain shade of color, is made that stone is
shipped long distances. This condition is due to the willingness
of most builders to use the stone at hand without regard to its
durability, rather than to the universal distribution of good build-
ing stone.

Hundreds of " brown-stone " buildings of New York City show
the lack of durability of this sandstone. Many of them have
been treated with various solutions to fill the pores and protect
the stone, but no satisfactory process of protecting poor stone has
yet been devised. Probably the best known process is to saturate
the exposed portion of the stone with a solution of water-glass,
several applications being necessary. When this is dried a solu-
tion of calcium chloride is applied.

The following estimate of the "life" of different kinds of building stone
in the climate of New York City is given in one of the volumes of the
Tenth Census of the United States. By life of a stone is meant the
period after which the decay becomes so offensive to the eye as to de-
mand repair or removal:

Life in years.

Brownstone coarse 5 to 15

Brownstone fine, compact 100 to 200

Sandstone best, silicious cement. . 100 to many centuries

Limestone coarse 20 to 40

Marble coarse 40

Marble fine 50 to 200

Granite 75 to 200

Gneiss 50 to many centuries

264 PHYSIOGRAPHY

In value of stone quarried the largest yearly return is obtained from
limestone. This is doubtless due to the many uses to which it is adapted.

Mineral Fertilizers. The most important mineral fertilizers
found in this country are the rock phosphates of Tennessee, South
Carolina, and Florida. These rocks owe their value chiefly to the
great number of bones of mastodons and other animals of which
they are largely composed. They are efficient fertilizers of lands
needing phosphorus. Sodium nitrate (Chile saltpeter) occurs in
beds several feet in thickness in the northern part of Chile, and
is also found in Humboldt County, Nevada, and in California.
It supplies about 15 per cent of its weight of nitrogen. One of
the most valuable mineral fertilizers used to supply potash is salt-
peter. Saltpeter is formed abundantly in certain soils in Spain,
Egypt, and Persia, and is formed in considerable quantities in the
soil of many caves in the Mississippi Valley. It yields about 45
per cent of its weight of potash, and 13 or 14 per cent of nitrogen.

Minerals. Only a few of the many minerals found in the bed
rock can be discussed here. For descriptions of others and for
more complete accounts of those mentioned below, the student
must be referred to works on Mineralogy and to such works as
the " Mineral Resources of the United States," published annually
by the United States Government.

Graphite, commonly called " black lead," is pure carbon, being
identical in composition with the diamond. Extensive graphite
mines are located at Ticonderoga, N. Y., and in several Western
States, but the quality of American graphite is not equal to that
found on the Island of Ceylon and in Siberia. Graphite is a soft
mineral with a metallic luster, and derives its name from its prop-
erty of leaving a mark on substances. It is used in making lead-
pencils, crucibles in which to fuse metals, stove polish, electric
light carbons, and as a lubricant.

Sulphur occurs uncombined about extinct as well as active vol-
canoes. In the United States it is mined in Nevada, Utah, Cali-
fornia, and Louisiana. Large quantities come from Vesuvius and
other active volcanoes. It is extensively used in manufacturing

THE BED ROCK 265

gunpowder, fireworks, matches, and sulphuric acid. It is also
burned to produce bleaching and disinfecting agents.

Gypsum. Like calcite and common salt, gypsum is one of the
minerals brought down to the sea in solution, but it is not absorbed
from the sea water by plants and animals, as calcite is. For this
reason it is found chiefly in beds formed by the evaporation of salt
water, and is usually associated with salt. Important mines are lo-
cated in western New York, at Alabaster and Grand Rapids, Mich-
igan, and in several of the Western States. It is not affected by
acid, is softer than calcite, and has not the glassy luster of calcite.
It is used as a fertilizer and as a substitute for marble in buildings,
and also in the manufacture of plaster of Paris and Portland cement.

Rock Salt is widely distributed, and some of its deposits are of
great thickness. It is used as the basis of many chemical manu-
facturing processes, e.g. making washing soda, soda ash, and hydro-
chloric acid; also in preserving meats and fish.

Mineral Resources of the United States. The following table
shows that we are extracting from the lithosphere material valued
at about $7,000,000 every working day of the year:

SUMMARY FOR 1907.
Metallic.

Pig Iron $529,958,000

Copper 173,799,300

Gold 90,435,700

Silver 37,299,700

All other metals 7 I )53 I )3S

$903,024,005
Non-Metallic.

Coal $614,798,898

Petroleum and Natural Gas 172,973,524

Cement 85,903,831

Building Stone 71,105,805

Abrasives 1,680,737

Fertilizers 10,661,987

Clay and Sand 1 73,435,358

Lime, Slate, etc 19,885,501

Mineral Paints 2,979,158

Unspecified 42,840,312

$1,166,265,191

$^,069,289,196

266

PHYSIOGRAPHY

FIG. 118. THE EFFECTS OF EROSION ON A HARD, SANDY CLAY
Foot of Scott's Bluff, Neb.

Destruction of the Bed Rock. The great mass of the mantle
rock impresses us with the fact that the present surface of the bed
rock must formerly have been covered with many feet of bed rock
that has now disappeared. But the mantle rock of the land tells
only a small part of the story. The sedimentary rocks were all
of them made of material from former bed rock, and the loose
material on the ocean floor, with the exception of that found in
the deepest places, was derived from the same source.

To produce this great mass, a quantity of solid rock much greater
than the sum of the masses of the mantle rock, the sedimentary
rock, and the material on the ocean floor, must have been worn
away; for sea water contains millions of tons of matter dissolved
from both bed rock and mantle rock, and portions of the bed
rock have been deposited and worn away again more than once.

If all this material came from the land at present known, it is
evident that the total is equivalent to the removal of thousands
of feet of bed rock from all the land of the earth. This great
quantity of material has not been removed with equal rapidity
in all parts. The rocks have been worn away more rapidly where

THE BED ROCK

267

they were weak or where the agents were particularly active, mak-
ing the land uneven, developing valleys, canons, gorges, and fiords ;
and leaving mountain peaks and ridges, or mesas and buttes.

We use the general term erosion to designate the act of wearing
away the land. The principal sub-processes of erosion are weather-
ing, corrosion, and transportation. Weathering differs from the

268 PHYSIOGRAPHY

other processes in that it disintegrates rock but does not transport
the rock waste, whereas each of the other processes disintegrates
the rock, transports the rock waste, and deposits it elsewhere.
Each of these processes has been discussed in other chapters, and
we have seen that each of them wears away rock in a character-
istic manner and forms physical features both by erosion and by
deposition which are easily distinguished.

FIG. 120. YOUNG MOUNTAINS, SHOWING SLIGHT EFFECTS OF EROSION

The Story Recorded in the Bed Rock. As one walks along the sea-
shore he notes that the beach is strewn with shells and sea weed, and
that many living species make their homes in the sands. Occasionally
one sees a fish that has been washed ashore, or finds relics from the
plant and animal life of the land, such as leaves blown by the wind or
bones brought by some predatory animal.

If such a beach should be consolidated the sandstone formed would
preserve many of the modern kinds of life and from them students of
the distant future could learn much about our forms of animals and
plants. The great series of sedimentary rocks contains just such a
record of the forms of plant and animal life of the past, and since each
layer is older than all of those deposited above it, we are able to deter-
mine the order in which the various types of life occupied the earth's
surface. The various layers may be likened to the leaves of a book,
each one of which bears a record of the kinds of life which dwelt upon
the earth at a given time.

A study of this record reveals some very interesting facts. The lowest
layers of the sedimentary rocks rest upon igneous or metamorphic rocks
containing few indications that there was any form of life on the earth
while they were being formed. In the lowest layers of the sedimentary
rocks, however, we find many shells and the remains of animals resem-
bling in some respects the horseshoe crab, but no remains of any higher
form. As we examine the layers above the lowest we find that these
simple forms are replaced by more and more complex species of the same
families, and then fish appear. As we ascend, layers are reached con-

THE BED ROCK 269

taining the remains of reptiles, small at first, but finally specimens of
gigantic size are reached. In succeeding layers reptiles decrease in
importance and are replaced by mammals related to the elephant and
the dog. In the upper layers the remains of modern animals are found,
but the remains of man and his implements and works are confined al-
most entirely to the mantle rock. This record of the rocks tells us that
animal life began with very simple forms which gradually increased in
complexity of structure and brain power and that by slow steps, through
long ages, the present forms of life have been developed.

QUESTIONS

1. Mention six gems that belong to the quartz group of minerals.

2. State two properties common to both quartz and feldspar. State
two properties, either one of which would distinguish quartz from feld-
spar.

3. Find five illustrations in this book showing stratified rock, and
three showing unstratified.

4. State four properties in which calcite differs from feldspar.

5. State two points of resemblance and two of difference between
sandstone and shale.

6. Which is the more common surface rock in the United States, sedi-
mentary, igneous or metamorphic? Which least common?

7. What States yield coal? See Fig. 117.

8. How could you determine whether a given specimen was sandstone
or limestone, if you had no acid?

9. Why is rock forming near the shore of some coral islands a pure
limestone?

10. How may slate be distinguished from shale?

11. Coarse-grained granite is now on the surface in New England;
what does this prove concerning the elevation of the surface in these
localities at the time when the granite was forming?

