Part 13

The Indian Ocean has an average depth of about 12,000 feet, and the deepest soundings have been taken on the eastern side. It is interesting to observe that the deepest sounding, about five and three-quarter miles, in the South Pacific somewhat exceeds the height of the highest mountain. Mount Everest has a height of 29,000 feet above the sea level. And it must also be noted that the mean height of the land, 1,000 feet, is only about one-twelfth the mean depth of the whole ocean, 12,000 feet. (See colored map showing comparative surfaces of land areas and ocean depths.)

INLAND AND BORDER WATERS. These enclosed basins belong to the structure of the continents, rather than to the oceans. All are shallow in comparison with the great basins with which they are connected, as is apparent from the depths given below.

The Gulf of Mexico is from 5,000 to 7,000 feet in depth. The deepest part of the Caribbean Sea, on a line connecting Porto Rico and Costa Rica, averages 7,000 feet, and near the latter it reaches a depth of 14,000; but the ocean, immediately outside of the Lesser Antilles, is more than 18,000 feet deep.

The Mediterranean is divided into two basins, by a rocky isthmus, from 50 to 500 feet below the surface, lying between Sicily and Cape Bon, in Africa. The western basin is over 9,000 feet in depth, and comparatively uniform; while the eastern is more irregular, varying from 6,000 near the center, to 13,000 feet, south of the Ionian Islands. The Red Sea has an irregular bottom, with an average depth of 3,000 feet, but in some places it reaches 6,000.

The Baltic Sea, being a simple depression in the great European plain, is but a few hundred feet deep. In the North Sea, the depth averages 300 feet, and rarely exceeds 600. The continent is here prolonged in the form of a submarine plain, whose highest portions form the British Isles.

The Border Seas of Asia, lying within the chain of continental islands, are only a few hundred feet in depth, while immediately without those islands, abrupt slopes descend to the great depths of the Pacific basin.

Smaller inlets are also of frequent occurrence, especially in districts where mountain ranges approach the borders of the ocean. Such are the _lochs_ of Scotland, the _voes_ of the Shetland Islands, and the _fiords_ of Norway and Greenland. The term _lagoon_ is usually applied to lake-like inlets.

SALT AND OTHER INGREDIENTS OF SEA-WATER. The waters of the ocean are salt, holding in solution various saline matters. The saline ingredients amount to rather more than thirty-five grains in a thousand grains of sea-water. The most abundant of these is chloride of sodium or common salt, which in general forms about a third of the whole. Besides this, sea-water contains some magnesia, lime, potash, and traces of iodine and bromine.

The following table exhibits the exact percentage composition of sea-water.

One hundred parts by weight of sea-water contain:

Water 96.470 Sodium Chloride 2.700 Magnesium Chloride .360 Potassium Chloride .070 Magnesium Sulphate .230 Calcium Sulphate .140 Calcium Carbonate .003 Magnesium Bromide .002 Traces of Iodides, Silica, etc., estimated .025 ------- 100.000

HOW THE SEA GETS ITS COLOR. The color of sea-water is due to the character of the skies and clouds above, and to vegetable and animal objects growing and living in it. The luminosity or phosphorescence of the ocean is due to the decay of animal and vegetable substances, but in some cases it arises from the presence of myriads of living animals, which, like the glow-worm and fire-fly of the land and air, have the power of emitting light.

OCEAN TEMPERATURE. The water of the ocean appears generally to agree with that of the climate in which it is situated. In warm latitudes the temperature of the deep sea diminishes with the depth below the surface until a certain depth is reached, below which it appears to retain an equable temperature, this being about 40 degrees Fahrenheit. In the Polar Seas, where the temperature of the surface is lower than 40 degrees the heat increases downward until it reaches that point. In latitude 70° the temperature of the ocean is considered to be the same at all depths.

[Illustration: =HOW TIDES ARE FORMED BY ACTION OF THE MOON=

The moon pulls the waters of the earth into a great double wave heaping it up on the side nearest to the moon and on the opposite side. As the earth rotates, this double wave moves round the earth, and the crests and troughs alternately produce high and low tide. Thus there are two high and two low tides daily, at intervals of about twelve hours, or half a Sun or day.]

