CHAPTER V.

THE ATMOSPHERE.

The firm-set portion of the earth, composed of materials which became

solid when the heat so far disappeared from the sphere that rocky

matter could pass from its previous fluid condition to the solid or

frozen state, is wrapped about by two great envelopes, the atmosphere

and the waters. Of these we shall first consider the lighter and more

universal air; in taking account of its peculiarities we shall have to

make some mention of the water with which it is greatly involved;

afterward we shall consider the structure and functions of that fluid.

Atmospheric envelopes appear to be common features about the celestial

spheres. In the sun there is, as we have noted, a very deep envelope

of this sort which is in part composed of the elements which form our

own air; but, owing to the high temperature of the sphere, these are

commingled with many substances which in our earth--at least in its

outer parts--have entered in the solid state. Some of the planets, so

far as we can discern their conditions, seem also to have gaseous

wraps; this is certainly the case with the planet Mars, and even the

little we know of the other like spheres justifies the supposition

that Jupiter and Saturn, at least, have a like constitution. We may

regard an atmosphere, in a word, as representing a normal and

long-continued state in the development of the heavenly orbs. In only

one of these considerable bodies of the solar system, the moon, do we

find tolerably clear evidence that there is no atmosphere.

The atmosphere of the earth is composed mainly of very volatile

elements, known as nitrogen and argon. This is commingled with oxygen,

also a volatile element. Into this mass a number of other substances

enter in varying but always relatively very small proportions. Of

these the most considerable are watery vapour and carbon dioxide; the

former of these rarely amounts to one per cent of the weight of the

air, considering the atmosphere as a whole, and the latter is never

more than a small fraction of one per cent in amount. As a whole, the

air envelope of the earth should be regarded as a mass of nitrogen and

argon, which only rarely, under the influence of conditions which

exist in the soil, enters into combinations with other elements by

which it assumes a solid form. The oxygen, though a permanent element

in the atmosphere, tends constantly to enter into combinations which

fix it temporarily or permanently in the earth, in which it forms,

indeed, in its combined state about one half the weight of all the

mineral substances we know. The carbon dioxide, or carbonic-acid gas,

as it is commonly termed, is a most important substance, as it affords

plants all that part of their bodies which disappear on burning. It is

constantly returned to the atmosphere by the decay of organic matter,

as well as by volcanic action.

In addition to the above-noted materials composing the air, all of

which are imperatively necessary to the wonderful work accomplished by

that envelope, we find a host of other substances which are

accidentally, variably, and always in small quantities contained in

this realm. Thus near the seashores, and indeed for a considerable

distance into the continent, we find the air contains a certain amount

of salt so finely divided that it floats in the atmosphere. So, too,

we find the air, even on the mountain tops amid eternal snows, charged

with small particles of dust, which, though not evident to the

unassisted eye, become at once visible when we permit a slender ray of

light to enter a dark chamber.

It is commonly asserted that the atmosphere does not effectively

extend above the height of forty-five miles; we know that it is

densest on the surface of the earth, the most so in those depressions

which lie below the level of the sea. This is proved to us by the

weight which the air imposes upon the mercury at the open end of a

barometric tube. If we could deepen these cavities to the extent of a

thousand miles, the pressure would become so great that if the pit

were kept free from the heat of the earth the gaseous materials would

become liquefied. Upward from the earth's surface at the sea level the

atoms and molecules of the air become farther apart until, at the

height of somewhere between forty and fifty miles, the quantity of

them contained in the ether is so small that we can trace little

effect from them on the rays of light which at lower levels are

somewhat bent by their action. At yet higher levels, however, meteors

appear to inflame by friction against the particles of air, and even

at the height of eighty miles very faint clouds have at times been

discerned, which are possibly composed of volcanic dust floating in

the very rarefied medium, such as must exist at this great elevation.

The air not only exists in the region where we distinctly recognise

it; it also occupies the waters and the under earth. In the waters it

occurs as a mechanical mixture which is brought about as the rain

forms and falls in the air, as the streams flow to the sea, and as the

waves roll over the deep and beat against the shores. In the realm of

the waters, as well as on the land, the air is necessary for the

maintenance of all animal forms; but for its presence such life would

vanish from the earth.

Owing to certain peculiarities in its constitution, the atmosphere of

our earth, and that doubtless of myriad other spheres, serves as a

medium of communication between different regions. It is, as we know,

in ceaseless motion at rates which may vary from the speed in the

greatest tempests, which may move at the rate of somewhere a hundred

and fifty miles an hour, to the very slow movements which occur in

caverns, where the transfer is sometimes effected at an almost

microscopic rate in the space of a day. The motion of the atmosphere

is brought about by the action of heat here and there, and in a

trifling way, by the heat from the interior of the earth escaping

through hot springs or volcanoes, but almost altogether by the heat of

the sun. If we can imagine the earth cut off from the solar radiation,

the air would cease to move. We often note how the variable winds fall

away in the nighttime. Those who in seeking for the North Pole have

spent winters in the long-continued dark of that region have noted

that the winds almost cease to blow, the air being disturbed only when

a storm originated in the sunlit realm forced its way into the

circumpolar darkness.

The sun's heat does not directly disturb the atmosphere; if we could

take the solid sphere of the world away, leaving the air, the rays

would go straight through, and there would be no winds produced. This

is due to the fact that the air permits the direct rays of heat, such

as come from the sun, to pass through it with very slight resistance.

In an aërial globe such as we have imagined, the rays impinging upon

its surface would be slightly thrown out of their path as they are in

passing through a lens, but they would journey on in space without in

any considerable measure warming the mass. Coming, however, upon the

solid earth, the heat rays warm the materials on which they are

arrested, bringing them to a higher temperature than the air. Then

these heated materials radiate the energy into the air; it happens,

however, that this radiant heat can not journey back into space as

easily as it came in; therefore the particles of air next the surface

acquire a relatively high temperature. Thus a thermometer next the

ground may rise to over a hundred degrees Fahrenheit, while at the

same time the fleecy clouds which we may observe floating at the

height of five or six miles above the surface are composed of frozen

water.

The effect of the heated air which acquires its temperature by

radiation from the earth's surface is to produce the winds. This it

brings about in a very simple manner, though the details of the

process have a certain complication. The best illustration of the mode

in which the winds are produced is obtained by watching what takes

place about an ordinary fire at the bottom of a chimney. As soon as

the fire is lit, we observe that the air about it, so far as it is

heated, tends upward, drawing the smoke with it. If the air in the

chimney be cold, it may not draw well at first; but in a few minutes

the draught is established, or, in other words, the heated lower air

breaks its way up the shaft, gradually pushing the cooler matter out

at the top. In still air we may observe the column from the flue

extending about the chimney-top, sometimes to the height of a hundred

feet or more before it is broken to pieces. It is well here to note

the fact that the energy of the draught in a chimney is, with a given

heat of fire and amount of air which is permitted to enter the shaft,

directly proportionate to the height; thus in very tall flues, between

two and three hundred feet high, which are sometimes constructed, the

uprush is at the speed of a gale.

Whenever the air next the surface is so far heated that it may

overcome the inertia of the cooler air above, it forces its way up

through it in the general manner indicated in the chimney flue. When

such a place of uprush is established, the hot air next the surface

flows in all directions toward the shaft, joining the expedition to

the heights of the atmosphere. Owing to the conditions of the earth's

surface, which we shall now proceed to trace, these ascents of heated

air belong in two distinct classes--those which move upward through

more or less cylindrical chimneys in the atmosphere, shafts which are

impermanent, which vary in diameter from a few feet to fifty or

perhaps a hundred miles, and which move over the surface of the earth;

and another which consists of a broad, beltlike shaft in the

equatorial regions, which in a way girdles the earth, remains in

about the same place, continually endures, and has a width of hundreds

of miles. Of these two classes of uprushes we shall first consider the

greatest, which occurs in the central portions of the tropical realm.

Under the equator, owing to the fact that the sun for a considerable

belt of land and sea maintains the earth at a high temperature, there

is a general updraught which began many million years ago, probably

before the origin of life, in the age when our atmosphere assumed its

present conditions. Into this region the cooler air from the north and

south necessarily flows, in part pressed in by the weight of the cold

air which overlies it, but aided in its motion by the fact that the

particles which ascend leave place for others to occupy. Over the

surfaces of the land within the tropical region this draught toward

what we may term the equatorial chimney is perturbed by the

irregularities of the surface and many local accidents. But on the

sea, where the conditions are uniform, the air moving toward the point

of ascent is marked in the trade winds, which blow with a steadfast

sweep down toward the equator. Many slight actions, such as the

movement of the hot and cold currents of the sea, the local air

movements from the lands or from detached islands, somewhat perturb

the trade winds, but they remain among the most permanent features in

this changeable world. It is doubtful if anything on this sphere

except the atoms and molecules of matter have varied as little as the

trade winds in the centre of the wide ocean. So steadfast and uniform

are they that it is said that the helm and sails of a ship may be set

near the west coast of South America and be left unchanged for a

voyage which will carry the navigator in their belt across the width

of the Pacific.

Rising up from the earth in the tropical belt, the air attains the

height of several thousand feet; it then begins to curve off toward

the north and south, and at the height of somewhere about three to

five miles above the surface is again moving horizontally toward

either pole; attaining a distance on that journey, it gradually

settles down to the surface of the earth, and ceases to move toward

higher latitudes. If the earth did not revolve upon its axis the

course of these winds along the surface toward the equator, and in the

upper air back toward the poles, would be made in what we may call a

square manner--that is, the particles of air would move toward the

point where they begin to rise upward in due north and south lines,

according as they came from the southern or northern hemisphere, and

the upper currents or counter trades would retrace their paths also

parallel with the meridians or longitude lines. But because the earth

revolves from west to east, the course of the trade winds is oblique

to the equator, those in the northern hemisphere blowing from

northeast to southwest, those in the southern from southeast to

northwest. The way in which the motion of the earth affects the

direction of these currents is not difficult to understand. It is as

follows:

Let us conceive a particle of air situated immediately over the

earth's polar axis. Such an atom would by the rotation of the sphere

accomplish no motion except, indeed, that it might turn round on its

own centre. It would acquire no velocity whatever by virtue of the

earth's movement. Then let us imagine the particle moving toward the

equator with the speed of an ordinary wind. At every step of its

journey toward lower latitudes it would come into regions having a

greater movement than those which it had just left. Owing to its

inertia, it would thus tend continually to lag behind the particles of

matter about it. It would thus fall off to the westward, and, in place

of moving due south, would in the northern hemisphere drift to the

southwest, and in the southern hemisphere toward the northwest. A good

illustration of this action may be obtained from an ordinary

turn-table such as is used about railway stations to reverse the

position of a locomotive. If the observer will stand in the centre of

such a table while it is being turned round he will perceive that his

body is not swayed to the right or left. If he will then try to walk

toward the periphery of the rotating disk, he will readily note that

it is very difficult, if not impossible, to walk along the radius of

the circle; he naturally falls behind in the movement, so that his

path is a curved line exactly such as is followed by the winds which

move toward the equator in the trades. If now he rests a moment on the

periphery of the table, so that his body acquires the velocity of the

disk at that point, and then endeavours to walk toward the centre, he

will find that again he can not go directly; his path deviates in the

opposite direction--in other words, the body continually going to a

place having a less rate of movement by virtue of the rotation of the

earth, on account of its momentum is ever moving faster than the

surface over which it passes. This experiment can readily be tried on

any small rotating disk, such as a potter's wheel, or by rolling a

marble or a shot from the centre to the circumference and from the

circumference to the centre. A little reflection will show the

inquirer how these illustrations clearly account for the oblique

though opposite sets of the trade winds in the upper and lower parts

of the air.