CHAPTER XIX
THE GROUND WATER

What Becomes of the Rain. We all know that rain either dries
up, or runs off in streams, or sinks into the ground. In Arizona,
for example, where the air is dry and warm, a large percentage is
evaporated. On steep hillsides a large percentage becomes run-off
into streams. The streams receiver large portion of spring rains
because the air, being cold and moist, evaporates but little, and
the ground, being frozen, cannot absorb it. But when rain falls
on loose soil, especially where it is level or gently sloping, a large
percentage enters the ground and is called ground water.

of water

-^^T lmpermeat>/e layer

FIG. 121. WATER TABLE WITH ITS HILLS AND VALLEYS

Importance of Ground Water. This water that enters the
ground supplies our wells and springs, keeps our rivers from run-
ning dry between rains, and deposits valuable ores and metals;
but its most important work is to dissolve rocks and minerals and
to furnish liquid food to plants, thus making agriculture possible.

Water Table. Surface water tends to sink into the ground
until it reaches the saturated portion. This saturated portion
extends deep into the earth, perhaps into the heated interior, for

THE GROUND WATER

271

steam forms a considerable part of the product of volcanoes.
The water table is the upper surface of the saturated portion of
the soil and rocks. The level of water in wells indicates the
height of the water table. In general, the water table has its
hills and valleys corresponding to those of the surface, but with

FIG. 122. WATER TABLE IN RELATION TO LAKE, MARSHES, AND
SPRINGS

less difference in elevation, for it is nearer the surface in valleys
than under hills. In fact, it frequently comes to the surface in
valleys, forming springs, swamps, and lakes. The beds of per-
manent streams reach below the dry- weather level of the water
table; streams whose beds are above the water table tend to sink
into the ground and disappear. The same is true of lakes and
swamps.

Soring ruin**
6y ditch

-Oitctl. - .

FIG. 123. WATER TABLE LOWERED BY DITCH

The water table rises after a heavy rainfall and sinks during a
dry season. Wherever its surface is inclined there is a constant
movement of the water toward the lower portions, causing a
renewal or change of water, not only in springs, but also in wells.

The water in a well sunk in coarse sand and gravel, in Madison,
Wisconsin, stands 52 feet above that of a lake 1,250 feet distant, show-
ing a slope of i foot in 24, while in Long Island a slope of i in 440 has
been found. When water is pumped rapidly from a well, the slope of
the surrounding water table becomes steep, and there is a more rapid
movement of water toward the well.

272

PHYSIOGRAPHY

Wells. Wells are holes dug or bored for the purpose of supply-
ing water. To be permanent they must sink below the dry-
weather level of the water table. They should be so situated and
so protected that surface filth and impurities cannot drain into
them. Clear, tasteless, odorless water may yet be dangerous.

;// fa i/s in dry season

Never -fof/ing we//

Wet weather spring.

FIG. 124. WATER TABLE, WELLS, AND SPRINGS

When water comes from great depths it is well filtered and freer
from surface impurities, such as typhoid fever germs.

Many small communities depend upon wells for their water
supply, but most large cities have abandoned them, using instead
the water from lakes and rivers.

Well, drained

FIG. 125. EFFECTS OF PUMPING ON WATER TABLE AND WELL

Artesian Wells. In ordinary wells the water level coincides
with the local water table; but this is not the case in artesian wells.
Artesian wells are wells in which the water level is independent of the
local water table and dependent upon pressure transmitted from some

THE GROUND WATER 273

distant water table. The first artesian wells, in Artois, near Paris,
overflowed, forming what are called flowing wells.

The conditions necessary for an artesian well are an impervious
layer overlying a porous layer, such as sand or gravel, that receives
water from some level higher than that of the bottom of the well.
There may be an underlying impervious layer, but this is not neces-
sary. The water in the well tends to rise to the level of the water
hi the porous layer, and it may overflow.

Heoef of coptire afieef of water

FIG. 126. DIAGRAM OF AN ARTESIAN WELL

These necessary conditions are frequently found on plains slop-
ing gently from higher land, such as the Great Plains sloping
eastward from the Rocky Mountains, and in the Atlantic and
Gulf plains in southeastern United States. At Atlantic City,
artesian wells 800 feet deep furnish water that fell miles away on
the mainland, and flowed under the salt water of the marshes off
the coast of New Jersey. Some of the water in the artesian wells
of the eastern shore of Maryland has passed beneath Chesapeake
Bay; in Calais, northern France, water is drunk that fell as rain
on the English side of the Strait of Dover.

To Illustrate the Action of an Artesian Well. Take a basin, represent-
ing the lower impervious layer, and partly fill it with a mixture of sand
and water. Force down into the mixture a second (tin) basin with a
hole in it. How high will the water spout?

274 PHYSIOGRAPHY

Springs. When ground water forms a small stream, it tends to
follow joints and cracks and the tops of impervious layers. When
the stream comes naturally to the surface it is a spring. Some-
times there is a line of springs along a hillside, just above an
impervious layer. A spring rising through sand may form a
dangerous or troublesome quicksand. In regions with plentiful
rainfall springs are numerous. A permanent spring is a valuable

FIG. 127. SPRING FROM CAVE FORMED BY STREAM FOLLOWING A FAULT

asset on any farm. In arid regions they are especially valuable,
for around them is found the only productive land. Oases in the
desert owe their fertility to springs and wells.

Hot Springs. Hot springs are formed when their waters come from
great depths or from near hot lava. Such springs are generally mineral
springs also, for warm water is a better solvent than cold water. Many
mineral springs have medicinal properties and become health and pleasure
resorts. Well-known examples are Saratoga Springs, New York; White
Sulphur Springs, Virginia; Hot Springs, Arkansas; Bath, England; Vichy,
France, and Karlsbad, Bohemia.

Geysers. Geysers are explosive springs of boiling water whose eruptions
occur at rather regular intervals. They are found in the volcanic regions

THE GROUND WATER

275

of Yellowstone Park, Iceland, and New Zealand. The water is boiling
hot because it comes in contact with highly heated rock. The boiling
point is above 212 Fahrenheit, owing to the pressure of the column of
water above. The irregularities of the tube probably make convectional
interchanges of water difficult and slow. When, in spite of the great
pressure, the boiling point is reached and steam is formed below, causing
some of the surface waters to overflow, the diminished pressure lowers
the boiling point of the water and enables great quantities much above
its boiling temperature to flash into steam. The explosion of the steam
expels the overlying water with great force, often sending the column 200
feet above the crater. The operation is repeated at intervals. Beauti-
ful white deposits are formed around geysers.

FIG. 128. DEPOSITS FROM HOT SPRINGS, YELLOWSTONE NATIONAL PARK

Destructive Action of Ground Water. Ground water absorbs
carbon dioxide from the air and acids from decaying plants and
animals so that it becomes a weak acid. When it passes through
the rocks it tends to weather and to dissolve it, weathering it to
mantle rock. By weathering the mantle rock it assists in the
formation of soil. This is the most important destructive work
of ground water. A more spectacular work is seen as it passes
through limestone. It slowly dissolves the limestone and forms
passages, and in some regions large caves.

276

PHYSIOGRAPHY

Mammoth Cave. The largest known cave in the world is Mammoth
Cave, Kentucky. It is over nine miles from entrance to farthest recess.
It has a network of numerous galleries and passages which cross and
recross one another, with a total length of over two hundred miles. It
has its own rivers and lakes, in which are found sightless crayfish.
Countless bats cling to its walls. Its blind rats have very long sensitive

FIG. 129. DIAGRAM SHOWING CAVE AND NATURAL BRIDGE

AA, Layers of limestone; BB ? sink holes; CC, vertical shafts or domes; and DD, hori-
zontal galleries with stalactites, stalagmites, and pillars.

whiskers for feelers. The many blind forms of anim?l life are probably
the descendants of normal animals that entered the cave long ago.
Skeletons of men have been found there.

The best preserved skeletons of prehistoric man and the best samples
of his hand work have been obtained from the limestone caves of France,
Belgium, and Spain.

In limestone regions the surface streams sometimes fall into
sink holes, that lead to underground passages. Where sink holes
are numerous there may be no surface streams, as in the Karst
region east of the Adriatic Sea. Occasionally, too, portions of the
roof of a cave fall in, leaving a portion that forms a natural bridge.
Natural bridges are also formed in other ways.

Underground streams do the same kinds of work that the sur-
face streams do. In addition to this, some of them, where they
are closely confined, wear away the roof above them.

Constructive Work of Ground Water. When water is evapo-
rated it deposits its dissolved mineral matter. An example of
this is the scale on the inside of a tea-kettle, or of a steam boiler,
in which hard water (from limestone regions) has been boiled. In
regions of limited rainfall the evaporation of the ground water
leaves on the surface deposits that render the soil unfit for agricul-
ture. In certain portions of New Mexico, where irrigation has
been introduced, these alkali deposits are removed by flooding.

THE GROUND WATER

277

Fro. 130. NATURAL BRIDGE IN SANDSTONE
Near Buffalo Gap, South Dakota.

2 7 8

PHYSIOGRAPHY

In some regions the matter deposited underground acts as a
cement, consolidating the mantle rock. Sometimes crevices in
the rocks are filled with minerals, from solutions, forming veins.
They are to be distinguished from dikes, in which the crevices
are filled with what was once molten matter. Hot water is, as
a rule, a better solvent than cold water. Water from deep in the

FIG. 131. THE SIDE OF A "SINK" AND ENTRANCE TO A CAVE IN WEST VIRGINIA
The stream that enters here reappears three-quarters of a mile away. (Photo by Johnson.)

ground is hot. When such water approaches the surface it cools
and deposits are formed. Many veins of ore originated in this way.

Where water containing dissolved limestone slowly drips from
the ceiling of a cave, the evaporation of the water and the loss of
the dissolved carbon dioxide cause the deposition of limestone,
both as stalactites, hanging like icicles of stone from the ceiling,
and as stalagmites, on the floor of the cave. A stalagmite may unite
with a stalactite to form a pillar.