CAUSE OF THE TIDES, WAVES AND CURRENTS OF THE OCEANS

The waters of the ocean are retained in their bed by the attraction of gravitation. This power is great in proportion to the mass; and as the earth is of much greater mass than the particles of water on its surface, it attracts them and keeps them in their assigned places. But the sun and moon also possess this power of attraction, and notwithstanding their distance, attract and draw them up to a certain elevation. The vast mass of the waters being drawn up by the moon into a mountain or curve of water forms what is called the “great primary or tidal wave.”

[Illustration: =VAST OCEAN CAVERN AT CAPRI, WIDELY KNOWN AS THE “BLUE GROTTO”=

This remarkable cavern, on the shore of the island of Capri, at the entrance of the Bay of Naples, is entered from the sea, and is one hundred and eighteen feet long and forty feet high, with a breadth of ninety-eight feet at its widest part. It derives its name from the wonderful blue reflection of the sun’s rays through the water, which gives the interior its marvelous beauty and majesty. The cavern has been created by the ceaseless action of the tide.]

EBB-TIDE AND FLOOD-TIDE. This drawing up of the waters of mid-ocean causes a recession from the shores, thus giving rise to ebb-tide, or low water. But when the temporary attraction ceases the waters flow back to their natural level, returning to shore and forming flood-tide, or high water. This culmination or rising of the waters in the great tidal wave takes place twice in twenty-four hours and fifty minutes. The combined influence of the sun and moon at new and full moon augments the size of this wave, and causes the “spring-tides” at those periods.

HEIGHT OF TIDES. High water at the various points along the coast is dependent on the return of this great wave, though some variations are caused by local peculiarities; and the height of the tide also varies greatly in different parts of the earth.

On the eastern coast of North America, the average rise of the tide is from nine to twelve feet. At the entrance to the Bay of Fundy, however, it rises eighteen feet, while at the head of that bay it reaches sixty, and in the highest spring tides, even seventy feet. At Bristol, in England, the spring tides rise to forty feet; and at St. Malo, on the south coast of the English Channel, they reach fifty feet.

THE MAELSTROM, CHARYBDIS AND HELL GATE

Differences in level, produced by high tides, cause currents which vary in force and direction with the condition of the tide, producing, in some cases, dangerous whirlpools. The famous Maelstrom, off the coast of Norway, is but a tidal current, which rushes with great violence between two of the Lofoden Islands, causing a whirling motion in the water which is reversed at each ebb and flow of the tide.

Such is, also, the famous whirlpool of Charybdis, in the Straight of Messina, and many others of less note. The powerful currents of Hell Gate, in the passage from Long Island Sound to New York Bay, are due to a similar cause, high water occurring at different hours in the bay and in the west end of the sound.

WHAT CAUSES THE WAVES OF THE OCEAN

The waves of the ocean, which are caused by the action of the wind, and which are called secondary or wind waves are of a totally different character from the tidal wave. The influence of the wind is supposed not to extend to a greater depth than forty or fifty feet, the deep sea, though raised in a great mass by the grand tidal movement, being free from agitation. Wind waves at a distance from the shore are comparatively low and long, but in shoal water they assume a greater curvature, and fall on the beach either in gentle ripples or in mighty breakers, according to the depth of the water and the force of the wind. The heavy swell which occasionally takes place, called the “ground sea,” is supposed to originate in distant storms of wind.

THE RIVERS IN THE SEA

Currents in the ocean arise from various causes. They may be produced by long-continued gales of wind, by the melting of polar ice, or by any cause that may give rise to onward movements of limited portions of the great mass of waters. Other currents, and of these only is it necessary to speak in this connection, are permanent. The most remarkable of these are the polar currents and the equatorial currents.

POLAR CURRENTS are produced by the perpetual movement of the waters from the poles to the equator. In accordance with the laws of mechanics, an accumulation of the waters takes place on that part of the globe which has the greatest velocity of motion; and as the earth in turning on its axis moves with far greater velocity at the equator, the waters continually flow toward that line from the poles.