The dominating effect of the tropical heat in controlling the

movements of the air currents extends, on the ocean surface, in

general about as far north and south as the parallels of forty

degrees, considerably exceeding the limits of the tropics, those lines

where the sun, because of the inclination of the earth's axis, at some

time of the year comes just overhead. Between these belts of trade

winds there is a strip or belt under the region where the atmosphere

is rising from the earth, in which the winds are irregular and have

little energy. This region of the "doldrums" or frequent calms is one

of much trouble to sailing ships on their voyages from one hemisphere

to another. In passing through it their sails are filled only by the

airs of local storms, or winds which make their way into that part of

the sea from the neighbouring continents. Beyond the trade-wind belt,

toward the poles, the movements of the atmosphere are dependent in

part on the counter trades which descend to the surface of the earth

in latitudes higher than that in which the surface or trade winds

flow. Thus along our Atlantic coast, and even in the body of the

continent, at times when the air is not controlled by some local

storm, the counter trade blows with considerable regularity.

The effect of the trade and counter-trade movements of the air on the

distribution of temperature over the earth's surface is momentous. In

part their influence is due to the direct heat-carrying power of the

atmosphere; in larger measure it is brought about by the movement of

the ocean waters which they induce. Atmospheric air, when deprived of

the water which it ordinarily contains, has very little

heat-containing capacity. Practically nearly all the power of

conveying heat which it possesses is due to the vapour of water which

it contains. By virtue of this moisture the winds do a good deal to

transfer heat from the tropical or superheated portion of the earth's

surface to the circumpolar or underheated realms. At first, the

relatively cool air which journeys toward the equator along the

surface of the sea constantly gains in heat, and in that process takes

up more and more water, for precisely the same reason that causes

anything to dry more rapidly in air which has been warmed next a fire.

The result is that before it begins to ascend in the tropical

updraught, being much moisture-laden, the atmosphere stores a good

deal of heat. As it rises, rarefies, and cools, the moisture descends

in the torrential rains which ordinarily fall when the sun is nearly

vertical in the tropical belt.

Here comes in a very interesting principle which is of importance in

understanding the nature of great storms, either the continuous storm

of the tropics or the local and irregular whirlings which occur in

various parts of the earth. When the moisture-laden air starts on its

upward journey from the earth it has, by virtue of the watery vapour

which it contains, a store of energy which becomes applied to

promoting the updraught. As it rises, the moisture in the air gathers

together or condenses, and in so doing parts with the heat which

caused it to evaporate from the ocean surface. For a given weight of

water, the amount of heat required to effect the evaporation is very

great; this we may roughly judge by observing what a continuous fire

is required to send a pint of water into the state of steam. This

energy, when it is released by the condensation of water into rain or

snow, becomes again heat, and tends somewhat, as does the fire in the

chimney, to accelerate the upward passage of the air. The result is

that the water which ascends in the equatorial updraught becomes what

we may term fuel to promote this important element in the earth's

aërial circulation. Trades and counter trades would doubtless exist

but for the efficiency of this updraught, which is caused by the

condensation of watery vapour, but the movement would be much less

than it is.

WHIRLING STORMS.

In the region near the equator, or near the line of highest

temperature, which for various reasons does not exactly follow the

equator, there is, as we have noticed, a somewhat continuous uprushing

current where the air passes upward through an ascending chimney,

which in a way girdles the sea-covered part of the earth. In this

region the movements of the air are to a great extent under the

control of the great continuous updraught. As we go to the north and

south we enter realms where the air at the surface of the earth is, by

the heat which it acquires from contact with that surface, more or

less impelled upward; but there being no permanent updraught for its

escape, it from time to time breaks through the roof of cold air which

overlies it and makes a temporary channel of passage. Going polarward

from the equator, we first encounter these local and temporary

upcastings of the air near the margin of the tropical belt. In these

districts, at least over the warmer seas, during the time of the year

when it is midsummer, and in the regions where the trade winds are not

strong enough to sweep the warm and moisture-laden air down to the

equatorial belt, the upward tending strain of the atmosphere next the

earth often becomes so strong that the overlying air is displaced,

forming a channel through which the air swiftly passes. As the

moisture condenses in the way before noted, the energy set free serves

to accelerate the updraught, and a hurricane is begun. At first the

movement is small and of no great speed, but as the amount of air

tending upward is likely to be great, as is also the amount of

moisture which it contains, the aërial chimney is rapidly enlarged,

and the speed of the rising air increased. The atmosphere next the

surface of the sea flows in toward the channel of escape; its passage

is marked by winds which are blowing toward the centre. On the

periphery of the movement the particles move slowly, but as they win

their way toward the centre they travel with accelerating velocity. On

the principle which determines the whirling movement of the water

escaping through a hole in the bottom of a basin, the particles of the

air do not move on straight lines toward the centre, but journey in

spiral paths, at first along the surface, and then ascending.

We have noted the fact that in a basin of water the direction of the

whirling is what we may term accidental--that is, dependent on

conditions so slight that they elude our observation--but in

hurricanes a certain fact determines in an arbitrary way the direction

in which the spin shall take place. As soon as such a movement of the

air attains any considerable diameter, although in its beginning it

may have spun in a direction brought about by local accidents, it will

be affected by the diverse rates of travel, by virtue of the earth's

rotation, of the air on its equatorial and polar sides. On the

equatorial side this air is moving more rapidly than it is on the

polar side. By observing the water passing from a basin this

principle, with a few experiments, can be made plain. The result is to

cause these great whirlwinds of the hurricanes of higher latitudes to

whirl round from right to left in the northern hemisphere and in the

reverse way in the southern. The general system of the air currents

still further affects these, as other whirling storms, by driving

their centres or chimneys over the surface of the earth. The principle

on which this is done may be readily understood by observing how the

air shaft above a chimney, through which we may observe the smoke to

rise during a time of calm, is drawn off to one side by the slight

current which exists even when we feel no wind; it may also be

discerned in the little dust whirls which form in the streets on a

summer day when the air is not much disturbed. While they spin they

move on in the direction of the air drift. In this way a hurricane

originating in the Gulf of Mexico may gradually journey under the

influence of the counter trades across the Antilles, or over southern

Florida, and thence pursue a devious northerly course, generally near

the Atlantic coast and in the path of the Gulf Stream, until it has

travelled a thousand miles or more toward the North Atlantic. The

farther it goes northward the less effectively it is fed with warm and

moisture-laden air, the feebler its movement becomes, until at length

it is broken up by the variable winds which it encounters.

A very interesting and, from the point of view of the navigator,

important peculiarity of these whirls is that at their centre there is

a calm, similar in origin and nature to the calm under the equator

between the trade-wind belts. Both these areas are in the field where

the air is ascending, and therefore at the surface of the earth does

not affect the sails of ships, though if men ever come to use flying

machines and sail through the tropics at a good height above the sea

it will be sensible enough. The difference between the doldrum of the

equator and that of the hurricane, besides their relative areas, is

that one is a belt and the other a disk. If the seafarer happens to

sail on a path which leads him through the hurricane centre, he will

first discern, as from the untroubled air and sea he approaches the

periphery of the storm, the horizon toward the disturbance beset by

troubled clouds, all moving in one direction. Entering beneath this

pall, he finds a steadily increasing wind, which in twenty miles of

sailing may, and in a hundred miles surely will, compel him to take in

all but his storm sails, and is likely to bring his ship into grave

peril. The most furious winds the mariner knows are those which he

encounters as he approaches the still centre. These trials are made

the more appalling by the fact that in the furious part of the whirl

the rain, condensing from the ascending air, falls in torrents, and

the electricity generated in the condensation gives rise to vivid

lightning. If the storm-beset ship can maintain her way, in a score or

two of miles of journey toward the centre, generally very quickly, it

passes into the calm disk, where the winds, blowing upward, cease to

be felt. In this area the ship is not out of danger, for the waves,

rolling in from the disturbed areas on either side, make a torment of

cross seas, where it is hard to control the movements of a sailing

vessel because the impulse of the winds is lost. Passing through this

disk of calm, the ship re-encounters in reverse order the furious

portion of the whirl, afterward the lessening winds, until it escapes

again into the airs which are not involved in the great torment.

In the old days, before Dove's studies of storms had shown the laws of

hurricane movement, unhappy shipmasters were likely to be caught and

retained in hurricanes, and to battle with them for weeks until their

vessels were beaten to pieces. Now the "Sailing Directions," which are

the mariner's guide, enable him, from the direction of the winds and

the known laws of motion of the storm centre, to sail out of the

danger, so that in most cases he may escape calamity. It is otherwise

with the people who dwell upon the land over which these atmospheric

convulsions sweep. Fortunately, where these great whirlwinds trespass

on the continent, they quickly die out, because of the relative lack

of moisture which serves to stimulate the uprush which creates them.

Thus in their more violent forms hurricanes are only felt near the

sea, and generally on islands and peninsulas. There the hurricane

winds, by the swiftness of their movement, which often attains a speed

of a hundred miles or more, apply a great deal of energy to all

obstacles in their path. The pressure thus produced is only less

destructive than that which is brought about by the tornadoes, which

are next to be described.