THE GROUND WATER 279

Precipitation. Sulphur springs are very common in some regions.
If the water from such a spring should mingle with water containing
dissolved compounds of silver, gold, copper, lead, or zinc, these metals
would be precipitated in combination with the sulphur as sulphides,
and would fill the channels through which the mingled streams
passed. Other deposits are due to the diminished pressure as the water
approaches the surface. This permits the escape of some of the dissolved
gases which, like carbon dioxide, are necessary to hold the mineral in
solution. Still other deposits from solution are caused by certain minute
plants called algae. A compact limestone is deposited about hot springs
in Yellowstone National Park. The white deposits about geysers are
believed to be due to the same action.

Petrifaction. Petrifaction consists in the slow solution of certain
organic substances in the earth, and the replacement of the dissolved
molecules with deposits of mineral. Many fossils, originally in lime-
stone, have been exactly reproduced in quartz or in iron pyrites.

Capillary Water. If the corner of a lump of sugar touches a
liquid, like coffee or water, the liquid quickly spreads through the
whole lump. In a similar way water spreads through soil, cover-
ing each soil particle with a thin film called capillary water. This
capillary water makes the soil moist, and even in a dry season
supplies water to the roots of plants above the water table. It
also transports water from the water table to the surface, where
it is evaporated.

Capillary Action. As the result of the attraction of water parti-
cles for one another and of glass for water particles, water is
attracted a short distance up the side of a glass of water. Water
rises about ^ of an inch in a glass tube with a bore of -fa of an
inch; but it rises 6 inches when the bore is y^o of an inch.
This shows that water rises higher in tubes of smaller bore.

In the soil, the small spaces practically form slender tubes or
passages through which water moves. If the passages are dimin-
ished in size, capillary action is facilitated, and the water tends to
collect there. Advantage is sometimes taken of this when a per-
son steps upon the place where seeds have just been planted.
The weight of the body presses the soil around the seeds, capillary
action is assisted, water collects, and the seed germinates more
quickly.

280 PHYSIOGRAPHY

On the other hand, if the spaces are increased, as for example
when soil is plowed or cultivated, capillary action is interfered
with. The ground water does not rise so rapidly and evaporation
is retarded, leaving more water in the ground to nourish the crop.
The loosened soil particles resulting from cultivation constitute
what is called a dust mulch.

Although the loss of ground water through evaporation is very
great, yet in the eastern part of the United States this is in large
part counterbalanced by plentiful rainfall. But even here many
a farmer has learned by experience the value of cultivating his
corn in a dry season, though there are no weeds to be killed.

Dry Farming. The value of the dust mulch is greatest where
rainfall is scanty. In the Great Plains, just east of the Rockies,
the rainfall is under 20 inches, not enough for agriculture under
old methods. But dust mulching after every rain has made parts
of this region, formerly called the Great American Desert, blossom
as the rose.

In portions of both the Great Plains and the Plateau Region
the rainfall is too scanty, even with careful dust mulching, to raise
a good crop every year. So the farmers not only cultivate after
every rain, but they also refrain for a season from planting any
crop. This cultivating without planting (called summer fallow)
stores the rainfall until there is sufficient ground water for a
bountiful crop.

The practice of alternate cropping and summer fallowing is, according
to the Year Book of the Department of Agriculture for 1907, a common
one in the semi-arid region. But the practice of allowing the soil to
remain bare during the entire season is questionable, for it must neces-
sarily result in an almost complete destruction of the organic matter in
the soil. A much better practice is to raise some kind of leguminous
crop which can be turned under while there is still a sufficient amount of
moisture in the plants and in the soil to cause rapid decomposition.

Not satisfied with preventing evaporation and conserving the
rain of two or more seasons, some farmers cause the ground water
to collect near the roots of the plants, where it will do the most

THE GROUND WATER

281

good. This is done by sub-surface packing. The sub-surface
packer forces the soil particles a little below the surface nearer
together. This facilitates capillary action so that the ground
water tends to rise and to collect in the packed earth. The dust
mulch is used to prevent its escape by evaporation.

Dry farming is the method by which crops are raised in regions
of deficient rainfall. It applies the principles of dust mulching,

SHADED AREAS

HAVE ENOUGH

RAIJf FOR FARMING

FIG. 132. DRY V ARMING is SUCCESSFUL IN THE AREAS UNSHADED IN THE MAP

summer fallowing, sub-surface packing, and the selection of drought-
resisting crops.

Forests. Regions with sufficient rainfall, well distributed throughout
the growing season, are usually forested. A forest is a growth of trees
sufficiently dense to form a fairly unbroken canopy of trees. Most of
our trees are deciduous, that is, they shed their leaves in winter. The
accumulation and decay of these leaves give to a forest a peculiarly rich
soil of its own making and covered with leaves that tend to prevent
evaporation of soil moisture.

Value of Forests. Trees furnish us all of our wood and lumber. They
furnish the fuel of the country, much of it directly, but part of it indirectly
through coal. They furnish food fruits, nuts, maple sugar, etc. They
furnish many valuable raw materials used in manufacturing paper,
tanning materials, wood alcohol, tar, pitch, turpentine, resin, fibres.
They improve the climate by setting oxygen free in the process of making
starch; as windbreaks they check the destructiveness of the winds; the

282

PHYSIOGRAPHY

evaporation of their moisture and their shade cool the air, making it
moister and subject to changes that are less and slower than in the
neighboring open country. They affect drainage; by retarding the
melting of the snow they prevent spring freshets; the leaves and loose
soil retain the rain that would otherwise flow off rapidly in floods; the
water so retained is doled out, so that the streams and their water power
are maintained during dry weather.

The removal of forests causes floods that destroy property and life,
remove the humus and fertile soils, and fill the streams with soil and rock
waste. The coarse rock waste of the freshets is spread over the low

LUMBER REGIONS

VMft Heavily Timbered

. ' ' ; j Moderately Timbered

FIG. 133. THE LUMBER REGIONS

ground, thus destroying what has been valuable farming land. The water
power is lessened or destroyed during dry seasons and may be too great
to be utilized during floods.

11 Forestry is the preservation of forests by wise use." Roosevelt.
Our 300,000 square miles of national forest are protected by the United
States Forest Service, which administers public property estimated to be
worth over $2,000,000,000. The Service aims to diffuse information, to
prevent the spread of fires, to destroy injurious insects and fungi, to
restrict cattle grazing in forests to certain seasons, to insist that each
tree cut be replaced by another of the same kind.

Forests cover 550,000,000 acres, about one-fourth of the United States,
and Forestry is becoming more and more important, for a timber famine,
especially in the hard woods, is upon us. Forestry is very promising as a
profession.

THE GROUND WATER 283

QUESTIONS

i. Would a rainy month in the spring cause floods of the same size as
an equally rainy month in the fall? Why?

-.2. Why is the water table not level? Would it be more nearly level
in sand, in gravel, or ordinary soil? Why?

3. Illustrate by means of a diagram properly labeled the position of
the water table and a spring, a permanent stream, a temporary stream,
a marsh, and a lake.

4. With the vertical scale i inch equals 100 feet, draw a diagram for
an artesian well that sends water 50 feet into the air. Name and label
the parts.

^5. Show the proper relative positions of a house, barn, well, and out-
buildings on a side hill. Give your reasons.

6. Why do cities generally depend for their water supply upon lakes
or rivers instead of upon wells?

7. Which would be more apt to be brackish or mineral, water from
springs in a desert, or from springs in a well- watered region? Which
might seem better? Why?

8. Illustrate relations of stalactite, stalagmite, and pillar.

9. Fill three flower pots of the same size with the same amount of soils
of uniform texture. Cover one with straw, another with dust mulch, and
"puddle" the top of the other (that is, wet it so that when it dries it
will cake as a dried mud pie) . When all are prepared weigh them. Set
each in a saucer with a weighed amount of water. As the water is
absorbed from the bottom receptacle, keep filling it up, being careful to
weigh or to measure carefully the amounts supplied. Compare the
amounts absorbed and state your generalization therefrom.

10. How would living in a cave without light affect the various senses
of animals compelled to live there for many generations?

sii. Why are regions which are believed to be the roots of worn down
mountains so frequently rich in. ores?

1 1. How may geysers choke themselves until they no longer erupt?
si3. How will the evaporation of water, furnished by irrigation, affect

the amount of soluble plant food in the soil below the surface and at
the surface?
vu4. "What's the use of cultivating corn when there are no weeds in it?"

CHAPTER XX
THE WORK OF RIVERS

The rivers of the United States furnish power to great manufac-
turing industries, supply water to cities, and transport every year
hundreds of thousands of people and millions of tons of merchan-
dise and farm products. They have figured in the history of our
country from the beginning, both in its peaceful settlement and
development and in war. The Hudson River was of vital strategic
importance during the Revolution, and the Mississippi and the
Tennessee during the Civil War.

In the economy of nature streams have many functions, the most
obvious of which is the removal of the surplus rainfall. But while
doing this, all streams, from the largest river to the tiniest brook,
are slowly wearing down the land.

The Formation and Development of a Gully. To study the
principal phenomena of the work of a river in wearing down the
land, it is not necessary to go farther than to the nearest bank of
soft earth and to note what happens during and immediately
after a heavy rain. The water collects in a stream and flows over
the edge of the bank and down its slope, quickly forming a minia-
ture valley or gully.

If the stream is swift and the bank soft and steep, the valley
deepens rapidly. The steepest place is at the edge of the bank,
and here the stream cuts into the earth and the valley lengthens
headwards. During a heavy rain the sides of the gully are washed
in and the valley widens rapidly.

