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 debris-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.

The tropical waters when they attain high latitudes are constantly cooled, and are overlaid by the warmer contributions of that tide, and are thus brought lower and lower in the sea. When they start downward they have, as observations show, a temperature not much above the freezing point of salt water. They do not congeal for the reason that the salt of the ocean lowers the point at which the water solidifies to near 28 deg. Fahr. The effect of this action is gradually to press down the surface cold water until it attains the very bottom in all the circumpolar regions. At the same time this descending water drifts along the bottom of the ocean troughs toward the equatorial realm. As this cold water is heavier than that which is of higher temperature and nearer the surface, it has no tendency to rise. Being below the disturbing influences of any current save its own, it does not tend, except in a very small measure, to mingle with the warmer overlying fluid. The result is that it continues its journey until it may come within the tropics without having gained a temperature of more than 35 deg. Fahr., the increase in heat being due in small measure to that which it receives from the earth's interior and that which it acquires from the overlying warmer water. Attaining the region of the tropical current, this drift water from the poles gradually rises, to take the place of that which goes poleward, becomes warm, and again starts on its surface journey toward the arctic and antarctic regions.

Nothing is known as to the rate of this bottom drift from the polar districts toward the equator, but, from some computation which he has made, the writer is of the opinion that several centuries is doubtless required for the journey from the Arctic Circle to the tropics. The speed of the movement probably varies; it may at times require some thousand years for its accomplishment. The effect of the bottom drift is to withdraw from seas in high latitudes the very cold water which there forms, and to convey it beneath the seas of middle latitudes to a realm where it is well placed for the reheating process. If all the cold water of circumpolar regions had to journey over the surface to the equator, the perturbing effect of its flow on the climates of various lands would be far greater than it is at present. Where such cold currents exist the effect is to chill the air without adding much to the rainfall; while the currents setting northward not only warm the regions near which they flow, but by so doing send from the water surfaces large quantities of moisture which fall as snow or rain. Thus the Gulf Stream, directly and indirectly, probably contributes more than half the rainfall about the Atlantic basin. The lack of this influence on the northern part of North America and Asia causes those lands to be sterilized by cold, although destitute of permanent ice and snow upon their surfaces.

We readily perceive that the effect of the oceanic circulation upon the temperatures of different regions is not only great but widely contrasted. By taking from the equatorial belt a large part of the heat which falls within that realm, it lowers the temperature to the point which makes the district fit for the occupancy of man, perhaps, indeed, tenable to all the higher forms of life. This same heat removed to high latitudes tempers the winter's cold, and thus makes a vast realm inhabitable which otherwise would be locked in almost enduring frosts. Furthermore, this distribution of temperatures tends to reduce the total wind energy by diminishing the trades and counter trades which are due to the variations of heat which are encountered in passing polarward from the equator. Still further, but for this circulation of water in the sea, the oceans about the poles would be frozen to their very bottom, and this vast sheet of ice might be extended southward to within the parallels of fifty degrees north and south latitude, although the waters under the equator might at the same time be unendurably hot and unfit for the occupancy of living beings.

A large part of the difficulties which geologists encounter in endeavouring to account for the changes of the past arise from the evidences of great climatal revolutions which the earth has undergone. In some chapters of the great stone book, whose leaves are the strata of the earth, we find it plainly written in the impressions made by fossils that all the lands beyond the equatorial belt have undergone changes which can only be explained by the supposition that the heat and moisture of the countries have been subjected to sudden and remarkable changes. Thus in relatively recent times thick-leaved plants which retained their vegetation in a rather tender state throughout the year have flourished near to the poles, while shortly afterward an ice sheet, such as now covers the greater part of Greenland, extended down to the line of the Ohio River at Cincinnati. Although these changes of climate are, as we shall hereafter note, probably due to entangled causes, we must look upon the modifications of the ocean streams as one of the most important elements in the causation. We can the more readily imagine such changes to be due to the alterations in the course and volume of the ocean current when we note how trifling peculiarities in the geography of the shores—features which are likely to be altered by the endless changes which occur in the form of a continent—affect the run of these currents. Thus the growth of coral reefs in southern Florida, and, in general, the formation of that peninsula, by narrowing the exit of the great current from the Gulf of Mexico, has probably increased its velocity. If Florida should again sink down, that current would go forth into the North Atlantic with the speed of about a mile an hour, and would not have momentum enough to carry its waters over half the vast region which they now traverse. If the lands about the western border of the Caribbean Sea, particularly the Isthmus of Darien, should be depressed to a considerable depth below the ocean level, the tropical current would enter the Pacific Ocean, adding to the temperature of its waters all the precious heat which now vitalizes the North Atlantic region. Such a geographic accident would not only profoundly alter the life conditions of that part of the world, but it would make an end of European civilization.