There is another effect from hurricanes which is even more destructive

to life than that caused by the direct action of the wind. In these

whirlings great differences in atmospheric pressure are brought about

in contiguous areas of sea. The result is a sudden elevation in the

level of one part of the water. These disturbances, where the shore

lands are low and thickly peopled, as is the case along the western

coast of the Bay of Bengal, may produce inundations which are terribly

destructive to life and property. They are known also in southern

Florida and along the islands of the Caribbean, but in that region are

not so often damaging to mankind.

Fortunately, hurricanes are limited to a very small part of the

tropical district. They occur only in those regions, on the eastern

faces of tropical lands, where the general westerly set of the winds

favours the accumulation of great bodies of very warm, moist air next

the surface of the sea. The western portion of the Gulf of Mexico and

the Caribbean, the Bay of Bengal, and the southeastern portion of Asia

are especially liable to their visitations. They sometimes develop,

though with less fury, in other parts of the tropics. On the western

coast of South America and Africa, where the oceans are visited by the

dry land winds, and where the waters are cooled by currents setting

in from high latitudes, they are unknown.

Only less in order of magnitude than the hurricanes are the circular

storms known as cyclones. These occur on the continents, especially

where they afford broad plains little interrupted by mountain ranges.

They are particularly well exhibited in that part of North America

north of Mexico and south of Hudson Bay. Like the hurricanes, they

appear to be due to the inrush of relatively warm air entering an

updraught which had been formed in the overlying, cooler portions of

the atmosphere. They are, however, much less energetic, and often of

greater size than the hurricane whirl. The lack of energy is probably

due to the comparative dryness of the air. The greater width of the

ascending column may perhaps be accounted for by the fact that,

originating at a considerable height above the sea, they have a less

thickness of air to break through, and so the upward setting column is

readily made broad.

The cyclones of North America appear generally to originate in the

region of the Rocky Mountains, though it is probable that in some

instances, perhaps in many, the upward set of the air which begins the

storm originates in the ocean along the Pacific coast. They gather

energy as they descend the great sloping plain leading eastward from

the Rocky Mountains to the central portion of the great continental

valley. Thence they move on across the country to the Atlantic coast.

Not infrequently they continue on over the ocean to the European

continent. The eastward passage of the storm centre is due to the

prevailing eastward movement of the air in its upper part throughout

that portion of the northern hemisphere. Commonly they incline

somewhat to the northward of east in their journey. In all cases the

winds appear to blow spirally into the common storm centre. There is

the same doldrum area or calm field in the centre of the storm that we

note between the trade winds and in the middle of a hurricane disk,

though this area is less defined than in the other instances, and the

forward motion of the storm at a considerable speed is in most cases

characteristic of the disturbance. On the front of one of these storms

in North America the winds commonly begin in the northeast, thence

they veer by the east to the southwest. At this stage in the movement

the storm centre has passed by, the rainfall commonly ceases, and

cold, dry winds setting to the northwestward set in. This is caused by

the fact that the ascending air, having attained a height above the

earth, settles down behind the storm, forming an anticyclone or mass

of dry air, which presses against the retreating side of the great

whirlwind.

In front of the storm the warm and generally moist relatively warm

air, pressing in toward the point of uprise and overlaid by the upper

cold air, is brought into a condition where it tends to form small

subordinate shafts up through which it whirls on the same principle,

but with far greater intensity than the main ascending column. The

reason for the violence of this movement is that the difference in

temperature of the air next the surface and that at the height of a

few thousand feet is great. As might be expected, these local

spinnings are most apt to occur in the season when the air next the

earth is relatively warm, and they are aptest to take place in the

half of the advancing front lying between the east and south, for the

reason that there the highest temperatures and the greatest humidity

are likely to coexist. In that part of the field, during the time when

the storm is advancing from the Rocky Mountains to the Atlantic, a

dozen or more of these spinning uprushes may be produced, though few

of them are likely to be of large size or of great intensity.

The secondary storms of cyclones, such as are above noted, receive the

name of tornadoes. They are frequent and terrible visitations of the

country from northern Texas, Florida, and Alabama to about the line of

the Great Lakes; they are rarely developed in the region west of

central Kansas, and only occasionally do they exhibit much energy in

the region east of the plain-lands of the Ohio Valley. Although known

in other lands, they nowhere, so far as our observations go, exhibit

the paroxysmal intensity which they show in the central portion of the

North American continent. There the air which they affect acquires a

speed of movement and a fury of action unknown in any other

atmospheric disturbances, even in those of the hurricanes.

The observer who has a chance to note from an advantageous position

the development of a tornado observes that in a tolerably still air,

or at least an air unaffected by violent winds--generally in what is

termed a "sultry" state of the atmosphere--the storm clouds in the

distance begin to form a kind of funnel-shaped dependence, which

gradually extends until it appears to touch the earth. As the clouds

are low, this downward-growing column probably in no case is observed

for the height of more than three or four thousand feet. As the funnel

descends, the clouds above and about it may be seen to take on a

whirling movement around the centre, and under favourable

circumstances an uprush of vapours may be noted in the centre of the

swaying shaft. As the whirl comes nearer, the roar of the disturbance,

which at a distance is often compared to the sound made by a threshing

machine or to that of distant musketry, increases in loudness until it

becomes overwhelming. When a storm such as this strikes a building, it

is not only likely to be razed by the force of the wind, but it may be

exploded, as by the action of gunpowder fired within its walls,

through the sudden expansion of the air which it contains. In the

centre of the column, although it rarely has a diameter of more than a

few hundred feet, the uprush is so swift that it makes a partial

vacuum. The air, striving to get into the space which it is eager to

occupy, is whirling about at such a rate that the centrifugal motion

which it thus acquires restrains its entrance. In this way there may

be, as the column rapidly moves by, a difference of pressure

amounting probably to what the mercury of a barometer would indicate

by four or five inches of fall. Unless the structure is small and its

walls strong, its roof and sides are apt to be blown apart by this

difference of pressure and the consequent expansion of the contained

air. In some cases where wooden buildings have withstood this curious

action the outer clapboards have been blown off by the expansion of

the small amount of air contained in the interspaces between that

covering and the lath and plaster within (see Fig. 9).

[Illustration: Fig. 9.--Showing effect of expansion of air contained

in a hollow wall during the passage of the storm.]

The blow of the air due to its rotative whirling has in several cases

proved sufficient to throw a heavy locomotive from the track of a

well-constructed railway. In all cases where it is intense it will

overturn the strongest trees. The ascending wind in the centre of the

column may sometimes lift the bodies of men and of animals, as well as

the branches and trunks of trees and the timber of houses, to the

height of hundreds of feet above the surface. One of the most striking

exhibitions of the upsucking action in a tornado is afforded by the

effect which it produces when it crosses a small sheet of water. In

certain cases where, in the Northwestern States of this country, the

path of the storm lay over the pool, the whole of the water from a

basin acres in extent has been entirely carried away, leaving the

surface, as described by an observer, apparently dry enough to plough.

Fortunately for the interests of man, as well as those of the lower

organic life, the paths of these storms, or at least the portion of

their track where the violence of the air movement makes them very

destructive, often does not exceed five hundred feet in width, and is

rarely as great as half a mile in diameter. In most cases the length

of the journey of an individual tornado does not exceed thirty miles.

It rarely if ever amounts to twice that distance.

In every regard except their small size and their violence these

tornadoes closely resemble hurricanes. There is the same broad disk of

air next the surface spirally revolving toward the ascending centre,

where its motion is rapidly changed from a horizontal to a vertical

direction. The energy of the uprush in both cases is increased by the

energy set free through the condensation of the water, which tends

further to heat and thus to expand the air. The smaller size of the

tornado may be accounted for by the fact that we have in their

originating conditions a relatively thin layer of warm, moist air next

the earth and a relatively very cold layer immediately overlying it.

Thus the tension which serves to start the movement is intense, though

the masses involved are not very great. The short life of a tornado

may be explained by the fact that, though it apparently tends to grow

in width and energy, the central spout is small, and is apt to be

broken by the movements of the atmosphere, which in the front of a

cyclone are in all cases irregular.

On the warmer seas, but often beyond the limits of the tropics,

another class of spinning storms, known as waterspouts, may often be

observed. In general appearance these air whirls resemble tornadoes,

except that they are in all cases smaller than that group of

whirlings. As in the tornadoes, the waterspout begins with a funnel,

which descends from the sky to the surface of the sea. Up the tube

vapours may be seen ascending at great speed, the whole appearing like

a gigantic pillar of swiftly revolving smoke. When the whirl reaches

the water, it is said that the fluid leaps up into the tube in the

form of dense spray, an assertion which, in view of the fact of the

action of a tornado on a lake as before described, may well be

believed. Like the tornadoes and dust whirls, the life of a waterspout

appears to be brief. They rarely endure for more than a few minutes,

or journey over the sea for more than two or three miles before the

column appears to be broken by some swaying of the atmosphere. As

these peculiar storms are likely to damage ships, the old-fashioned

sailors were accustomed to fire at them with cannon. It has been

claimed that a shot would break the tube and end the little

convulsion. This, in view of the fact that they appear to be easily

broken up by relatively trifling air currents, may readily be

believed. The danger which these disturbances bring to ships is

probably not very serious.

The special atmospheric conditions which bring about the formation of

waterspouts are not well known; they doubtless include, however, warm,

moist air next the surface of the sea and cold air above. Just why

these storms never attain greater size or endurance is not yet known.

These disturbances have been seen for centuries, but as yet they have

not been, in the scientific sense, observed. Their picturesqueness

attracts all beholders; it is interesting to note the fact that

perhaps the earliest description of their phenomena--one which takes

account in the scientific spirit of all the features which they

present--was written by the poet Camoëns in the Lusiad, in which he

strangely mingles fancy and observation in his account of the great

voyage of Vasco da Gama. The poet even notes that the water which

falls when the spout is broken is not salt, but fresh--a point which

clearly proves that not much of the water which the tube contains is

derived from the sea. It is, in fact, watery vapour drawn from the air

next the surface of the ocean, and condensed in its ascent through the

tube. In this and other descriptions of Nature Camoëns shows more of

the scientific spirit than any other poet of his time. He was in this

regard the first of modern writers to combine a spiritual admiration

for Nature with some sense of its scientific meaning.