The materials washed out of the gully are, in part, deposited at
the foot of the bank, where the slope becomes gentler. Here the
stream is building up instead of cutting down. When the rain
slackens it may be possible to see the stream diminish in size after

THE WORK OF RIVERS

285

a time, and the stream -deposits extend up the gully, partially
filling it and making the miniature valley flat-bottomed.

Sometimes for several days a small stream, fed by some tem-
porary spring, will flow down this flat-bottomed valley in a wind

FIG. 134. GULLY AND ALLUVIAL CONE, FORMED IN A SINGLE SHOWER
Near Baraboo, Wisconsin. Note coarse stones in gully. (Eliot Blackwelder.)

ing course, cutting into its outer or concave bank at every curve.
Here the banks are generally higher and steeper and the stream
deeper. The convex inner bank is lower and more gently sloping,
with deposits in front.

If the gully is examined after the stream has disappeared, it
will be seen that in places the slope is too steep for any deposits
to be made, but where deposits are made in steep places coarse
materials predominate, whereas deposits made on gentler slopes
are not so coarse. The result is an assorting of deposited mate-
rials. This is noticeable in the deposits at the foot of the slope.

Sometimes a stone in the path of the stream causes a fall to
form. Just below the fall the water wears out a hole deeper than

286

PHYSIOGRAPHY

the average of the other portions of the stream. A short distance

below the fall there is frequently a deposit in the bed of the stream.

Most of these phenomena of the gully can be seen without much

FIG. 135. MAN MEASURING STREAM FLOW

difficulty along every stream, in some portion of its course. Gully
and stream work may be summarized in a sentence: Streams drain
away the surplus rainfall, and in so doing wear down the land, and
transport, comminute, and finally deposit the waste so formed.

DRAINAGE

Drainage. Our rivers remove the equivalent of about 10 inches
of rainfall per year from the whole United States. The Mississippi
annually carries to the sea about one-ninth of the rainfall of the
whole country, an estimated total of 44.7 cubic miles. This is
enough to make a lake the size of the State of Illinois, and four feet
deep.

THE WORK OF RIVERS

287

The economic value of this
water is such that the National
Government is seeking to de-
termine the best methods of
storing and conserving it for
irrigation and power pur-
poses. Government officials
have estimated that the
streams of the Southern
Appalachians alone have
1,400,000 undeveloped horse-
power, worth, at $20 each,
$28,000,000 per year.

By saving and doling out
in dry seasons the water of
floods, mills can be kept
going that otherwise might
be compelled to close. If
water power, often called
white coal, could be used
instead of coal for power purposes, our diminishing coal deposits
would be conserved.

CORRASION

Corrasion. Streams wear away their beds and banks, forming
most of the valleys of the world and obtaining materials that are
eventually deposited in the sea. Stream corrasion is the wearing
away of rocks by running water. Part of this is due to the solvent
action of water. Clear water alone is a poor corrading agent.
Just as paper by itself is a poor abrading agent but becomes an
efficient one when covered with sand as sand-paper, so water
supplied with sand and rock fragments as tools becomes an effi-
cient corrading agent. A load of sand thrown into the clear water
of the Niagara River from the bridge just above the American
Falls soon scours away the moss that the water alone is not able
to remove from the rocks in the bed of the river.

136. SOME INSTRUMENTS USED IN
MEASURING STREAM FLOW

288

PHYSIOGRAPHY

The rate of corrasion depends upon the resistance of the materials
forming the stream bed, the volume and velocity of the water, and
the kind and amount of material transported by the stream and used
as corrading tools. In 1906 the Colorado River, which supplied
water to an irrigating ditch leading to the Imperial Valley region
of southern California, got beyond control. In the weak deposits

8.000

7,000

6.000

6,000

4,000

3,000

2,000

1,000

FIG. 137. STREAM FLOW IN REGION OF LITTLE RAINFALL

Discharge of West Gallatin River at Salesville, Montana, for 1899.

(U. S. Geological Survey.)

of the old delta that the Colorado had built across the Gulf of
California, it soon changed a small irrigating ditch to a channel
large enough to receive half of the river. In the depression that
had once been the head of the Gulf, the Salton Sea was formed, a
lake larger at one time than Lake Champlain. A fall over ninety
feet high and 1,500 feet wide cut back toward the Colorado at
the rate of half a mile a day. After threatening the destruction
of $100,000,000 worth of property, the river was, with much diffi-
culty and at great expense, finally brought under control.

(a) Downward Corrasion. Every stream is in some part of its
course corrading its bed, thus forming or deepening its valley. The

THE WORK OF RIVERS 289

best example in the world of the work of downward corrasion is
the Grand Canon of the Colorado, 300 miles long and in places
over a mile deep, all cut out little by little by the Colorado River.

etc. 138. GORGE CORRADED IN SHALES, LAKE KEUKA, N. Y.

The deepening of Us valley is the first work of a river, for the
water brings its corrading tools into direct contact with its bed
and wears the bed away. The stream tends to cut down verti-
cally and to form narrow, deep valleys with precipitous sides,

2 9

PHYSIOGRAPHY

called gorges or canons. An overhanging side may be the result
either of curving and consequent undercutting by the stream, or
it may be due to the rock structure.

The widening of a river valley is largely the result of weathering.
The side walls are disintegrated and the particles of rock fall and
are washed into the stream and carried away. If the materials
of the sides of the valley are sand or gravel, the sides cannot be
very steep, otherwise the materials would roll down the slope.

FIG. 139. DIAGRAM OF GRADUAL WIDENING OF VALLEY

The rate of widening determines the shape of the valley as seen in
cross-section. Torrents may retain for a time valleys with almost
vertical sides, such as is seen at AAA in Fig. 139; but as down-
cutting becomes less rapid, and the weathering agents widen the
valley, it becomes V-shaped in cross-section, as B B B. As the
valley widens more and more, it assumes in turn shapes more
like C C C, D D D, and E E E, becoming wider and wider, until
its sides have a very gentle slope.

When the valley is cut through rocks of different hardness, the
weaker rocks are worn away more rapidly than the more resistant,
which may stand out as cliffs, their upper surfaces forming rock
terraces, as in Fig. 140.

A river valley becomes longer, just as a gully develops, by corra-
sion at the very head of its valley. This headward corrasion

THE WORK OF RIVERS

2QI

HARD
SOFT

HARD

TERRACE

SOFT

HARD

SOFT

FIG. 140. ROCK TERRACES

increases the length and decreases the steepness of the stream, as
is shown in the series of profiles in Fig. 141. A river profile is a
line showing the slope of the surface of a river from its source to its
mouth, according to definite vertical and horizontal scales. The

FIG. 141. PROGRESSIVE HEAD WARD CORRASION

profile A E M shows a steep slope near the source, becoming gen-
tler down stream. Profiles BFEM, CGFEM, andDGFEM
show progressive headward corrasion.

When a stream encounters in its bed rocks of different hardness,
it wears away the weaker rocks more rapidly, producing at the
hard rocks falls and rapids. At the foot of the falls the water
swirls stones and bowlders and tends to wear there a depression
called a pothole.

Most land surfaces are so uneven that the course to be taken

292 PHYSIOGRAPHY

by rains falling on them is predetermined. In such regions it is
easy to locate the water parting or divide. A divide is the line
separating two adjacent river basins. Sometimes the area between
two streams is so nearly level that the course rainfall will take is
doubtful. In such a region divides are not well marked. But no

FIG. 142. DIVIDES AND STREAMS
IN AUSTRIAN ALPS. (Hachure Map.)

FIG. 143. DIVIDES IN PRECEDING REGION

matter how level the region, with the headward advance of streams
divides must eventually be developed between principal streams,
with subdivides between tributaries.

BASE

LEVEL-

FIG. 144. DIVIDES, VALLEY, AND STREAMS

Divide A between streams X and Y is adjusted. Divide S between streams Y and Z, shifting
from i to 5, disappears at 6, causing Z to be captured by a tributary of Y.

THE WORK OF RIVERS

293

If one stream is more actively deepening its valley than a neigh-
boring stream, the divide between them shifts toward the weaker
stream, as divide S in Fig. 144. But when two streams reach
the point of lowering their basins at the same rate, the divide
between them is adjusted, and instead of shifting sinks vertically
as the region is worn down, as divide A in Fig. 144,

Stream Capture. The effects of shifting of divides are seen in
the Shenandoah River Valley, a region of rocks less resistant than
those of the Blue Ridge to the east of it. The master stream of

FIG. 145. STREAM ARRANGEMENT IN WEST VIRGINIA AND VIRGINIA
Three stages in the capture of Beaver Dam Creek, B, by the Shenandoah, S.

the Shenandoah, the Potomac, cuts through the Blue Ridge at
Harper's Ferry, forming a water gap. Beaver Dam Creek formerly
had its course west of the Blue Ridge, as is shown in Fig. 145.
It then flowed through the mountain ridge at Snicker's Gap, a
battleground of the Civil War.

The Potomac, being larger, corraded its gap deeper than did
the Beaver Dam; consequently the Shenandoah, a tributary of
the Potomac, was able to corrade more deeply in the weak rocks
of its valley than the Beaver Dam through the resistant rocks of
the Blue Ridge. The divide between the two shifted nearer and
nearer to the Beaver Dam, until finally the Upper Beaver Dam

294

PHYSIOGRAPHY

was captured and became a tributary of the Shenandoah. The
present Beaver Dam is a beheaded stream; and the abandoned
water gap is a wind gap. From Snicker's Gap to the Shenandoah
the stream is reversed in direction. This action by the Shenandoah
illustrates one method of stream capture or river piracy.

Tributaries. When several streams flowing down the same gen-
eral slope unite, they form a tree-like system of drainage in which

FIG. 146. MEANDERING STREAM IN A NARROW FLOOD PLAIN
Canon del Muerte, viewed from Mummy Cave.

is illustrated the general rule that tributaries join their master
stream at an acute angle pointing down stream.