EQUATORIAL CURRENTS. This accumulation of the waters at the equator tends to produce the equatorial currents, which consist of the continuous progression of the tropical seas in a westerly direction. When the wave brought by the polar currents arrives--coming as it does from regions where it naturally has less velocity--it does not at once acquire the velocity of the earth’s motion at the equator; and since the rotation of the earth is from west to east, this portion of the water lagging behind forms a stream or current which has an apparent motion from east to west, that is to say, apparent as regards the earth, but real in relation to the adjacent land and water. The trade winds, which in this zone blow constantly in the same direction, lend their aid in maintaining the equatorial current.

THE GREAT SYSTEMS OF OCEAN CURRENTS

An extensive system of currents appears to arise in the Antarctic Ocean. A current of cold water flowing northward joins the equatorial current in the Pacific. Entering the Indian Ocean, it maintains its westerly course until it approaches the shores of Africa; then bending southward it rushes through the Mozambique Channel, and doubling the Cape of Good Hope travels northward until it arrives at the Bight of Benin. This current then joins the equatorial current, and crossing the Atlantic from the coast of Guinea to that of Brazil, it is divided into two branches by the projecting headland of Cape San Roque, one flowing southward and the other northward.

THE GULF STREAM. After passing the Island of Trinidad, this great oceanic current enters the Gulf of Mexico, and there acquires a high temperature, and sweeping round that sea it again pours forth into the Atlantic, forming the most powerful of known currents, called the Gulf Stream. Issuing from the Gulf of Mexico, this current of warm water rushes with considerable force through the Bahama Channel; then taking a northerly course it travels along the eastern shores of North America, and at Newfoundland is turned to the eastward by an opposing cold current which sets in from Baffin’s Bay. It now maintains an easterly direction, and crossing the Atlantic arrives at the Azores in about twenty-eight days, and divides its waters on the coast of France and Spain: one portion goes southward and at length joins the grand current which sets from the coast of Guinea; and another portion travels northward and skirts the western coasts of Europe. These currents are seldom more than 500 feet deep.

ATMOSPHERE, CLIMATE AND WEATHER

The atmosphere is the vast ocean of air that envelops the earth and makes life possible on our globe. It absorbs the heat and vapors caused by the action of the sun upon the surface of both land and water, and is the medium through which the ever-changing phenomena of _climate_ and _weather_ are produced. The two great forces of nature acting in connection with it are _gravitation_ and _heat_, or solar radiation; and the results of their ceaseless action may be summed up as follows: (1) _Temperature_, or heat, which we soon learn to know by our senses, and to measure by the thermometer. (2) _Evaporation_, which changes the weight of the air by carrying invisible moisture through it. This change of _weight_ is indicated by the barometer. (3) _Condensation_, producing fog, dew, rain, hail, and snow; all estimated accurately by the rain gauge or pluviometer. (4) _Motions_, as in the winds, varying from the gentle breeze to the awful cyclone, the force and velocity of which are indicated by the anemometer. (5) _Electricity_, producing lightning, thunder, magnetic and chemical changes in the atmosphere. (6) _Optical Phenomena_, such as rainbows, haloes, coronas, mirage, and the auroras.

_THE ATMOSPHERE: ITS EXTENT, CHARACTER, USE AND EFFECT_

The Earth is enveloped in its own atmosphere, which like a transparent covering surrounds it, and revolves with it. This atmosphere does not extend to more than forty or fifty miles above the earth’s surface, and is higher at the equator than at the poles.

[Illustration]

WHAT THE ATMOSPHERE IS COMPOSED OF

The atmosphere is an elastic fluid consisting of a mixture (not a compound) of oxygen and nitrogen, in the proportions of about twenty-one of the first to seventy-nine parts of the last named. It also contains a small quantity of carbonic acid gas, and a yet smaller quantity of ammonia; and water in the form of invisible vapor is always present in it, though the quantity is subject to great variations. All these substances move freely among each other, and are continually changing places: the oxygen being ever ready to perform the office assigned to it of sustaining life and combustion; the carbonic acid to promote the growth of vegetation; the nitrogen to perfect the fruits of the earth, and the vapor to descend to the thirsty ground, in the form of showers and dew.