In treating of the atmosphere, meteorologists base their studies

largely on changes in the weight of that medium, which they determine

by barometric observations. In fact, the science of the air had its

beginning in Pascal's admirable observation on the changes in the

height of a column of mercury contained in a bent tube as he ascended

the volcanic peak known as Puy de Dome, in central France. As before

noted, it is to the disturbances in the weight of the air, brought

about mainly by variations in temperature, that we owe all its

currents, and it is upon these winds that the features we term climate

in largest measure depend. Every movement of the winds is not only

brought about by changes in the relative weight of the air at certain

points, but the winds themselves, owing to the momentum which the air

attains by them, serve to bring about alterations in the quantity of

air over different parts of the earth, which are marked most

distinctly by barometric variations. These changes are exceedingly

complicated; a full account of them would demand the space of this

volume. A few of the facts, however, should be presented here. In the

first place, we note that each day there is normally a range in the

pressure which causes the barometer to be at the lowest at about four

o'clock in the morning and four o'clock in the afternoon, and highest

at about ten o'clock in those divisions of the day. This change is

supposed to be due to the fact that the motes of dust in the

atmosphere in the night, becoming cooled, condense the water vapour

upon their surfaces, thus diminishing the volume of the air. When the

sun rises the water evaporated by the heat returns from these little

storehouses into the body of the atmosphere. Again in the evening the

condensation sets in; at the same time the air tends to drift in from

the region to the westward, where the sun is still high, toward the

field where the barometer has been thus lowered; the current gradually

attains a certain volume, and so brings about the rise of the

barometer about ten o'clock at night.

In the winter time, particularly on the well-detached continent of

North America, we find a prevailing high barometer in the interior of

the country and a corresponding low state of pressure on the Atlantic

Ocean. In the summer season these conditions are on the whole

reversed.

Under the tropics, in the doldrum belt, there is a zone of low

barometer connected to the ascending currents which take place along

that line. This is a continuous manifestation of the same action which

gives a large area of a disklike form in the centre or eye of the

hurricane and in the middle portion of the tornado's whirl. In

general, it may be said that the weight of the air is greatest in the

regions from which it is blowing toward the points of upward escape,

and least in and about those places where the superincumbent air is

rising through a temporary or permanent line of escape. In other

words, ascending air means generally a relatively low barometer, while

descending air is accompanied by greater pressure in the field upon

which it falls.

In almost every part of the earth which is affected by a particular

physiography we find that the movements of the atmosphere next the

surface are qualified by the condition which it encounters. In fact,

if a person were possessed of all the knowledge which could be

obtained concerning winds, he could probably determine as by a map the

place where he might chance to find himself, provided he could extend

his observations over a term of years. In other words, the regimen of

the winds--at least those of a superficial nature--is almost as

characteristic of the field over which they go as is a map of the

country. Of these special winds a number of the more important have

been noted, only a few of which we can advert to. First among these

may well come the land and sea breezes which are remarked about all

islands which are not continuously swept by permanent winds. One of

the most characteristic instances of these alternate winds is perhaps

that afforded on the island of Jamaica.

The island of Jamaica is so situated within the basin of the Caribbean

that it does not feel the full influence of the trades. It has a range

of high mountains through its middle part. In the daytime the surface

of the land, which has the sun overhead twice each year, and is always

exposed to nearly vertical radiation, becomes intensely hot, so that

an upcurrent is formed. The formation of this current is favoured by

the mountains, which apply a part of the heat at the height of about a

mile above the surface of the sea. This action is parallel to that we

notice when, in order to create a draught in the air of a chimney, we

put a torch some distance up above the fireplace, thus diminishing the

height of the column of air which has to be set in motion. It is

further shown by the fact that when miners sought to make an upcurrent

in a shaft, in order to lead pure air into the workings through other

openings, they found after much experience that it was better to have

the fire near the top of the shaft rather than at the bottom.

The ascending current being induced up the mountain sides of Jamaica,

the air is forced in from the sea to the relatively free space. Before

noon the current, aided in its speed by a certain amount of the

condensation of the watery vapour before described, attains the

proportions of a strong wind. As the sun begins to sink, the earth's

surface pours forth its heat; the radiation being assisted by the

extended surfaces of the plants, cooling rapidly takes place.

Meanwhile the sea, because of the great heat-storing power of water,

is very little cooled, the ascent of the air ceases, the temporary

chimney with its updraught is replaced by a downward current, and the

winds blow from the land until the sun comes again to reverse the

current. In many cases these movements of the daily winds flowing into

and from islands induce a certain precipitation of moisture in the

form of rain. Generally, however, their effect is merely to ameliorate

the heat by bringing alternately currents from the relatively cool sea

and from the upper atmosphere to lessen the otherwise excessive

temperature of the fields which they traverse.

Although characteristic sea and land winds are limited to regions

where the sun's heat is great, they are traceable even in high

latitudes during the periods of long-continued calm attended with

clear skies. Thus on the island of Martha's Vineyard, in

Massachusetts, the writer has noted, when the atmosphere was in such a

state, distinct night and day, or sea and land, breezes coming in

their regular alternation. During the night when these alternate winds

prevail the central portion of the island, at the distance of three

miles from the sea, is remarkably cold, the low temperature being due

to the descending air current. To the same physical cause may be

attributed the frequent insets of the sea winds toward midday along

the continental shores of various countries. Thus along the coast of

New England in the summer season a clear, still, hot day is certain to

lead to the creation of an ingoing tide of air, which reaches some

miles into the interior. This stream from the sea enters as a thin

wedge, it often being possible to note next the shore when the

movement begins a difference of ten degrees of temperature between the

surface of the ground to which the point of the wedge has attained,

and a position twenty feet higher in the air. This is a beautiful

example to show at once how the relative weight of the atmosphere,

even when the differences are slight, may bring about motion, and also

how masses of the atmosphere may move by or through the rest of the

medium in a way which we do not readily conceive from our observations

on the transparent mass. Very few people have any idea how general is

the truth that the air, even in continuous winds, tends to move in

more or less individualized masses. This, however, is made very

evident by watching the gusts of a storm or the wandering patches of

wind which disturb the surface of an otherwise smooth sea.

[Illustration: _South shore, Martha's Vineyard, Massachusetts, showing

a characteristic sand beach with long slope and low dunes. Note the

three lines of breakers and the splash flows cutting little bays in

the sand._]

Among the notable local winds are those which from their likeness to

the Föhn of the Swiss valleys receive that name. Föhns are produced

where a body of air blowing against the slope of a continuous mountain

range is lifted to a considerable height, and, on passing over the

crest, falls again to a low position. In its ascent the air is cooled,

rarefied, and to a great extent deprived of its moisture. In

descending it is recondensed, and by the process by which its atoms

are brought together its latent heat is made sensible. There being but

little watery vapour in the mass, this heat is not much called for by

that heat-storing fluid, and so the air is warmed. So far Föhn winds

have only been remarked as conspicuous features in Switzerland and on

the eastern face of the Rocky Mountains. In the region about the head

waters of the Missouri and to the northward their influence in what

are called the Chinook winds is distinctly to ameliorate the severe

winter climate of the country.

In almost all great desert regions, particularly in the typical

Sahara, we find a variety of storm belonging to the whirlwind group,

which, owing to the nature of the country, take on special

characteristics. These desert storms take up from the verdureless

earth great quantities of sand and other fine _débris_, which often so

clouds the air as to bring the darkness of night at midday. Their

whirlings appear in size to be greater than those which produce

tornadoes or waterspouts, but less than hurricanes or cyclones.

Little, however, is known about them. They have not been well

observed by meteorologists. In some ways they are important, for the

reason that they serve to carry the desert sand into regions

previously verdure-clad, and thus to extend the bounds of the desolate

fields in which they originate. Where they blow off to the seaward,

they convey large quantities of dust into the ocean, and thus serve to

wear down the surface of the land in regions where there are no rivers

to effect that action in the normal way.

Notwithstanding its swift motion when impelled by differences in

weight, the movements of the air have had but little direct and

immediate influence on the surface of the earth. The greater part of

the work which it does, as we shall see hereafter, is done through the

waters which it impels and bears about. Yet where winds blow over

verdureless surfaces the effect of the sand which they sweep before

them is often considerable. In regions of arid mountains the winds

often drive trains of sand through the valleys, where the sharp

particles cut the rocks almost as effectively as torrents of water

would, distributing the wearing over the width of the valley. The dust

thus blown, from a desert region may, when it attains a country

covered with vegetation, gradually accumulate on its surface, forming

very thick deposits. Thus in northwestern China there is a wide area

where dust accumulations blown from the arid districts of central Asia

have gradually heaped up in the course of ages to the depth of

thousands of feet, and this although much of the _débris_ is

continually being borne away by the action of the rain waters as they

journey toward the sea. Such dust accumulations occur in other parts

of the world, particularly in the districts about the upper

Mississippi and in the valleys of the Rocky Mountains, but nowhere are

they so conspicuous as in the region first mentioned.

Where prevailing winds from the sea, from great lakes, and even from

considerable rivers, blow against sandy shores or cliffs of the same

nature, large quantities of sand and dust are often driven inland

from the coast line. In most cases these wind-borne materials take on

the form of dunes, or heaps of sand, varying from a few feet to

several hundred feet in height. It is characteristic of these hills of

blown sand that they move across the face of the country. Under

favourable conditions they may journey scores of miles from the shore.

The marching of a dune is effected through the rolling up of the sand

on the windward side of the elevation, when it is impelled by the

current of air to the crest where it falls into the lee or shelter

which the hill makes to the wind. In this way in the course of a day

the centre of the dune, if the wind be blowing furiously, may advance

a measurable distance from the place it occupied before. By fits and

starts this ongoing may be indefinitely continued. A notable and

picturesque instance of the march of a great dune may be had from the

case in which one of them overwhelmed in the last century the village

of Eccles in southeastern England. The advancing sand gradually crept

into the hamlet, and in the course of a decade dispossessed the people

by burying their houses. In time the summit of the church spire

disappeared from view, and for many years thereafter all trace of the

hamlet was lost. Of late years, however, the onward march of the sands

has disclosed the church spire, and in the course of another century

the place may be revealed on its original site, unchanged except that

the marching hill will be on its other side.

In the region about the head of the Bay of Biscay the quantity of

these marching sands is so great that at one time they jeopardized the

agriculture of a large district. The French Government has now

succeeded, by carefully planting the surface of the country with

grasses and other herbs which will grow in such places, in checking

the movement of the wind-blown materials. By so doing they have merely

hastened the process by which Nature arrests the march of dunes. As

these heaps creep away from the sea, they generally come into regions

where a greater variety of plants flourish; moreover, their sand

grains become decayed, so that they afford a better soil. Gradually

the mat of vegetation binds them down, and in time covers them over so

that only the expert eye can recognise their true nature. Only in

desert regions can the march of these heaps be maintained for great

distances.