Tributaries, though smaller, are generally steeper and swifter
than their master stream, and may be corrading their beds more
rapidly; but it is clear that they cannot corrade deeper than their
master stream where they join. This tendency of tributaries to
corrade their beds at such a rate as to join their master stream at
its grade, or level, is known as Playfair's Law.

THE WORK OF RIVERS

295

Stream capture, as we have seen, may result in causing tribu-
taries to join the capturing stream at a right angle, or even at an
acute angle pointing up stream. The Potomac and the Delaware
have tributaries joining' them at right angles; and Schoharie
Creek, N. Y.,, and the Maumee River, in Ohio, have tributaries
that join at acute angles pointing up stream.

(b) Lateral Corrasion. Every stream is cutting into its outer
bank at every curve, because water obeys the law of motion that

CUTTING

FIG. 147. DEVELOPMENT OF A MEANDER

bodies in motion continue in motion in a straight line and at a
uniform rate unless acted upon by some outside force.

If any curve in any stream is examined, it is found that the main
channel and the swiftest current approach the outside of the curve.
Let this be indicated in Fig. 147 by a dotted line, and let an arrow
indicate the direction of flow. Just above and below the curve, the
main channel, at A and D, is in the middle of the stream. But
between B and C the water (obeying the law of motion) ap-
proaches B, and the swiftest current and deepest water are found
nearer B than C. The swift current cuts into the bank at B and
tends to undermine it. Soon a portion of the bank at B falls into
the stream, and is washed away. This operation continues, and
the channel moves more and more toward B.

296

PHYSIOGRAPHY

On the opposite side, at C, on the inside of the curve where the
bank is convex, the velocity of the water is less and the stream
deposits a part of its load, building out this side.

This cutting and filling action continues at every curve, and the
stream tends to develop a series of winding curves called meanders.

This tendency to meander is best seen where the materials

FIG. 148. MEANDERING STREAMS, LARAMIE CREEK, WYOMING
Notice nearest curve bank, high on our right, low on left.

composing the banks of the stream are most easily corraded. These
conditions are found in the low " bottom " lands near streams
that the water overflows when the streams are in flood. These
lands, called flood plains, are composed of materials that the
stream has brought there and deposited from its muddy waters.

FIG. 149. How CUT-OFF AND OXBOW LAKES ARE FORMED

THE WORK OF RIVERS

297

A river may become so curved, especially where it is meander-
ing over a large flood plain, that two curves approach each other.
At some flood stage the water cuts through the intervening nar-

FIG. 150. OXBOW LAKES, SAND DEPOSITS, AND MAIN CHANNEL OF PORTION OF THE
MISSISSIPPI RIVER

row neck of land and forms a cut-off, as at X in Fig. 147. For
a time the water flows through both channels; but the new is
shorter and the current through it consequently swifter, so that
it rapidly increases in size until all of the water passes through it.
In this way streams tend to straighten themselves. Fig. 149.

298 PHYSIOGRAPHY

The ends of the old channel are almost at right angles to the
new channel; water entering the old is checked in velocity and
deposits materials that slowly close the entrances to the aban-
doned channel, changing it to an oxbow or crescent lake.

The crescent lakes of large rivers are arcs of larger circles than
are the crescent lakes of smaller streams. There is a relation
between the size of a stream and the size of the curves it makes
on its flood plain. The curves of the lower Mississippi are arcs
of circles of approximately five to ten miles in diameter. A line
along the outside of the curves on the east side of the Mississippi
is about fifteen miles from the corresponding line on the west
side, making the meander belt or meander zone of the Mississippi
about fifteen miles wide. Meanders move down stream.

TRANSPORTATION

Transportation. A glass of water dipped from any muddy
stream will become clear after standing for a longer or shorter
period, and layers of sediment will form on the bottom of the
glass. This solid material distributed through the water, in spite
of its greater density, is said to be carried in suspension.

The particles have been obtained by corrasion of the bed or of
the banks, or may have been washed into the stream. It is pos-
sible that the particles may travel to the mouth of the stream
without a stop, but under ordinary conditions they settle to the
bottom when the current slackens, to be picked up again and
carried farther when the current is again increased.

The finest particles of rock waste are heavier than water, and
would settle in water at rest or in water moving in lines parallel
to the surface. But the irregularities in the stream beds are con-
tinually sending numerous currents upward, thus counteracting
the tendency of the particles to settle. In comparison with a
large particle a small particle has a larger area, and it must there-
fore set a relatively larger mass of water in motion in order to
sink.

The transporting power of a stream depends mainly on the
volume and the velocity of the stream. A stone weighs less in

THE WORK OF RIVERS 29^

water to the extent of the weight of water it displaces; this causes
most common rocks to lose about one-third of their weight in
water. The transporting power of streams increases as the sixth
power of the velocity. For example, if the velocity is trebled, the
transporting power, instead of being trebled, is increased 729
times, that is, 3X3X3X3X3X3- Torrents, in their steep upper
courses, are able to roll along bowlders of many tons weight.
A change in velocity, even if slight, makes a great change in the
amount of sediment that may be transported.

In swift streams much rock waste too coarse to be held in
suspension is rolled along the bottom, and sometimes in mountain
torrents the collisions between the stones thus moved produce
a loud and almost continuous noise. In the lower portion of rivers
the amount of sand and small pebbles that is rolled along the
bottom is probably a very important part of the total amount
of rock waste transported.

Work in many streams is done at flood time only. During the
summer most streams are low and do little work.

Water in passing over soluble substances dissolves them in
part. In limestone regions the water becomes " hard," that is,
with soap it does not easily form lather or suds, and when boiled
a scale forms on the inside of the boiler or of the kettle.

The amount of mineral matter carried in solution by streams
varies greatly. It depends, among other things, upon the nature
of the region over which the streams flow. It is well known that
the water of streams in sandstone regions is softer than that of
streams in limestone regions ; that is to say, they contain a smaller
percentage of mineral matter in solution. The total amount of
mineral matter carried to the ocean in solution is about one-third
as great as that carried in suspension.

A small amount of rock waste is transported on the surface of
streams, lodged in ice or attached to the roots of trees or to other
floating objects, called drift. Drift tends to go toward shore
when a stream is rising and the water surface in the middle of the
channel is highest. But when a flood is subsiding, the water
surface is concave and the drift tends to leave the banks and to

300 PHYSIOGRAPHY

seek the middle of the channel. Lumbermen take advantage of
this in floating out their logs.

Hilly, forested portions of the land should not be stripped of
their forests and cultivated, because this causes the streams to
wash the soil away. Neglect of these precautions has done much
damage in some of the older States. The Forest Service of the
National Government is endeavoring to avert the fate that has
overtaken certain portions of Spain and China.

The Mississippi River removes yearly, by rolling along its bed,
enough waste to cover a square mile to a depth of 19 feet; in
suspension waste enough for 241 feet more; in solution 50 feet
more if it were all limestone a total of 310 feet. This is enough
waste to lower the level of the whole Mississippi River Basin at
the rate of i foot in about 4,000 years.

The Po removes enough waste to lower its whole basin at the
rate of i foot in every 729 years.

COMMINUTION OF LOAD

Grinding and Polishing. Streams push and roll angular frag-
ments of rock along their beds and over one another, colliding as
they go, until their corners are knocked off and they are rounded
and worn smooth. In this way large angular stones become small
pebbles, characteristically smooth and rounded; just as boys'
marbles may be made by placing small pieces of marble in a cyl-
inder, the rotation of which causes the pieces to wear one another
round.

DEPOSITION

Deposition. A river carrying waste tends to deposit its waste
whenever its velocity is diminished. The velocity is diminished
by decreasing either the slope of the bed or the depth or volume of
the water. A slight check in velocity causes only the coarsest
materials to come to rest. Further checking deposits materials
Hot quite so coarse. By this process deposits of different sized
materials tend to form in different places at the same time, and in
the same place at different times. As a result, stream deposits

THE WORK OF RIVERS

301

3 02

PHYSIOGRAPHY

are generally assorted according to size and stratified, that is, ar-
ranged in layers.

Alluvial Fans and Cones. The effect of a sudden change of
slope is well illustrated in Fig. 134, of the gully. The water
loses velocity as it approaches the level land, and here the coarsest
materials are dropped. As the velocity diminishes the particles
deposited are smaller and smaller, gravel will be carried farther

FIG. 152. ALLUVIAL CONE AT MOUTH OF AZTEC GULCH, COLORADO

than the bowlders, sand farther than the gravel, and finally clay
farther than the sand. From the foot of the slope the deposits
spread out in a semicircular form, made up of concentric bands of
assorted materials called alluvial fans, if of gentle slope, but allu-
vial cones if the slope is steep.

Streams from mountains sometimes form fans which join later-
ally, forming plains. Because such plains from the Sierra Nevada
are higher than those from the Coast Ranges of southern Cali-
fornia, 'the San Joaquin River lies nearer to the Coast Ranges
than to the Sierras. For a similar reason the Po lies nearer to
the Apennines than to the Alps.

Sand Bars. A decrease of slope within the bed of a stream
causes deposition forming sand bars. These bars sometimes begin

THE WORK OF RIVERS

303

about obstructions that check the velocity of the water. Their
formation and growth resemble that of snowdrifts and sand
dunes. They have a gentle slope up which sand and pebbles are
rolled, and like dunes, they migrate. They are most noticeable
in times of low water; during high water they may be scoured
out, but they may form again about the same place.