In the chapter on climatal changes further attention will be given to the action of ocean currents from the point of view of their influence on the heat and moisture of different parts of the world. We now have to consider the last important influence of ocean currents—that which they directly exercise on the development of organic life. The most striking effect of this nature which the sea streams bring about is caused by the ceaseless transportation to which they subject the eggs and seeds of animals and plants, as well as the bodies of the mature form which are moved about by the flowing waters. But for the existence of these north and south flowing currents, due to the presence of the continental barriers, the living tenants of the seas would be borne along around the earth, always in the same latitude, and therefore exposed to the same conditions of temperature. In this state of affairs the influences which now make for change in organic species would be far less than they are. Journeying in the great whirlpools which the continental barriers make out of the westward setting tropical currents, these organic species are ever being exposed to alterations in their temperature conditions which we know to be favourable to the creation of those variations on which the advance of organic life so intimately depends. Thus the ocean currents not only help to vary the earth by producing changes in the climate of both sea and land, breaking up the uniformity which would otherwise characterize regions at the same distance from the equator, but they induce, by the consequences of the migrations which they enforce, changes in the organic tenants of the sea.

Another immediate effect of ocean streams arises where their currents of warm water come against shores or shallows of the sea. At these points, if the water have a tropical temperature, we invariably find a vast and rapid development of marine animals and plants, of which the coral-making polyps are the most important. In such positions the growth of forms which secrete solid skeletons is so rapid that great walls of their remains accumulate next the shore, the mass being built outwardly by successive growths until the realm of the land may be extended for scores of miles into the deep. In other cases vast mounds of this organic debris may be accumulated in mid ocean until its surface is interspersed with myriads of islands, all of which mark the work due to the combined action of currents and the marine life which they nourish. Probably more than four fifths of all the islands in the tropical belt are due in this way to the life-sustaining action of the currents which the trade winds create.

There are many secondary influences of a less important nature which are due to the ocean streams. The reader will find on most wall-maps of the world certain areas in the central part of the oceans which are noted as Sargassum seas, of which that of the North Atlantic, west and south of the Azore Islands, is one of the most conspicuous. In these tracts, which in extent may almost be compared with the continents, we find great quantities of floating seaweed, the entangled fronds of which often form a mass sufficiently dense to slightly restrain the speed of ships. When the men on the caravels of Columbus entered this tangle, they were alarmed lest they should be unable to escape from its toils. It is a curious fact that these weeds of the sea while floating do not reproduce by spores the structures which answer to the seeds of higher plants, but grow only by budding. It seems certain that they could not maintain their place in the ocean but for the action of the currents which convey the bits rent off from the shores where the plant is truly at home. This vast growth of plant life in the Sargassum basins doubtless contributed considerable and important deposits of sediment to the sea floors beneath the waters which it inhabits. Certain ancient strata, known as the Devonian black shale, occupying the Ohio valley and the neighbouring parts of North America to the east and north of that basin, appear to be accumulations which were made beneath an ancient Sargassum sea.

The ocean currents have greatly favoured and in many instances determined the migrations not only of marine forms, but of land creatures as well. Floating timber may bear the eggs and seeds of many forms of life to great distances until the rafts are cast ashore in a realm where, if the conditions favour, the creatures may find a new seat for their life. Seeds of plants incased in their often dense envelopes may, because they float, be independently carried great distances. So it comes about that no sooner does a coral or other island rise above the waters of the sea than it becomes occupied by a varied array of plants. The migrations of people, even down to the time of the voyages which discovered America, have in large measure been controlled by the run of the ocean streams. The tropical set of the waters to the westward helped Columbus on his way, and enabled him to make a journey which but for their assistance could hardly have been accomplished. This same current in the northern part of the Gulf Stream opposed the passage of ships from northern Europe to the westward, and to this day affects the speed with which their voyages are made.