Characteristic dunes occur from point to point all along the Atlantic

coast from the State of Maine to the northern coast of Florida. They

also occur along the coasts of our Great Lakes, being particularly

well developed at the southern end of Lake Michigan, where they form,

perhaps, the most notable accumulations within the limits of the

United States.

When blown sands invade a forest and the deposit is rapidly

accumulated, the trees are often buried in an undecayed condition. In

this state, with certain chemical reactions which may take place in

the mass, the woody matter is apt to become replaced by silex

dissolved from the sand, which penetrates the tissues of the plants.

In this way salicified forests are produced, such as are found in the

region of the Rocky Mountains, where the trunks of the trees, now very

hard stone, so perfectly preserve their original structure that when

cut and polished they may be used for decorative purposes. Conspicuous

as is this work of the dunes, it is in a geological way much less

important than that accomplished by the finer dust which drifts from

one region of land to another or into the sea. Because of their

weight, the sand grains journey over the surface of the earth, except,

indeed, where they are uplifted by whirl storms. They thus can not

travel very fast or far. Dust, however, rises into the air, and

journeys for indefinite distances. We thus see how slight differences

in the weight of substances may profoundly affect the conditions of

their deportation.

THE SYSTEM OF WATERS.

The envelope of air wraps the earth completely about, and, though

varying in thickness, is everywhere present over its surface. That of

the waters is much less equally distributed. Because of its weight, it

is mainly gathered in the depths of the earth, where it lies in the

interstices of the rocks and in the great realm of the seas. Only a

very small portion of the fluid is in the atmosphere or on the land.

Perhaps less than a ten thousandth part of the whole is at any one

time on this round from the seas through the air to the land and back

to the great reservoir.

The great water store of the earth is contained in two distinct

realms--in the oceans, where the fluid is concentrated in a quantity

which fills something like nine tenths of the hollows formed by the

corrugations of the earth's surface; and in the rocks, where it is

stored in a finely divided form, partly between the grains of the

stony matter and partly in the substance of its crystals, where it

exists in a combination, the precise nature of which is not well

known, but is called water of crystallization. On the average, it

seems likely that the materials of the earth, whether under the sea or

on the land, have several per cent of their mass of the fluid.

It is not yet known to what depth the water-bearing section of the

earth extends; but, as we shall see more particularly hereafter when

we come to consider volcanoes, the lavas which they send up to the

surface are full of contained water, which passes from them in the

form of steam. The very high temperature of these volcanic ejections

makes it necessary for us to suppose that they come from a great

depth. It is difficult to believe that they originate at less than a

hundred miles below the earth's surface. If, then, the rocks contain

an average of even five per cent of water to the depth of one hundred

miles, the quantity of the fluid stored within the earth is greater

than that which is contained in the reservoir of the ocean. The

oceans, on the average, are not more than three miles deep; spread

evenly over the surface of the whole earth, their depth would be less

than two miles, while the water in the rocks, if it could be added to

the seas, would make the total depth seven miles or more. As we shall

note hereafter, the processes of formation of strata tend to imprison

water in the beds, which in time is returned to the earth's surface by

the forces which operate within the crust.

Although the water in the seas is, as we have seen, probably less than

one half of the store which the earth possesses, the part it plays in

the economy of the planet is in the highest measure important. The

underground water operates solely to promote certain changes which

take place in the mineral realm. Its effect, except in volcanic

processes, are brought about but slowly, and are limited in their

action. The movements of this buried water are exceedingly gradual;

the forces which impel it about and which bring it to do its work

originate in the earth. In the seas the fluid has an exceeding freedom

of motion; it can obey the varied impulses which the solar energy

imposes upon it. The rôle of these wonderful actions which we are

about to trace includes almost everything which goes on upon the

surface of the planet--that which relates to the development of animal

and vegetable life, as well as to the vast geological changes which

the earth is undergoing.

If the surface of the earth were uniformly covered with water to the

depth of ten thousand feet or more, every particle of fluid would, in

a measure, obey the attraction of the sun, of the moon, and,

theoretically, also of all the other bodies in space, on the principle

that every particle of matter in the universe exercises a gravitative

effect on every other. As it is, owing to the divided condition of the

water on the earth's surface, only that which is in the ocean and

larger seas exhibits any measurable influence from these distant

attractions. In fact, only the tides produced by the moon and sun are

of determinable magnitude, and of these the lunar is of greater

importance, the reason being the near position of our satellite to our

own sphere. The solar tide is four tenths as great as the lunar. The

water doubtless obeys in a slight way the attraction of the other

celestial bodies, but the motions thus imparted are too small to be

discerned; they are lost in the great variety of influences which

affect all the matter on the earth.

Although the tides are due to the attraction of the solar bodies,

mainly to that of the moon, the mode in which the result is brought

about is somewhat complicated. It may briefly and somewhat

incompletely be stated as follows: Owing to the fact that the

attracting power of the earth is about eighty times greater than that

of the moon, the centre of gravity of the two bodies lies within the

earth. About this centre the spheres revolve, each in a way swinging

around the other. At this point there is no centrifugal motion arising

from the revolution of the pair of spheres, but on the side of the

earth opposite the moon, some six thousand miles away, the centrifugal

force is considerable, becoming constantly greater as we pass away

from the turning point. At the same time the attraction of the moon on

the water becomes less. Thus the tide opposite the satellite is

formed. On the side toward the moon the same centrifugal action

operates, though less effectively than in the other case, for the

reason that the turning point is nearer the surface; but this action

is re-enforced by the greater attraction of the moon, due to the fact

that the water is much nearer that body.

In the existing conditions of the earth, what we may call the normal

run of the tides is greatly interrupted. Only in the southern ocean

can the waters obey the lunar and solar attraction in anything like a

normal way. In that part of the earth two sets of tides are

discernible, the one and greater due to the moon, the other, much

smaller, to the sun. As these tides travel round at different rates,

the movements which they produce are sometimes added to each other

and sometimes subtracted--that is, at times they come together, while

again the elevation of one falls in the hollow of the other. Once

again supposing the earth to be all ocean covered, computation shows

that the tides in such a sea would be very broad waves, having,

indeed, a diameter of half the earth's circumference. Those produced

by the moon would have an altitude of about one foot, and those by the

sun of about three inches. The geological effects of these swayings

would be very slight; the water would pass over the bottom to and fro

twice each day, with a maximum journey of a hundred or two feet each

way from a fixed point. This movement would be so slow that it could

not stir the fine sediment; its only influence would perhaps be to

help feed the animals which were fixed upon the bottom by drawing the

nurture-bringing water by their mouths.

Although the divided condition of the ocean perturbs the action of the

tides, so that except by chance their waves are rarely with their

centres where the attracting bodies tend to make them, the influence

of these divisions is greatly to increase the geological or

change-bringing influences arising from these movements. When from the

southern ocean the tides start to the northward up the bays of the

Atlantic, the Pacific, or the Indian Ocean, they have, as before

noted, a height of perhaps less than two feet. As they pass up the

narrowing spaces the waves become compressed--that is, an equal volume

of moving water has less horizontal room for its passage, and is

forced to rise higher. We see a tolerably good illustration of the

same principle when we observe a wind-made wave enter a small recess

of the shore, the sides of which converge in the direction of the

motion. With the diminished room, the wave gains in height. It thus

comes about that the tide throughout the Atlantic basin is much higher

than in the southern ocean. On the same principle, when the tide rolls

in against the shores every embayment of a distinct kind, whose sides

converge toward the head, packs up the tidal wave, often increasing

its height in a remarkable way. When these bays are wide-mouthed and

of elongate triangular form, with deep bottoms, the tides which on

their outer parts have a height of ten or fifteen feet may attain an

altitude of forty or fifty feet at the apex of the triangle.

We have already noted the fact that the tide, such as runs in the

southern ocean, exercises little or no influence upon the bottom of

the sea over which it moves. As the height of the confined waters

increases, the range of their journey over the bottom as the wave

comes and goes rapidly increases. When they have an elevation of ten

feet they can probably stir the finer mud on the ocean floor, and in

shallow water move yet heavier particles. In the embayments of the

land, where a great body of water journeys like an alternating river

into extensive basins, the tidal action becomes intense; the current

may be able to sweep along large stones quite as effectively as a

mountain torrent. Thus near Eastport, Me., where the tides have a

maximum rise and fall of over twenty feet, the waters rush in places

so swiftly that at certain stages of the movement they are as much

troubled as those at the rapids of the St. Lawrence. In such portions

of the shore the tides do important work in carving channels into the

lands.

Along the shores of the continents about the North Atlantic, where the

tides act in a vigorous manner, we almost everywhere find an

underwater shelf extending from the shore with a declivity of only

five to ten feet to the mile toward the centre of the sea, until the

depth of about five hundred feet is attained; from this point the

bottom descends more steeply into the ocean's depth. It is probable

that the larger part of the material composing these continental

shelves has been brought to its position by tidal action. Each time

the tidal wave sweeps in toward the shore it urges the finer particles

of sediment along with it. When it moves out it drags them on the

return journey toward the depths of the sea. If this shelf were

perfectly horizontal, the two journeys of the sand and mud grains

would be of the same length; but as the movement takes place up and

down a slope, the bits will travel farther under the impulse which

leads them downward than under that which impels them up. The result

will be that the particles will travel a little farther out from the

shore each time it is swung to and fro in the alternating movement of

the tide.

The effect of tidal movement in nurturing marine life is very great.

It aids the animals fixed on the bottoms of the deep seas to obtain

their provision of food and their share of oxygen by drawing the water

by their bodies. All regions which are visited by strong tides

commonly have in the shallows near the shores a thick growth of

seaweed which furnishes an ample provision of food for the fishes and

other forms of animal life.

A peculiar effect arising from tidal action is believed by students of

the phenomena to be found in the slowing of the earth's rotation on

its axis. The tides rotate around the earth from east to west, or

rather, we should say, the solid mass of the earth rubs against them

as it spins from west to east. As they move over the bottom and as

they strike against the shores this push of the great waves tends in a

slight measure to use up the original spinning impulse which causes

the earth's rotation. Computation shows that the amount of this action

should be great enough gradually to lengthen the day, or the time

occupied by the earth in making a complete revolution on the polar

axis. The effect ought to be great enough to be measurable by

astronomers in the course of a thousand years. On the other hand, the

records of ancient eclipses appear pretty clearly to show that the

length of the day has not changed by as much as a second in the course

of three thousand years. This evidence does not require us to abandon

the supposition that the tides tend to diminish the earth's rate of

rotation. It is more likely that the effect of the reduction in the

earth's diameter due to the loss of heat which is continually going on

counterbalances the influence of the tidal friction. As the diameter

of a rotating body diminishes, the tendency is for the mass to spin

more rapidly; if it expands, to turn more slowly, provided in each

case the amount of the impulse which leads to the turning remains the

same. This can be directly observed by whirling a small weight

attached to a string in such a manner that the cord winds around the

finger with each revolution; it will be noted that as the line

shortens the revolution is more quickly accomplished. We can readily

conceive that the earth is made up of weights essentially like that

used in the experiment, each being drawn toward the centre by the

gravitative stress, which is like that applied to the weight by the

cord.