Nearly all streams, large and small, are undergoing this scour-
and-fill process. The scouring effect is produced artificially in

FIG. 153. ALLUVIAL CONE, WITH TRIBUTARY AND DISTRIBUTARY STREAMS
Note contours on cone. (U. S. Geological Survey.)

South Pass, one of the mouths of the Mississippi, by means of
jetties, which narrow the channel. The water above the jetties
rises. The higher head of water causes the water to flow through
the narrower space with greater velocity, scouring out the deposits
and preventing the formation of a bar that would impede navi-
gation.

Braided Streams. Some rivers have in the dry season very
wide beds with but small volume of water. The wide and shallow
stream cannot then carry the load brought by its tributaries and
deposits much of the load on its own bed, forming numerous
interlacing channels. Such a stream is said to be braided; in dry

304 PHYSIOGRAPHY

seasons much of its water flows underground. The Platte River
is an example of a braided stream.

Deposits on Flood Plains. Water particles move about each
other with much less friction than they move over solids, and
also with less friction than between water and air. This
accounts for the fact that the deepest portion of the cross-
section of a stream is the swiftest portion, and also for the
further fact that in this deepest portion the velocity at the bot-
tom is less than that near the top.

When the water of a river spreads out in shallow sheets, as it
does when it overflows its flood plain, the velocity on the flood

FIG. 154. SECTION ACROSS ALLUVIAL PLAIN ON ONE SIDE OF A LARGE RIVER
Vertical scale exaggerated.

plain is much diminished by friction, whereas the velocity in the
channel, where the water is deep, is greater than the velocity in
the same place at low water. This not only causes deposition on
the flood plain, but is likely to cause corrasion in the bed else-
where, thus increasing the available materials for deposition on
the flood plain. The deposit is called alluvium, or silt. The
lands subject to Hoods tend to build up to the level of the floods,
and are very properly called alluvial or flood plains. Because every
flood renews the fertility of the flood plain, we find here the most
fertile lands. The flood plain of the Nile was the granary of the
ancient world. When the current is swift, it may wash fertile
soil away, or cover it with gravel and bowlders.

When water leaves the main channel and spreads over the
flood plain its velocity is checked the most close to the stream,
and consequently more and coarser deposits are made here than
farther back. This excess of sandy deposits on the flood plain
close to the stream is called a natural levee.

THE WORK OF RIVERS

305

In the lower Mississippi, below the mouth of the Red River,
the slope is so gentle that sediment is deposited along the bed of
the river, raising the level of the surface of the river. Not infre-
quently the surface of the river is higher than the land back of
the natural levees, and this gives the river the appearance of

INDIANA

_ DEPOSITING.
ROWS SHOW MAIN
CHANNEL AND EDDY

FIG. 155. THE MEANDERING OF THE GREAT MIAMI RIVER

In a wide alluvial plain at its junction with the Ohio River. At different periods it has
consecutively entered the Ohio through the different mouths as indicated by the dotted
lines. As late as 1786 it occupied the bed numbered 3 in the map. Most of the surround-
ing plain is covered several times a year by water in times of flood, sometimes to the depth
of 15 feet. The amount of sediment deposited is remarkable.

A stone monument that marks the Ohio-Indiana state line at this point when set up was of a
height that a man on horseback could barely reach the top; at the present time the top is
but two feet above the surface of the plain.

Another feature of this deposition of sediment is that the older deposits now buried to the
depth of many feet are far better suited to agricultural requirements than the present sur-
face sediments for the reason that the earlier deposits come from the forest-clad hills rich
in humus, while the present are the impoverished wastings of newly-tilled, bare fields.

306

PHYSIOGRAPHY

flowing along in the top of a ridge it has built across its flood
plain.

The slope of the flood plain, away from the river, is usually
steeper than the general slope of the river toward its mouth.
As a result of this, when the Mississippi overflows its banks and
spreads out over its flood plain., those lands most remote from the

FIG. 156. MOUTHS OF THE MISSISSIPPI RIVER
Only the natural levee portions of the deposits appear above water. (U. S. Geological Survey.)

river are first and most deeply drowned, in some places to a depth
of more than thirty feet.

As the river continues to build up its front lands, there comes
a time when the lower swampy back lands offer a more favorable
route for the river than its normal meandering course. The river at
some flood stage seeks this new and more favorable route. This
sudden change of the river is called migration, and is to be distin-
guished from the gradual shifting of the channel called meandering,
which is confined to the meander zone.

THE WORK OF RIVERS 307

From Memphis to Vicksburg the Yazoo River, on the east side
of the flood plain, occupies an abandoned channel of the Missis-
sippi. The Yazoo is not capable of forming meanders as large as
those it follows. South of Vicksburg, where the Mississippi fol-
lows the east side of its flood plain, there is a similar abandoned
channel on the west side of the flood plain, now occupied by the
Tensas River, which is likewise incompetent to produce the wide
meanders of its course.

Protection from Overflow. Two methods are advocated for
protecting the flood plain of the Mississippi from overflow. One
is to maintain outlets to distribute the floods as quickly as possible.
On the east side, just below Baton Rouge, an outlet could be main-
tained into Lake Pontchartrain, by way of Bayou Manchac, a
former distributary; on the west side, one through the Atcha-
falaya, one through Bayou Plaquemine, and one through Bayou
La Fourche.

The other method is to build levees sufficiently high to restrain
the highest floods. This involves the building of high levees along
both sides of the Mississippi (except where it flows near the high
land, where but one levee is necessary) and along all important
tributaries and distributaries.

Levees are built of flood plain materials, the largest being about
40 feet high and 200 feet wide at the base. In times of danger
the height of the levee may be temporarily raised by bags of
earth. The levees are built alongside the river where the banks
are convex; but where the banks are concave the levee is built
farther back because of the cutting and caving of the banks on
this side.

When the water is critically high, State guards patrol the levees
on the lookout for leaks and to prevent tampering with the levees.
Steamboats are required to keep as far away as possible from
the levees, lest the waves generated by them cause the levees to
break. The greatest natural enemies of the levees are the cray-
fish and the muskrat.

Because the flood plain is highest near the river, small streams

308 PHYSIOGRAPHY

starting near the main stream flow away from it toward the back
swamp. The Yazoo River enters the flood plain and flows along the
back swamp region parallel to the main stream until captured by
the migration of the Mississippi to that side of its flood plain at
Vicksburg. In Louisiana the Atchafalaya continues along the back
swamps to the sea. The Red River, receiving the drainage from
the Tensas Basin of the Mississippi flood plain to the north, is,
more and more, sending its waters to the Gulf by way of the
Atchafalaya.

When a river empties into a quieter body of water, as the sea
or a lake, its velocity is checked and waste is deposited in and
around its mouth, forming a delta.

The several channels into which the main stream divides in
the delta are called distributaries. Those at the mouth of the Mis-
sissippi, called " passes," are bordered by continuations of the
natural levees of the flood plain.

The river-borne waste, which consists of the finest materials,
may be carried far to sea before being deposited, sometimes as far
as 200 miles from shore.

QUESTIONS

1. On the U. S. Topographic Maps of your vicinity, study carefully
your nearest or most interesting river or stream. Apply a string care-
fully along its course and determine the length of the stream in miles
and kilometers. Determine its limiting divides. Estimate the area
of its basin in square miles and in square kilometers. Map it on manila
paper on as large a scale as is convenient.

2. Proceed similarly for your State, mapping the canals as well as the
principal rivers and lakes.

3. On a U. S. Base Map, mark the principal divides and guess at, or
estimate, the areas of the principal drainage regions in percentages of
the whole.

4. After you have studied a real gully, describe how, in your opinion,
it was formed.

5. If the average annual output of the Mississippi River is 44.7 cubic
miles of water, how deep a lake would this make if its area was that of
your county? your State?

THE WORK OF RIVERS 309

6. Draw or trace the tributaries of the Maumee River of Ohio, Scho-
harie Creek, New York, and of some other stream system, and account
for the ways the tributaries join their master streams.

7 . Give at least two reasons why some streams do not corrade their beds.

8. With a diagram explain how meanders and oxbow lakes are formed.

9. Draw a top view, showing tributaries, distributaries, and contours
of an alluvial cone or fan. Draw cross section showing location of
coarsest and finest deposits.

10. Draw a portion of the Platte River showing a braided stream,
n. Draw and label an ideal cross section of a flood plain, showing a

river flowing along in its bed in the top of a ridge it has built for itself.
The bottom of the bed is to be below sea level; the river bank full and
higher than the back swamps, which are beginning to fill with water.

12. Summarize in tabular form under headings, (i) kinds, (2) places,
and (3) products, the five different ways in which streams work.

13. On transparent paper trace the divides and subdivides in Fig.
142. Then without consulting the map, draw in, in blue, the streams
where you think they should be. Then compare your work with
Figs. 142 and 143.

14. Similarly trace divides and subdivides in the mountainous
portion of Fig. 153. Then draw in, in blue, the streams where you
think they should be. Compare Fig. 153.

CHAPTER XXI
LIFE HISTORY OF A RIVER

Base Level. The life work of a river, with reference to the
region it drains, is to wear down the land and to carry it into
the sea. Its work will never be finished until the region is worn
down to sea level. The level of the sea is the base level below
which the lands cannot be eroded; but we may also have local
base levels, such as a lake or the level of a stream into which a
tributary empties.

Stages of Development. It is convenient to speak of the dif-
ferent stages of a river's development as youth, maturity, and old
age, and to characterize the general features of a region as young,
mature, or old. These terms are relative, and do not lend them-
selves to expression in numbers of years.

