title: Physiography for high schools,
text: ok. V V *> 0^ .^ '^ ^ <, A** '% * / >*"\ > ■■ - ,-. a ^ ^ - <- ^ .0 o "V. * n i \ * A % , "P^ 4? h :»'\ &' -A* -? ', "> Digitized by the Internet Archive in 2011 with funding from The Library of Congress http://www.archive.org/details/physiographyforhOOarey Yosemite Falls from Sentinel Hotel. A flood plain in the foreground. By permission of Oliver Lippincott. Frontispiece. PH YSIOGR A PHY 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. MORREY, A.M. BUSHWICK HIGH SCHOOL NEW YORK CITY °XKc D. C. HEATH & CO., PUBLISHERS BOSTON NEW YORK CHICAGO 6 s Vh .,' Copyright, 191 1, By D. C. Heath & Co. S* ©CIA297068 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 PAGE 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 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 18)^ 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 ~suri% noon raw onJuneZlit Fig. 1. — Method of Eratosthenes A, a vertical pillar at Syene, is 23)2° 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 21st. 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 i. 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 -Wile — > ' « — niiie- —£ curvature of the Earth's surface 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 i. 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 f Aries, the Ram. — Libra, the Balance. 8 Taurus, the Bull. W Scorpio, the Scorpion. n Gemini, the Twins. * Sagittarius, the Archer. The Summer Signs The Winter Signs 2> Cancer, the Crab. V3 Capricornus, the Goat, ft Leo, the Lion. & Aquarius, the Waterman. ^ Virgo, the Virgin. * Pisces, the Fishes. lo PHYSIOGRAPHY Change of Seasons. — As the earth moves forward around the sun, its axis is always tipped 23 ^ 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 (1) 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 shorter 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 II Fig. s — Four Positions of the Earth Corresponding to the Four Seasons In the winter and summer positions two different views are shown, one looking along a line vertical to the earth's orbit and the other looking parallel to the earth s orbit. 12 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 Mjd-tfay Sit/, 11 zemth Nadir , Fig. 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 Chapter VII 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. zenith 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 Zjeni_rh_ Nadir Fig. 8. — Showing Position of Son'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 IS of rotation and revolution. Make a free-hand sketch to show the motion and describe it. 7. A degree of longitude in latitude 40 equals about 53 miles. How Zenith Tie I t>o> Nadir Fig o. — 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? 11. 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 1st? March 1st? July 1st? September 1st? 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? 1 5. State whether at these different dates the sun rises north or south of east and sets north or south of west. 16. 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? 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 equator 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' represents a meridian section of the earth, N S being the axis and E E' the equator. H H' is the plane of the horizon with the observer at O. O N' extends north and is parallel to the axis N S. The point Z is the zenith directly over the observer. i8 PHYSIOGRAPHY The angle 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', the zenith distance of the north pole of the sky plus the angle HON' equals a right angle, or 90 degrees, since Z O is the perpendicular to H H'. Angle N C plus angle E' C 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 E, Equals the Latitude of Observer 0, Angle C E' Angles N C 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 HON, 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, 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. 11. — 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, i3^° 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 zenith 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 1 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 5, and E' is the position of the earth a day later. M represents the moon in its orbit about the earth, and M' 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 O. 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. 22 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. Fig. i 2- -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', and the observer at is brought to 0'. The same star has again come to the observer's meridian and one sidereal day has ended. A sidereal day may then be defined 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 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 0, Fig. 12, to 0" 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 23 degrees to bring the moon again to the observer's meridian, that is, the earth turns through 373 degrees from O to O'", 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 lime. 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 Fig. 13. — Son Dial 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 1 80th 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 180th meridian. International Date Line. — The 180th 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 801 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. 