The fact that the days remain of the same length through vast periods

of time is probably due to this balance between the effects of tidal

action and those arising from the loss of heat--in other words, we

have here one of those delicate arrangements in the way of

counterpoise which serve to maintain the balanced conditions of the

earth's surface amid the great conflicts of diverse energies which are

at work in and upon the sphere.

It should be understood that the effects of the attraction which

produces tides are much more extensive than they are seen to be in the

movements of the sea. So long as the solar and planetary spheres

remain fluid, the whole of their masses partake of the movement. It is

a consequence of this action, as the computations of Prof. George

Darwin has shown, that the moon, once nearer the earth than it is at

present, has by a curious action of the tidal force been pushed away

from the centre of our sphere, or rather the two bodies have repelled

each other. An American student of the problem, Mr. T.J.J. See, has

shown that the same action has served to give to the double stars the

exceeding eccentricity of their orbits.

Although these recent studies of tidal action in the celestial sphere

are of the utmost importance to the theory of the universe, for they

may lead to changes in the nebular hypotheses, they are as yet too

incomplete and are, moreover, too mathematical to be presented in an

elementary treatise such as this.

* * * * *

We now turn to another class of waves which are of even more

importance than those of the tides--to the undulations which are

produced by the action of the wind on the surface of the water. While

the tide waves are limited to the open ocean, and to the seas and bays

which afford them free entrance, wind waves are produced everywhere

where water is subjected to the friction of air which flows over it.

While tidal waves come upon the shores but twice each day, the wind

waves of ordinary size which roll in from the ocean deliver their

blows at intervals of from three to ten seconds. Although the tidal

waves sometimes, by a packing-up process, attain the height of fifty

feet, their average altitude where they come in contact with the shore

probably does not much exceed four feet; usually they come in gently.

It is likely that in a general way the ocean surges which beat against

the coast are of greater altitude.

Wind waves are produced and perform their work in a manner which we

shall now describe. When the air blows over any resisting surface, it

tends, in a way which we can hardly afford here to describe, to

produce motions. If the particle is free to move under the impulse

which it communicates, it bears it along; if it is linked together in

the manner of large masses, which the wind can not transport, it tends

to set it in motion in an alternating way. The sounds of our musical

instruments which act by wind are due to these alternating vibrations,

such as all air currents tend to produce. An Æolian harp illustrates

the action which we are considering. Moving over matter which has the

qualities that we denote by the term fluid, the swayings which the air

produces are of a peculiar sort, though they much resemble those of

the fiddle string. The surface of the liquid rises and falls in what

we term waves, the size of which is determined by the measure of

fluidity, and by the energy of the wind. Thus, because fresh water is

considerably lighter than salt, a given wind will produce in a given

distance for the run of the waves heavier surges in a lake than it

will in the sea. For this reason the surges in a great storm which

roll on the ocean shore, because of the wide water over which they

have gathered their impetus, are in size very much greater than those

of the largest lakes, which do not afford room for the development of

great undulations.

To the eye, a wave in the water appears to indicate that the fluid is

borne on before the wind. Examination, however, shows that the amount

of motion in the direction in which the wind is blowing is very

slight. We may say, indeed, that the essential feature of a wave is

found in the transmission of impulse rather than in the movement of

the fluid matter. A strip of carpet when shaken sends through its

length undulations which are almost exactly like water waves. If we

imagine ourselves placed in a particle of water, moving in the

swayings of a wave in the open and deep sea, we may conceive ourselves

carried around in an ellipse, in each revolution returning through

nearly the same orbit. Now and then, when the particle came to the

surface, it would experience the slight drift which the continual

friction of the wind imposes on the water. If the wave in which the

journey was made lay in the trade winds, where the long-continued,

steadfast blowing had set the water in motion to great depths, the

orbit traversed would be moving forward with some rapidity; where also

the wind was strong enough to blow the tops of the waves over, forming

white-caps, the advance of the particle very near the surface would be

speedy. Notwithstanding these corrections, waves are to be regarded

each as a store of energy, urging the water to sway much in the manner

of a carpet strip, and by the swaying conveying the energy in the

direction of the wave movement.

The rate of movement of wind waves increases with their height.

Slight undulations go forward at the rate of less than half a mile an

hour. The greater surges of the deeps when swept by the strongest

winds move with the speed which, though not accurately determined, has

been estimated by the present writer as exceeding forty miles an hour.

As these surges often have a length transverse to the wind of a mile

or more, a width of about an eighth of a mile, and a height of from

thirty-five to forty-five feet, the amount of energy which they

transmit is very great. If it could be effectively applied to the

shores in the manner in which the energy of exploding gunpowder is

applied by cannon shot, it is doubtful whether the lands could have

maintained their position against the assaults of the sea. But there

are reasons stated below why the ocean waves can use only a very small

part of their energy in rending the rocks against which they strike on

the coast line.

In the first place, we should note that wind waves have very little

influence on the bottom of the deep sea. If an observer could stand on

the sea floor at the depth of a mile below a point over which the

greatest waves were rolling, he could not with his unaided senses

discern that the water was troubled. He would, indeed, require

instruments of some delicacy to find out that it moved at all. Making

the same observations at the depth of a thousand feet, it is possible

that he would note a slight swaying motion in the water, enough

sensibly to affect his body. At five hundred feet in depth the

movement would probably be sufficient to disturb fine mud. At two

hundred feet, the rasping of the surge on the bottom would doubtless

be sufficient to push particles of coarse sand to and fro. At one

hundred feet in depth, the passage of the surge would be strong enough

to urge considerable pebbles before it. Thence up the slope the

driving action would become more and more intense until we attained

the point where the wave broke. It should furthermore be noted that,

while the movement of the water on the floor of the deep sea as the

wave passes overhead would be to and fro, with every advance in the

shallowing and consequent increased friction on the bottom, the

forward element in the movement would rapidly increase. Near the coast

line the effect of the waves is continually to shove the detritus up

the slopes of the continental shelf. Here we should note the fact that

on this shelf the waves play a part exactly the opposite of that

effected by the tides. The tides, as we have noted, tend to drag the

particles down the slope, while the waves operate to roll them up the

declivity.

As the wave in advancing toward the shore ordinarily comes into

continually shallowing water, the friction on the bottom is

ever-increasing, and serves to diminish the energy the surge contains,

and therefore to reduce its proportions. If this action operated

alone, the subtraction which the friction makes would cause the surf

waves which roll in over a continental shelf to be very low, probably

in height less than half that which they now attain. In fact, however,

there is an influence at work to increase the height of the waves at

the expense of its width. Noting that the friction rapidly increases

with the shallowing, it is easy to see that this resistance is

greatest on the advancing front of the wave, and least on its seaward

side. The result is that the front moves more slowly than the rear, so

that the wave is forced to gain in height; but for the fact that the

total friction which the wave encounters takes away most of its

impetus, we might have combers a hundred feet high rolling upon the

shelving shores which almost everywhere face the seas.

As the wave shortens its width and gains in relative height, though

not in actual elevation, another action is introduced which has

momentous consequences. The water in the bottom of the wave is greatly

retarded in its ongoing by its friction over the sea floor, while the

upper part of the surge is much less affected in this way. The result

is that at a certain point in the advance, the place of which is

determined by the depth, the size, and the speed of the undulation,

the front swiftly steepens until it is vertical, and the top shoots

forward to a point where it is no longer supported by underlying

water, when it plunges down in what is called the surf or breaker. In

this part of the wave's work the application of the energy which it

transmits differs strikingly from the work previously done. Before the

wave breaks, the only geological task which it accomplishes is

effected by forcing materials up the slope, in which movement they are

slightly ground over each other until they come within the battering

zone of the shore, where they may be further divided by the action of

the mill which is there in operation.

When the wave breaks on the shore it operates in the following manner:

First, the overturning of its crest sends a great mass of water, it

may be from the height of ten or more feet, down upon the shore. Thus

falling water has not only the force due to its drop from the summit

of the wave, but it has a share of the impulse due to the velocity

with which the surge moved against the shore. It acts, in a word, like

a hammer swung down by a strong arm, where the blow represents not

only the force with which the weight would fall of itself, but the

impelling power of the man's muscles. Any one who will expose his body

to this blow of the surf will recognise how violent it is; he may, if

the beach be pebbly, note how it drives the stones about; fragments

the size of a man's head may be hurled by the stroke to the distance

of twenty feet or more; those as large as the fist may be thrown clear

beyond the limits of the wave. So vigorous is this stroke that the

sound of it may sometimes be heard ten miles inland from the coast

where it is delivered.

Moving forward up the slope of a gently inclined beach, the fragments

of the wave are likely to be of sufficient volume to permit them to

regather into a secondary surge, which, like the first, though much

smaller, again rises into a wall, forming another breaker. Under

favourable conditions as many as four or five of these successive

diminishing surf lines may be seen. The present writer has seen in

certain cases as many as a dozen in the great procession, the lowest

and innermost only a few inches high, the outer of all with a height

of perhaps twenty feet.

Along with the direct bearing action of the surf goes a to-and-fro

movement, due to the rushing up and down of the water on the beach.

This swashing affects not only the broken part of the waves, but all

the water between the outer breaker and the shore. These swayings in

the surf belt often swing the _débris_ on the inner margin over a

range of a hundred feet or more, the movement taking place with great

swiftness, affecting the pebbles to the depth of several inches, and

grinding the bits together in a violent way. Listening to the turmoil

of a storm, we can on a pebbly beach distinctly hear the sound of the

downward stroke, a crashing tone, and the roar of the rolling stones.

As waves are among the interesting things in the world, partly on

account of their living quality and partly because of their immediate

and often exceeding interest to man, we may here note one or two

peculiar features in their action. In the first place, as the reader

who has gained a sense of the changes in form of action may readily

perceive, the beating of waves on the shore converts the energy which

they possess into heat. This probably warms the water during great

storms, so that by the hand we may note the difference in temperature

next the coast line and in the open waters. This relative warmth of

the surf water is perhaps a matter of some importance in limiting the

development of ice along the shore line; it may also favour the

protection of the coast life against the severe cold of the winter

season.