Youth. A stream that is degrading and has most of its work
before it is said to be young. Young streams are characterized
by steep slope, rapid current, and great power to corrade their beds.
Young streams have narrow V-shaped valleys. Lakes are character-
istic of young rivers, disappearing before maturity. At first a
young stream has few tributaries, especially on plains and pla-
teaus, where the tributaries may begin as mere gullies. The
divides are not well marked, especially on plateaus and plains,
where large level interstream areas are found. Rapids and falls
may be present in a young stream, but they, too, disappear before
maturity. Falls, rapids, and lakes give a young stream a profile
that is in places convex upwards. Young streams are usually
clear. The upper course of all great rivers is young.

Maturity. A river or any part of it is said to be graded or
mature when it has a slope just suited to its load and volume.

LIFE HISTORY OF A RIVER 311

It has so destroyed its falls, rapids, and lakes, and so aggraded or
built up its too gentle slopes, that it has just the right slope to
carry its load of waste with its volume of water. Its profile is
called the profile of equilibrium. This perfect adjustment of slope,
volume, and load is difficult to attain. If attained, any change in
any one of the three factors disturbs their balance. Although no
river is graded throughout its entire course, most rivers have
graded portions. In maturity tfye divides are well defined and
adjusted, the valleys broad, and the numerous tributaries obey
Playfair's Law of entering their main stream at the
level of the main stream. The river tends to meander
over flood plains, becoming wider toward the mouth.
The profile of equilibrium is a curve, concave upward,

SEA LEVEL

SIMILES FROM MOUTH > 69 MILES - > 47MILES 4-2MILES > Z9M.

FIG. 157. PROFILE OF PASSAIC, SHOWING CHARACTERISTICS OF YOUTH
Rapids at R; lake when floods at L; falls at F.

steeper near the source, becoming more gently sloping, and pass-
ing imperceptibly into the base level of erosion at the mouth.
The middle course of great rivers is in the graded or mature stage.

Old Age. In some rivers, and especially near the mouths of
large rivers, the slope becomes too gentle for the stream to carry
all its load of waste, and so a part is deposited. Old streams have
generally wide, fiat-bottomed, shallow valleys, wide flood plains over
which the streams meander, forming oxbow lakes. Since the flood
plain is highest near the river, streams formed on the flood plain
are usually prevented from joining the river. The deposits tend to
build up the stream bed and, at the mouth, to form a delta that
extends or prolongs the flood plain, and the river breaks up into
distributaries. The lower course of many great rivers is in the
old-age stage.

When all the streams of a region have reached old age, and

312 PHYSIOGRAPHY

when the region is worn down nearly to base level, it forms what
is called a peneplain. A peneplain is a region that is "almost-a-
plain," and is the last stage of an eroded mountain or plateau.

Normal River Cycle. Because every stream tends to pass
through youth, maturity, and old age, these stages constitute
the normal cycle, and their record the life history of a river. Many
streams never complete their normal cycle because it is inter-
rupted in some way.

Interrupted River Cycles. The normal cycle of river develop-
ment may be interrupted by change of slope, resulting from de-
pression or elevation, and by change of climate from moist to dry or
dry to moist, or from warm to glacial or glacial to warm. The gen-
eral effect of elevation is to lengthen the cycle, of depression to
shorten it.

Effects of Depression. If depression occurs at the mouth of a
river, the sea will enter the lower portions of the river valleys,
drowning them and producing bays, estuaries, or fiords. Tribu-
taries near the mouth of a river enter bays, and the master stream
is said to be dismembered. The lower Susquehanna, with its tribu-
taries, the James, the York, and the Potomac, when drowned and
dismembered, becomes Chesapeake Bay, with its many branching
bays.

Effects of Elevation. Elevation of a region at the mouth of a
river lengthens the river. When two or more rivers thus length-
ened unite, they form an engrafted river. Rivers may also be en-
grafted by the extension of their deltas into the same bay. The
tributaries of the Mississippi River below Cairo have been en-
grafted upon it.

If elevation takes place at the source, the slope is increased, and
the river is rejuvenated. A meandering river, if rejuvenated, forms
entrenched meanders. Rivers entrenching themselves in flood
plains sometimes leave portions of the old flood plain persisting
as alluvial terraces.

If the uplift is across its course, the river may corrade down-
ward as rapidly as the uplift is made, thus producing water gaps.
The Green River passes through the Uinta Mountains and the

LIFE HISTORY OF A RIVER 313

Hudson through the Highlands. Because such rivers had their
approximate location before the mountains were uplifted they are
called antecedent rivers.

Effects of Change of Climate from Moist to Arid. In gen-
eral the river cycle is lengthened. When the annual rainfall of a
region diminishes the rivers become smaller and eventually cease
flowing, except immediately after rains. Forests disappear except
along stream courses. When forests are removed there is little to
retain the run-off, and the streams, quickly flooded, quickly subside.
Their dry stream beds are called gulches or wadies. The lakes by
evaporation become salt, and generally decrease in size, finally
becoming salinas or salt plains. When the inflowing streams bring
much sediment, playas or mud plains may be formed.

The region between southern Russia and Pekin, China, exhibits
these phenomena in different stages. Increasing dryness in what
is now the desert portions of the Chinese Empire may have caused
the great migrations of the Asiatics which resulted in the invasion
of Europe by the Huns and Mongols.

Similar climatic changes have taken place in the Great Basin
portion of the United States between the Rockies and the Sierra
Nevada Mountains, especially in Utah and Nevada.

From Arid to Moist. Increased rainfall shortens the river
cycle. The increased rainfall brings about reforestation. The
rain and the forests restore and enlarge the rivers and make them
in volume more nearly uniform throughout the year. The playas
and salinas become lakes, and salt lakes, when filled to overflowing,
become fresh. Plants and animals gradually return and increase.

The deposits of rock salt in regions now moist, as in New York,
Michigan, Kansas, and Louisiana, indicate former arid conditions
in these regions.

From Warm to Glacial Climate. As the climate becomes colder,
plants and animals adapt themselves to the cooler climate, migrate,
or perish. With increased cold and increased snowfall, more snow
may fall than melts during the year. This occurs first upon the
higher lands; but the areas of permanent snow gradually spread
until the entire region is snow covered. The rivers get smaller and

314 PHYSIOGRAPHY

smaller and finally disappear, except near the borders of the
glacier.

From Glacial Climate to Warm. The disappearance of the
glacial ice produces floods in all rivers. The rivers become longer
as the glaciers retreat. The melting ice leaves irregular deposits,
whose depressions are occupied by lakes and swamps. A new
system of drainage must be developed where the old has been
obliterated. In general, the river cycles begin anew. Some rivers
reoccupy their preglacial valleys in part, and in part develop new
channels, with falls and rapids. The lower Hudson occupies its
preglacial channel. The Genesee has developed a new channel,
with falls and rapids at Rochester.

QUESTIONS

1 . Make a comparative table of the characteristics of young, mature,
and old streams. In the characteristics column, place such items as
steepness, swiftness, corrasion of bed with products; corrasion of banks,
relations to falls, rapids, and lakes; profiles, divides, tributaries, trans-
portation methods, deposition and uses to man.

2. How, from an ordinary map, can you tell the stage of a river? .

3. Is a muddy stream more apt to be old or young? a clear stream?
Why?

4. Compare the effects of elevation and of depression on the length
of the river cycle, with examples.

5. Do the same for a change of climate from moist to arid, from arid
to moist.

6. Similarly for from warm or temperate to glacial, with change from
glacial.

7. In what ways, and with what results, may a normal river cycle be
interrupted?

8. State the advantages and disadvantages to man of the different
stages in the life history of a river.

9. What are portages?

10. What is it to rectify a stream?

1 1 . Account for sunken or incised meanders.

12. Why may a river valley be in some portions young and in other
portions mature or old? *

13. What is imperfect drainage?

14. What would be the effect on Lakes Erie and Ontario if eastern
Canada should be slowly uplifted?

15. Distinguish an estuary and a delta.

CHAPTER XXH
FALLS, RAPIDS, AND LAKES

FALLS AND RAPIDS

Location of Falls. Falls and rapids are numerous among moun-
tains and plateaus. They are characteristic of the upper courses
of great rivers, and of young streams among hills. In many
rivers falls and rapids mark the head of navigation. Small and
light boats, like canoes, can be unloaded and carried around them;
but large boats pass them by means of canals with locks.

Economic Importance of Falls. Falls and rapids furnish valu-
able water power. This is the foundation of the manufacturing
interests of New England. The establishment of mills at falls
quickly develops villages, which may become flourishing cities, as
Lowell, Rochester, and Minneapolis have done.

Electric power developed from water power at Niagara Falls is
transmitted as far as Syracuse, 180 miles from the falls, and this
illustration of the use of water power at a distance from the falls
has done much to increase the value of undeveloped falls that are
located in mountainous regions, or where manufacturing establish-
ments would be at a disadvantage for some other reason.

When a fall is at the head of navigation of a river, it becomes a
railroad center, and the loading and unloading of vessels furnish
labor, which aids in the development of a city.

Some Important Falls. At Niagara Falls, the outlet of Lake
Erie plunges over a precipice 160 feet high on its way to Lake
Ontario. Goat Island divides the stream, making two falls; the
larger, on the Canadian side, is called from its shape the Horse-
shoe Fall; the smaller, the American Fall, enters the side of the
gorge.

PHYSIOGRAPHY

FIG. 158. NIAGARA FALLS.

FALLS, RAPIDS, AND LAKES

317

The enormous volume of water passing over this fall gives
Niagara its grandeur and impressiveness and makes it one of the
wonders of the world.