The atmosphere is elastic, and therefore capable of expansion and compression; and is also a ponderable body. The consequence of these properties is, that it is much lighter and thinner in the upper regions than nearer the earth’s surface; for at the sea-level its whole weight presses on its lower strata and gives it greater density. Ascending from the earth’s surface it becomes gradually lighter and thinner, and at great elevations is so rarefied as to be unsusceptible of sustaining life.

HOW THE ATMOSPHERE IS WEIGHED AND MEASURED

The weight of the atmosphere at the level of the sea is equal to about fourteen and one-half pounds on every square inch of surface. This weight is balanced by a column of mercury thirty inches in height; but at an elevation of 18,000 feet it would be balanced by a column of only fifteen inches in height, and at 36,000 by one only seven and one-half inches in height. It is on this principle that the mercurial barometer has been constructed; and since the mercury in the barometer stands at the same point at all places at the sea-level, and falls in a regular ratio on ascending therefrom, this instrument forms a most useful standard for measuring altitudes.

As we ascend from the sea the atmosphere becomes colder; but, as with the density, the temperature does not appear to pass through regular gradations of change. From experiment, however, it has been assumed that the atmosphere loses one degree of heat by Fahrenheit’s thermometer for every 350 feet of ascent; and hence even in the hotter regions very lofty mountains are covered with perpetual ice and snow.

DISTRIBUTION OF TEMPERATURE OVER THE EARTH

The amount of heat produced by the sun upon the Earth’s surface, is greatest near the Equator, and diminishes gradually towards the Poles. Three general causes, each referable to the spherical form of the Earth, combine to produce the gradual diminution of temperature from the Equator to the Poles.

1. The angle at which the Sun’s rays strike the surface. In the Equatorial regions they are perpendicular to the surface of the sphere, and there produce their maximum effect; but, on account of the curved outline of the globe, they fall more and more obliquely with increasing latitude, and the intensity of action diminishes proportionately. At the Poles their effect is practically nothing.

2. The area on which a given amount of heating power is expended, is least at the Equator, consequently the resulting heat is greatest. The area covered increases, and the effect diminishes, with the increasing obliquity of the Sun’s rays in higher latitudes, which, as we have seen above, results from the spherical form of the Earth.

3. The absorption of heat by the atmosphere, as the Sun’s rays pass through it, is least where they fall perpendicularly,--that is, in the Equatorial regions,--and increases, with their increasing obliquity, towards the Poles.

EFFECT OF THE MOTIONS OF THE EARTH

The Earth revolves constantly around the Sun, and at the same time rotates upon an axis inclined twenty-three and one-half degrees towards the plane of its orbit. In consequence of the inclination of the axis, the declination of the Sun, or its angular distance from the Equator, varies with the advance of the Earth in its orbit, causing periodical variations in the length of day and night, and, consequently, in temperature.

VERNAL EQUINOX. On the twentieth of March, at mid-day, the Sun is vertical at the Equator. Rising directly in the east it ascends the heavens to the zenith, and, descending, sets directly in the west.

The illuminated hemisphere extends from pole to pole, and embraces half of every parallel of latitude; hence every point on the Earth’s surface is under the rays of the Sun during half of the diurnal rotation; the days and nights are equal all over the globe; and the heating power of the Sun is the same in both the northern and the southern hemisphere.

SUMMER SOLSTICE. As the Earth advances in its orbit the vertical Sun declines northward; and on the twenty-first of June, at the Summer Solstice, it is over the northern Tropic, twenty-three and one-half degrees from the Equator.

The illuminated hemisphere, extending ninety degrees on each side of the parallel of the vertical Sun, reaches twenty-three and one-half degrees beyond the North Pole; but, at the south, it barely touches the Antarctic circle. It embraces more than half of each parallel north of the Equator, hence throughout the northern hemisphere the day is longer than the night, the difference in their duration increasing with the latitude; and all points within the Arctic circle are in the light during the entire rotation.