The waves which successively come against the shore in any given time,

particularly if a violent wind is blowing on to the coast, are usually

of about the same size. When, however, in times of calm an old sea, as

it is called, is rolling in, the surges may occasionally undergo very

great variations in magnitude. Not infrequently these occasional waves

are great enough to overwhelm persons who are upon the rocks next the

shore. These greater surges are probably to be accounted for by the

fact that in the open sea waves produced by winds blowing in different

directions may run on with their diverse courses and varied intervals

until they come near the shore. Running in together, it very well

happens that two of the surges belonging to different sets may combine

their forces, thus doubling the swell. The danger which these

conjoined waves bring is obviously greatest on cliff shores, where, on

account of the depth of water, the waves do not break until they

strike the steep.

* * * * *

Having considered in a general way the action of waves as they roll in

to the shore, bearing with them the solar energy which was contributed

to them by the winds, we shall now take up in some detail the work

which goes on along the coast line--work which is mainly accomplished

by wave action.

On most coast lines the observer readily notes that the shore is

divided into two different kinds of faces--those where the inner

margin of the wave-swept belt comes against rocky steeps, and those

bordered by a strand altogether composed of materials which the surges

have thrown up. These may be termed for convenience cliff shores and

wall-beach shores. We shall begin our inquiry with cliff shores, for

in those sections of the coast line the sea is doing its most

characteristic and important work of assaulting the land. If the

student has an opportunity to approach a set of cliffs of hard rock in

time of heavy storm, when the waves have somewhere their maximum

height, he should seek some headland which may offer him safe foothold

whence he can behold the movements which are taking place. If he is so

fortunate as to have in view, as well may be the case, cliffs which

extend down into deep water, and others which are bordered by rude

and generally steeply sloping beaches covered with large stones, he

may perceive that the waves come in against the cliffs which plunge

into deep water without taking on the breaker form. In this case the

undulation strikes but a moderate blow; the wave is not greatly

broken. The part next the rock may shoot up as a thin sheet to a

considerable height; it is evident that while the ongoing wave applies

a good deal of pressure to the steep, it does not deliver its energy

in the effective form of a blow as when the wave overturns, or in the

consequent rush of the water up a beach slope. It is easy to perceive

that firm-set rock cliffs, with no beaches at their bases, can almost

indefinitely withstand the assaults. On the steep and stony beach,

because of its relatively great declivity, the breaker or surf forms

far in, and even in its first plunge often attains the base of the

precipice. The blow of the overfalling as well as that of the inrush

moves about stones of great size; those three feet or more in diameter

are often hurled by the action against the base of the steep, striking

blows, the sharp note of which can often be heard above the general

roar which the commotion produces. The needlelike crags forming isles

standing at a distance from the shore, such as are often found along

hard rock coasts, are singularly protected from the action of

effective waves. The surges which strike against them are unarmed with

stones, and the water at their bases is so deep that it does not sway

with the motion with sufficient energy to move them on the bottom.

Where a cliff is in this condition, it may endure until an elevation

of the coast line brings its base near the level of the sea, or until

the process of decay has detached a sufficient quantity of stone to

form a talus or inclined plane reaching near to the water level.

As before noted, it is the presence of a sloping beach reaching to

about the base of the cliff which makes it possible for the waves to

strike at with a hammer instead of with a soft hand. Battering at the

base of the cliff, the surges cut a crease along the strip on which

they strike, which gradually enters so far that the overhanging rock

falls of its own weight. The fragments thus delivered to the sea are

in turn broken up and used as battering instruments until they are

worn to pieces. We may note that in a few months of heavy weather the

stones of such a fall have all been reduced to rudely spherical forms.

Observations made on the eastern face of Cape Ann, Mass., where the

seas are only moderately heavy, show that the storms of a single

winter reduce the fragments thrown into the sea from the granite

quarries to spheroidal shapes, more than half of their weight commonly

being removed in the form of sand and small pebbles which have been

worn from their surfaces.

We can best perceive the effect of battering action which the sea

applies to the cliffs by noting the points where, owing to some chance

features in the structure in the rock, it has proved most effective.

Where a joint or a dike, or perhaps a softer layer, if the rocks be

bedded, causes the wear to go on more rapidly, the waves soon excavate

a recess in which the pebbles are retained, except in stormy weather,

in an unmoved condition. When the surges are heavy, these stones are

kept in continuous motion, receding as the wave goes back, and rushing

forward with its impulse until they strike against the firm-set rock

at the end of the chasm. In this way they may drive in a cut having

the length of a hundred feet or more from the face of the precipice.

In most cases the roofs over these sea caves fall in, so that the

structure is known as a chasm. Occasionally these roofs remain, in

which case, for the reason that the floor of the cutting inclines

upward, an opening is made to the surface at their upper end, forming

what is called in New England a "spouting horn"; from the inland end

of the tunnel the spray may be thrown far into the air. As long as the

cave is closed at this inner end, and is not so high but that it may

be buried beneath a heavy wave, the inrushing water compresses the

air in the rear parts of the opening. When the wave begins to retreat

this air blows out, sending a gust of spray before it, the action

resembling the discharge of a great gun from the face of a

fortification. It often happens that two chasms converging separate a

rock from the cliff. Then a lowering of the coast may bring the mass

to the state of a columnar island, such as abound in the Hebrides and

along various other shores.

If a cliff shore retreats rapidly, it may be driven back into the

shore, and its face assumes the curve of a small bay. With every step

in this change the bottom is sure to become shallower, so that the

waves lose more and more of their energy in friction over the bottom.

Moreover, in entering a bay the friction which the waves encounter in

running along the sides is greater than that which they meet in

coming in upon a headland or a straight shore. The result is, with the

inward retreat of the steep it enters on conditions which diminish the

effectiveness of the wave stroke. The embayment also is apt to hold

detritus, and so forms in time a beach at the foot of the cliff, over

which the waves rarely are able to mount with such energy as will

enable them to strike the wall in an effective manner. With this

sketch of the conditions of a cliff shore, we will now consider the

fate of the broken-tip rock which the waves have produced on that

section of the coast land.

By observation of sea-beaten cliffs the student readily perceives that

a great amount of rocky matter has been removed from most cliff-faced

shores. Not uncommonly it can be shown that such sea faces have

retreated for several miles. The question now arises, What becomes of

the matter which has been broken up by the wave action? In some part

the rock, when pulverized by the pounding to which it is subjected,

has dissolved in the water. Probably ninety per cent of it, however,

retains the visible state, and has a fate determined by the size of

the fragments of which it is composed. If these be as fine as mud, so

that they may float in the water, they are readily borne away by the

currents which are always created along a storm-swept shore,

particularly by the undertow or bottom outcurrent--the "sea-puss," as

it is sometimes called--that sweeps along the bottom from every shore,

against which the waves form a surf. If as coarse as sand grains, or

even very small pebbles, they are likely to be drawn out, rolling over

the bottom to an indefinite distance from the sea margin. The coarser

stones, however, either remain at the foot of the cliff until they are

beaten to pieces, or are driven along the shore until they find some

embayment into which they enter. The journey of such fragments may,

when the wind strikes obliquely to the shore, continue for many miles;

the waves, running with the wind, drive the fragments in oscillating

journeys up and down the beach, sometimes at the rate of a mile or

more a day. The effect of this action can often be seen where a vessel

loaded with brick or coal is wrecked on the coast. In a month

fragments of the materials may be stretched along for the distance of

many miles on either side of the point where the cargo came ashore.

Entering an embayment deep enough to restrain their further journey,

the fragments of rock form a boulder beach, where the bits roll to and

fro whenever they are struck by heavy surges. The greater portion of

them remain in this mill until they are ground to the state of sand

and mud. Now and then one of the fragments is tossed up beyond the

reach of the waves, and is contributed to the wall of the beach. In

very heavy storms these pebbles which are thrown inland may amount in

weight to many tons for each mile of shore.

The study of a pebbly beach, drawn from crest to the deep water

outside, will give an idea as to the history of its work. On either

horn of the crescent by which the pebbles are imported into the pocket

we find the largest fragments. If the shore of the bay be long, the

innermost part of the recess may show even only very small pebbles, or

perhaps only fine sand, the coarser material having been worn out in

the journey. On the bottom of the bay, near low tide, we begin to find

some sand produced by the grinding action. Yet farther out, below

high-tide mark, there is commonly a layer of mud which represents the

finer products of the mill.

Boulder beaches are so quick in answering to every slight change in

the conditions which affect them that they seem almost alive. If by

any chance the supply of detritus is increased, they fill in between

the horns, diminish the incurve of the bay, and so cause its beach to

be more exposed to heavy waves. If, on the other hand, the supply of

grist to the mill is diminished, the beach becomes more deeply

incurved, and the wave action is proportionately reduced. We may say,

in general, that the curve of these beaches represents a balance

between the consumption and supply of the pebbles which they grind up.

The supply of pebbles brought along the shore by the waves is in many

cases greatly added to by a curious action of seaweeds. If the bottom

of the water off the coast is covered by these fragments, as is the

case along many coast lines within the old glaciated districts, the

spores of algæ are prone to take root upon them. Fastening themselves

in those positions, and growing upward, the seaweeds may attain

considerable size. Being provided with floats, the plant exercises a

certain lifting power on the stone, and finally the tugging action of

the waves on the fronds may detach the fragments from the bottom,

making them free to journey toward the shore. Observing from near at

hand the straight wall of the wave in times of heavy storm, the

present writer has seen in one view as many as a dozen of these

plant-borne stones, sometimes six inches in diameter, hanging in the

walls of water as it was about to topple over. As soon as they strike

the wave-beaten part of the shore these stones are apt to become

separated from the plants, though we can often notice the remains or

prints of the attachments adhering to the surface of the rock. Where

the pebbles off the shore are plenty, a rocky beach may be produced

by this process of importation through the agency of seaweeds without

any supply being brought by the waves along the coast line.