The upper sixty feet of the face of the fall is a hard limestone,
in nearly horizontal layers; below this is a hardened mud or shale
with occasional thin bedded limestones, which is very easily cor-
raded. At the foot of the Horseshoe Fall the water is some 200

FIG. 159. LOCK IN ST. MARY'S CANAL

feet deep, the soft rocks at the base being worn away to this depth
by the force with which the water strikes it and by bowlders which
the water whirls around. Below the falls the river follows a
gorge some seven miles long. Only a small portion of the water
of the Niagara River is diverted from the falls for power
purposes.

The Genesee Falls. The Genesee River flows over the same
rock formations as the Niagara, but the volume of the water is
less, and we have here three separate falls, each of which has at
its crest a hard limestone or sandstone and beneath this an easily

PHYSIOGRAPHY

eroded shale. Below the falls the river flows through a gorge
similar to that at Niagara, but narrower.

St. Anthony's Falls. The Mississippi River at Minneapolis,
Minn., flows over a precipice capped by a somewhat thinner layer
of limestone than that at Niagara; and as the volume of water is
large and the cap rock was not resistant enough to preserve the
fall, it was therefore necessary to build a wall of cement under-

~TUVER

FIG. 160. How NIAGARA FALLS WERE FORMED

neath the Fall of St. Anthony in order to preserve the falls and
their valuable water power. Here again the river, below the falls,
flows through a gorge several miles long.

Shoshone Falls. The Snake River in Idaho flows over a hori-
zontal sheet of hard lava which overlies softer rocks, forming
this fall.

A Line of Falls. In the southeastern part of the United States
falls are found in all streams at the outer margin of the piedmont
plateau, where they enter the coastal plain. The term fall line
has been applied to a line connecting the falls and rapids at the
heads of navigation in these rivers.

How Falls are Formed. It will be noted that in each of the
four falls described above the cap rock of the precipice over which
the water falls consists of nearly horizontal layers of rock that
resists corrasion well, and that underneath this in each case is a

FALLS, RAPIDS, AND LAKES 319

soft rock. This is the structure which has led to the formation of
most falls. The weak rocks are corraded more rapidly than the
resistant rocks, increasing the slope of the stream at the point
where the hard and soft rocks meet, until finally a fall results.

The Meaning of the Gorge. Many falls are, like Niagara, sit-
uated at the up-stream end of a gorge of considerable length,
which has been formed by the recession of the fall. It has been
shown by careful surveys that the center of the Horseshoe Fall
at Niagara is travelling toward Lake Erie at the rate of about
five feet a year. Similar though less rapid recession takes place
in all falls of this structure.

In Fig. 1 60 a section of Niagara is shown. The water swirling
around at the foot of the fall cuts backward into the face of the
precipice, and spray, frost, and ice assist in the undermining, form-
ing a cave. The cap rock is worn but slightly as a rule, but under-
mining proceeds with comparative rapidity, causing the cap rock
to fall of its own weight. The crest of the fall thus travels up-
stream until it disappears.

LAKES

A lake or a pond will always be formed in all depressions in the
land if rainfall or inflow exceeds evaporation and possible seepage.
If the yearly rainfall and inflow exceeds the yearly evaporation
from the surface of the lake, the lake is permanent, and water
accumulates in the basin until it finally overflows at the lowest
point in the rim of the basin. Temporary lakes are formed where
the water reaching the depression temporarily exceeds the loss
during the given time, but where the yearly supply is less than can
be evaporated.

Two conditions are therefore necesssary for the formation of a
permanent lake a basin without an outlet that reaches to the
bottom of the basin, and an excess of water received over that
lost. In arid regions there are many basins that are not lakes
because of insufficient rainfall or tributary streams, but in well-
watered regions every basin contains a lake. Lakes always indi-
cate imperfect drainage.

3 20

PHYSIOGRAPHY

Origin of Lake Basins. Some lake basins, Lake Superior and
the Caspian Sea, for example, are believed to be the result of the
uplift of intervening land masses that separated them from the
ocean. Some of the early myths and legends of the Greeks seem
to indicate a former passage through the Black and Caspian Seas
to the Arctic Ocean. Other lakes are believed to be due to the
depression of their basins. Examples of this type are Lake Baikal,
over a mile deep, and the lakes in the Great Rift Valley, extend-
ing from the Sea of Galilee through the Dead and Red Seas into
the Lake region of Africa.

Lake basins are formed by obstructing river valleys by lava
flows, landslides, or glacial deposits. The Finger lakes of central
New York are the unfilled portions of pre-glacial river valleys.

Other lake basins are formed by the natural processes of rivers.
Lake Pipin, in the Mississippi River, is formed by delta deposits
which accumulated in its valley at the mouth of the Chippewa
River of Wisconsin. Oxbow lakes are abandoned portions of
streams that have been closed at one or both ends. The lakes
along the Red River of Louisiana are made by the more rapid
building up of the flood plain of the Red than of the flood plains
of its tributaries.

The craters of some dormant and some extinct volcanoes become
partially filled with water. Such lakes are generally deep, circular,
and with precipitous banks. Crater Lake, Oregon, is a typical
example in the United States. Lake Avernus, on whose shores
the ancients believed was situated the entrance to the Lower
World, is one of several crater lakes west of Naples. Other crater
lakes are found near Rome. In southern Germany are found older
crater lakes, with low, gently sloping banks.

At the base of Mt. Shasta, and several other volcanoes, lake
basins are found in depressions between lava flows.

Many lake basins are found in and among the uneven deposits
of till left by glaciers. The numerous "kettle lakes," such as
Lake Ronkonkoma on Long Island, belong to this class. In some
instances basins have also been scoured out of the solid bed
rock by a glacier.

FIG. 161. DEVELOPMENT OF GREAT LAKES AT END OF ICE AGE

Stages indicated by outlet: In i, separate; in 2, Lake Maumee into Lake Chicago only;
in 3, through the Mohawk; in 4, through the Ottawa.

3 22

PHYSIOGRAPHY

FIG. 162. DELTA BUILT INTO A LAKE
Silvaplana, Switzerland.

REFERENCE TABLE OF PRINCIPAL LAKES

AREA IN ALTITUDE MAXIMUM

NAME

Caspian

SQ. MI.
[7O.OOO

IN FT.

8<?

DEPTH.
3.2OO

COMPARISONS

California . . .

158 ooo

Superior. .

3 1 , 200

6O2

1, 008

So. Carolina

30,000

Victoria Nyanza .

26,OOO

800

240

West Virginia

. 2 5 ,OOO

Michigan

22 5OO

c;8i

87O

Huron

22 32O

7OO

Baikal

I3,OOO

1,700

5,6OO

Maryland

I2,2OO

Erie
Ontario.

9,960

7 2OO

573

247

2OO
738

New Hampshire ....
New Jersey

9,300
7 800

Tchad (dry season)

6 OOO

QOO

(wet season)

4.O OOO

Titicaca

32OO

12 ^OO

7OO

Nicaragua

2 8OO

Delaware

2 OOO

Great Salt Lake

2 2OO

Champlain

4.8O

New York City. . .

. 327

Dead Sea . .

160

1.268

1.300

Destruction of Lakes. Lakes are temporary features in the
early stages of the development of the drainage of a region, and
disappear as the river system develops. Many lakes have been
drained by corrasion of the outlet channel; in time all lakes whose

PLATE I. CONTOUR MAP. DELTA AT THE HEAD OP SENECA LAKE, N. Y.

From U. S. Geol. Survey.
Scale: 1 inch = 1 mile. Contour interval. 20 feet.

FALLS, RAPIDS, AND LAKES 323

bottoms are above sea level must disappear through this action,
unless some other agent destroys them first. Other lakes have dis-
appeared through the opening of a new outlet, for example, former
Lakes Chicago, Agassiz, and Warren (Fig. 161) were drained when
the melting of the glacial ice uncovered new and lower outlets.

FIG. 163. How VEGETATION DESTROYS A LAKE

Pond lilies in center, smartweed at edge, farther back cat-tails, blue flags, sweet flags and
sedges; still farther back soft turf with grass, moss, sedge and milkweed.

A second method of destroying lakes is by filling. Some five
miles of the southern end of Seneca Lake, New York, (Plate I)
has been filled with sediment brought in by streams; and deltas
are forming in nearly all lakes where streams enter, diminishing
their size. Lake St. Clair, between Lakes Huron and Erie, has
been greatly diminished by the growth of a delta. If sufficient
time were allowed, this cause alone would also destroy all lakes.

Some lakes are filled with vegetable matter; a certain kind of
moss sometimes grows on the surface of the water and holds wind-
blown sand and dust, which gradually spreads over the lake,
forming a floating bog. A railroad line in Minnesota crossed such
a bog. Cattle grazed upon it before the line was built; but the

3 2 4

PHYSIOGRAPHY

engineers discovered that the floating bog was a mass four feet
thick of moss and dust, and that beneath it was twenty feet of
water. Eel grass and wild rice also assist in filling many lakes.

FIG. 164. A, LAKE. B, LILIES AND BUSHES. C, BEGINNING or SPHAGNOUS GROWTH

D, BOG CLIMBING HILLSIDE. E, DISINTEGRATED PEAT

(U. S. Geological Survey.)

Marl deposits, which form in some lakes to a depth of many feet,
also assist in filling them. Marl consists chiefly of the shells v>f
animals and the remains of lime-secreting plants.

These methods of filling gradually convert a lake into a swamp
or marsh, and many of our fresh water marshes are former lakes
destroyed in this way. The student will doubtless be able to find
examples of such marshes near his home.

1,000

FIG. 165. STREAM FLOW VERY IRREGULAR; NOT INFLUENCED BY LAKES
Discharge of Neuse River at Selnia, N. C., for 1899.

FALLS, RAPIDS, AND LAKES

325

A third method of destroying lakes is by evaporation. The great
Lake Bonneville, that once covered a part of the Great Basin as
large