In the southern hemisphere, less than half of each parallel being illuminated, the night is longer than the day, and within the Antarctic circle there is constant night. The heating power of the Sun is now at the maximum in the northern hemisphere, while in the southern it is at the minimum.

AUTUMNAL EQUINOX. On the twenty-second of September, the distribution of light and heat upon the two hemispheres is the same as at the Vernal, and at the _Winter Solstice_, on the twenty-second of December, it is the reverse of that at the Summer Solstice.

=WHAT CAUSES THE SEASONS AND DAY AND NIGHT=

[Illustration: =FIGURE ILLUSTRATING THE CHANGE OF SEASONS THROUGHOUT THE YEAR=

The change of seasons is caused by the _revolution_ of the earth around the sun, and the inclinations of the planes of the equator and ecliptic. These causes also account for the difference in the length of the days and nights and the difference in the height of the midday sun. The exact duration of the seasons we get by observing the dates of equinoxes and solstices.]

[Illustration: =FIGURE SHOWING THE CAUSE OF DAY AND NIGHT=

The _revolution_ of the earth gives us the length of the year; its _rotation_ on its axis, the length of the day and night, by causing the risings and settings and daily apparent motion of the sun and stars.]

EFFECT OF UNEQUAL DAYS AND NIGHTS ON TEMPERATURE

The inequality in the length of the days in different parts of the year, occasioned by the inclination of the Earth’s axis, is of itself sufficient to produce a marked variation in temperature.

During the day the Earth receives from the Sun more heat than it radiates into space; while during the night it radiates more than it receives. Hence a succession of long days and short nights results in an accumulation of heat, raising the average temperature and producing summer; while long nights and short days result in a temperature below the average, producing winter.

Again, the heating power of the Sun in each hemisphere is greatest at the period of the longest days, because of its greater altitude in the heavens; and least at the period of shortest days. Thus long days and a high sun operate together to produce the high temperature of summer; while long nights and a low sun cause the low temperature of winter.

The following table gives the length of the longest day, excluding the time of twilight, and of the shortest night, in the different latitudes, with the difference of duration in hours and minutes, thus exhibiting more clearly the above law.

TABLE OF UNEQUAL DAYS AND NIGHTS

=============+=============+================+============ =LATITUDE= |=Longest Day=|=Shortest Night=|=Difference= -------------+-------------+----------------+------------ Equator | 12.0 hours | 12.0 hours | 00.0 hours 10° | 12.7 „ | 11.3 „ | 1.4 „ 20° | 13.3 „ | 10.7 „ | 2.6 „ Tropics | 13.5 „ | 10.5 „ | 3.0 „ 30° | 14.0 „ | 10.0 „ | 4.0 „ 35° | 14.5 „ | 9.5 „ | 5.0 „ 40° | 15.0 „ | 9.0 „ | 6.0 „ 45° | 15.6 „ | 8.4 „ | 7.2 „ 50° | 16.3 „ | 7.7 „ | 8.6 „ 55° | 17.3 „ | 6.7 „ | 10.6 „ 60° | 18.7 „ | 5.3 „ | 13.4 „ Polar Circles| 24.0 „ | 0.0 „ | 24.0 „ 67-1/2° | 1 month | 0.0 „ | ... 69-1/2° | 2 months | 0.0 „ | ... 73.3° | 3 „ | 0.0 „ | ... 78.3° | 4 „ | 0.0 „ | ... 84° | 5 „ | 0.0 „ | ... North Pole | 6 „ | 0.0 „ | ... -------------+-------------+----------------+------------

LAW OF VARIATION OF DAY AND NIGHT

The inequality of day and night increases slowly in the tropical regions, but more and more rapidly towards the polar circles. Beyond these circles the Sun, in the hemisphere in which it is vertical, makes the entire circuit of the heavens, without sinking below the horizon, for a period varying from twenty-four hours to six months; while in the opposite hemisphere there is a corresponding period of continuous night.

RESULT OF THIS LAW IN DIFFERENT ZONES

In the tropical regions, where the days and nights vary little in length, the temperature is nearly uniform throughout the year; while the increasing inequality of day and night towards the Poles, causes an increasing difference between the summer and the winter temperature.