Returning to sand beaches, we enter the most interesting field of

contact between seas and lands. Probably nine tenths of all the coast

lines of the open ocean are formed of arenaceous material. In general,

sand consists of finely broken crystals of silica or quartz. These

bits are commonly distinctly faceted; they rarely have a spherical

form. Not only do accumulations of sand border most of the shore line,

but they protect the land against the assaults of the sea, and this in

the following curious manner: When shore waves beat pebbles against

each other, they rapidly wear to bits; we can hear the sound of the

wearing action as the wave goes to and fro. We can often see that the

water is discoloured by the mud or powdered rock. When, however, the

waves tumble on a sandy coast, they make but a muffled sound, and

produce no mud. In fact, the particles of sand do not touch each other

when they receive the blow. Between them there lies a thin film of

water, drawn in by the attraction known as capillarity, which sucks

the fluid into a sponge or between plates of glass placed near

together. The stroke of the waves slightly compresses this capillary

water, but the faces of the grains are kept apart as sheets of glass

may be observed to be restrained from contact when water is between

them. If the reader would convince himself as to the condition of the

sand grains and the water which is between them, he may do so by

pressing his foot on the wet beach which the wave has just left. He

will observe that it whitens and sinks a little under the pressure,

but returns in good part to its original form when the foot is lifted.

In the experiment he has pushed a part of the contained water aside,

but he has not brought the grains together; they do not make the sound

which he will often hear when the sand is dry. The result is that the

sand on the seashore may wear more in going the distance of a mile in

the dry sand dune than in travelling for hundreds along the wet shore.

If the rock matter in the state of sand wore as rapidly under the

heating of the waves as it does in the state of pebbles, the

continents would doubtless be much smaller than they are. Those coasts

which have no other protection than is afforded by a low sand beach

are often better guarded against the inroads of the sea than the

rock-girt parts of the continents. It is on account of this remarkable

endurance of sand of the action of the waves that the stratified rocks

which make up the crust of the earth are so thick and are to such an

extent composed of sand grains.

The tendency of the _débris_-making influences along the coast line is

to fill in the irregularities which normally exist there; to batter

off the headlands, close up the bays and harbours, and generally to

reduce the shores to straight lines. Where the tide has access to

these inlets, it is constantly at work in dragging out the detritus

which the waves make and thrust into the recesses. These two actions

contend with each other, and determine the conditions of the coast

line, whether they afford ports for commerce or are sealed in by sand

bars, as are many coast lines which are not tide-swept, as that of

northern Africa, which faces the Mediterranean, a nearly tideless sea.

The same is the case with the fresh-water lakes; even the greater of

them are often singularly destitute of shelters which can serve the

use of ships, and this because there are no tides to keep the bays and

harbours open.

THE OCEAN CURRENTS.

The system of ocean currents, though it exhibits much complication in

detail, is in the main and primarily dependent on the action of the

constant air streams known as the trade winds. With the breath from

the lips over a basin of water we can readily make an experiment which

shows in a general way the method in which the winds operate in

producing the circulation of the sea. Blowing upon the surface of the

water in the basin, we find that even this slight impulse at once sets

the upper part in motion, the movement being of two kinds--pulsating

movements or waves are produced, and at the same time the friction of

the air on the surface causes its upper part to slide over the under.

With little floats we can shortly note that the stream which forms

passes to the farther side of the vessel, there divides, and returns

to the point of beginning, forming a double circle, or rather two

ellipses, the longer sides of which are parallel with the line of the

air current. Watching more closely, aiding the sight by the particles

which float at various distances below the surface, we note the fact

that the motion which was at first imparted to the surface gradually

extends downward until it affects the water to the depth of some

inches.

In the trade-wind belt the ocean waters to the depth of some hundreds

of feet acquire a continuous movement in the direction in which they

are impelled by those winds. This motion is most rapid at the surface

and near the tropics. It diminishes downwardly in the water, and also

toward the polar sides of the trade-wind districts. Thus the trades

produce in the sea two broad, slow-moving, deep currents, flowing in

the northern hemisphere toward the southwest, and in the southern

hemisphere toward the northwest. Coming down upon each other

obliquely, these broad streams meet about the middle of the tropical

belt. Here, as before noted, the air of the trade winds leaves the

surface and rises upward. The waters being retained on their level,

form a current which moves toward the west. If the earth within the

tropics were covered by a universal sea, the result of this movement

would be the institution of a current which, flowing under the

equator, would girdle the sphere.

With a girdling equatorial current, because of the intense heat of the

tropics and the extreme cold of the parallels beyond the fortieth

degree of latitude, the earth would be essentially uninhabitable to

man, and hardly so to any forms of life. Its surface would be visited

by fierce winds induced by the very great differences of temperature

which would then prevail. Owing, however, to the barriers which the

continents interpose to the motions of these windward-setting tropical

currents, all the water which they bear, when it strikes the opposing

shores, is diverted to the right and left, as was the stream in the

experiment with the basin and the breath, the divided currents seeking

ways toward high latitudes, conveying their store of heat to the

circumpolar lands. So effective is this transfer of temperature that a

very large part of the heat which enters the waters in the tropical

region is taken out of that division of the earth's surface and

distributed over the realms of sea and land which lie beyond the

limits of the vertical sun. Thus the Gulf Stream, the northern branch

of the Atlantic tropical current, by flowing into the North Atlantic,

contributes to the temperature of the region within the Arctic Circle

more heat than actually comes to that district by the direct influx

from the sun.

The above statements as to the climatal effect of the ocean streams

show us how important it is to obtain a sufficient conception as to

the way in which these currents now move and what we can of their

history during the geologic ages. This task can not yet be adequately

done. The fields of the sea are yet too imperfectly explored to afford

us all the facts required to make out the whole story. Only in the

case of our Gulf Stream can we form a full conception as to the

journey which the waters undergo and the consequence of their motion.

In the case of this current, observations clearly show that it arises

from the junction near the equatorial line of the broad stream created

by the two trade-wind belts. Uniting at the equator, these produce a

westerly setting current, having the width of some hundred miles and a

depth of several hundred feet. Its velocity is somewhat greater than a

mile an hour. The centre of the current, because of the greater

strength of the northern as compared with the southern trades, is

considerably south of the equator. When this great slow-moving stream

comes against the coast of South America, it encounters the projecting

shoulder of that land which terminates at Cape St. Roque. There it

divides, as does a current on the bows of an anchored ship, a

part--rather more than one half--of the stream turning to the

northward, the remainder passing toward the southern pole; this

northerly portion becomes what is afterward known as the Gulf Stream,

the history of which we shall now briefly follow.

Flowing by the northwesterly coast of South America, the northern

share of the tropical current, being pressed in against the land by

the trade winds, is narrowed, and therefore acquires at once a swifter

flow, the increased speed being due to conditions like those which add

to the velocity of the water flowing through a hose when it comes to

the constriction of the nozzle. Attaining the line of the southeastern

or Lesser Antilles, often known as the Windward Islands, a part of

this current slips through the interspaces between these isles and

enters the Gulf of Mexico. Another portion, failing to find sufficient

room through these passages, skirts the Antilles on their eastern and

northern sides, passes by and among the Bahama Islands, there to

rejoin the part of the stream which entered the Caribbean. This

Caribbean portion of the tide spreads widely in that broad sea, is

constricted again between Cuba and Yucatan, again expands in the Gulf

of Mexico, and is finally poured forth through the Straits of Florida

as a stream having the width of forty or fifty miles, a depth of a

thousand feet or more, and a speed of from three to five miles an

hour, exceeding in its rate of flow the average of the greatest

rivers, and conveying more water than do all the land streams of the

earth. In this part of its course the deep and swift stream from the

Gulf of Mexico, afterward to be named the Gulf Stream, receives the

contribution of slower moving and shallower currents which skirted the

Antilles on their eastern verge. The conjoined waters then move

northward, veering toward the east, at first as a swift river of the

sea having a width of less than a hundred miles and of great depth;

with each step toward the pole this stream widens, diminishing

proportionately in depth; the speed of its current decreases as the

original impetus is lost, and the baffling winds set its surface

waters to and fro in an irregular way. Where it passes Cape Hatteras

it has already lost a large share of its momentum and much of its

heat, and is greatly widened.

Although the current of the Gulf Stream becomes more languid as we go

northward, it for a very long time retains its distinction from the

waters of the sea through which it flows. Sailing eastward from the

mouth of the Chesapeake, the navigator can often observe the moment

when he enters the waters of this current. This is notable not only in

the temperature, but in the hue of the sea. North of that line the

sharpness of the parting wall becomes less distinct, the stream

spreads out broadly over the surface of the Atlantic, yet its

thermometric effects are distinctly traceable to Iceland and Nova

Zembla, and the tropical driftwood which it carries affords the

principal timber supply of the inhabitants of the first-named isle.

Attaining this circumpolar realm, and finally losing the impulse which

bore it on, the water of the Gulf Stream partly returns to the

southward in a relatively slight current which bears the fluid along

the coast of Europe until it re-enters the system of tropical winds

and the currents which they produce. A larger portion stagnates in the

circumpolar region, in time slowly to return to the tropical district

in a manner afterward to be described. Although the Gulf Stream in the

region north of Cape Hatteras is so indistinct that its presence was

not distinctly recognised until the facts were subjected to the keen

eye of Benjamin Franklin, its effects in the way of climate are so

great that we must attribute the fitness of northern Europe for the

uses of civilized man to its action. But for the heat which this

stream brings to the realm of the North Atlantic, Great Britain would

be as sterile as Labrador, and the Scandinavian region, the

cradle-land of our race, as uninhabitable as the bleakest parts of

Siberia.

It is a noteworthy fact that when the equatorial current divides on

the continents against which it flows, the separate streams, although

they may follow the shores for a certain distance toward the poles,

soon diverge from them, just as the Gulf Stream passes to the seaward

from the eastern coast of the United States. The reason for this

movement is readily found in the same principle which explains the

oblique flow of the trades and counter trades in their passage to and

from the equatorial belt. The particle of water under the equator,

though it flows to the west, has, by virtue of the earth's rotation,

an eastward-setting velocity of a thousand miles an hour. Starting

toward the poles, the particle is ever coming into regions of the sea

where the fluid has a less easterly movement, due to the earth's

rotation on its axis. Consequently the journeying water by its

momentum tends to move off in an easterly course. Attaining high

latitudes and losing its momentum, it abides in the realm long enough

to become cooled.

We have already noted the fact that only a portion of the waters sent

northward in the Gulf Stream and the other currents which flow from

the equator to the poles is returned by the surface flow which sets

toward the equator along the eastern side of the basins. The largest

share of the tide effects its return journey in other ways. Some

portion of this remainder sets equatorward in local cold streams, such

as that which pours forth through Davis Strait into Baffin Bay,

flowing under the Gulf Stream waters for an unknown distance toward

the tropics. There are several of these local as yet little known

streams, which doubtless bring about a certain amount of circulation

between the polar regions and the tropical districts. Their effect is,

however, probably small as compared with that massive drift which we

have now to note.