THE CIRCUIT OF THE RAIN.

We have now to consider those movements of the water which depend upon the fact that at ordinary temperatures the sea yields to the air a continued and large supply of vapour, a contribution which is made in lessened proportion by water in all stages of coldness, and even by ice when it is exposed to dry air. This evaporation of the sea water is proportional to the temperature and to the dryness of the air where it rests upon the ocean. It probably amounts on the average to somewhere about three feet per annum; in regions favourably situated for the process, as on the west coast of northern Africa, it may be three or four times as much, while in the cold and humid air about the poles it may be as little as one foot. When contributed to the air, the water enters on the state of vapour, in which state it tends to diffuse itself freely through the atmosphere by virtue of the motion which is developed in particles when in the vaporous or gaseous state.

The greater part of the water evaporated from the seas probably finds its way as rain at once back into the deep, yet a considerable portion is borne away horizontally until it encounters the land. The precipitation of the water from the air is primarily due to the cooling to which it is subjected as it rises in the atmosphere. Over the sea the ascent is accomplished by the simple diffusion of the vapour or by the uprise through the aerial shaft, such as that near the equator or over the centres of the whirling storms. It is when the air strikes the slopes of the land that we find it brought into a condition which most decidedly tends to precipitate its moisture. Lifted upward, the air as it ascends the slopes is brought into cooler and more rarefied conditions. Losing temperature and expanding, it parts with its water for the same reason that it does in the ascending current in the equatorial belt or in the chimneys of the whirl storms. A general consequence of this is that wherever moisture-laden winds from the sea impinge upon a continent they lay down a considerable part of the water which they contain.

If all the lands were of the same height, the rain would generally come in largest proportion upon their coastal belt, or those portions of the shore-line districts over which the sea winds swept. But as these winds vary in the amount of the watery vapour which they contain, and as the surface of the land is very irregular, the rainfall is the most variable feature in the climatal conditions of our sphere. Near the coasts it ranges from two or three inches in arid regions—such as the western part of the Sahara and portions of the coast regions of Chili and Peru—to eight hundred inches about the head waters of the Brahmapootra River in northern India, where the high mountains are swept over by the moisture-laden airs from the neighbouring sea. Here and there detached mountainous masses produce a singular local increase in the amount of the rainfall. Thus in the lake district in northwestern England the rainfall on the seaward side of mountains, not over four thousand feet high, is very much greater than it is on the other slope, less than a score of miles away. These local variations are common all over the world, though they are but little observed.

In general, the central parts of continents are likely to receive much less rainfall than their peripheral portions. Thus the central districts of North America, Asia, and Australia—three out of the five continental masses—have what we may call interior deserts. Africa has one such, though it is north of the centre, and extends to the shores of the Mediterranean and the Atlantic. The only continent without this central nearly rainless field is South America, where the sole characteristic arid district is situated on the western slope of the Cordilleran range. In this case the peculiarity is due to the fact that the strong westerly setting winds which sweep over the country encounter no high mountains until they strike the Andean chain. They journey up a long and rather gradual slope, where the precipitation is gradually induced, the process being completed when they strike the mountain wall. Passing over its summit, they appear as dry winds on the Pacific coast.

Even while the winds frequently blow in from the sea, as along the western coast of the Americas, they may come over water which is prevailingly colder than the land. This is characteristically the case on the western faces of the American continent, where the sea is cooled by the currents setting toward the equator from high latitudes. Such cool sea air encountering the warm land has its temperature raised, and therefore does not tend to lay down its burden of moisture, but seeks to take up more. On this account the rainfall in countries placed under such conditions is commonly small.

By no means all the moisture which comes upon the earth from the atmosphere descends in the form of rain or snow. A variable, large, though yet undetermined amount falls in the form of dew. Dew is a precipitation of moisture which has not entered the peculiar state which we term fog or cloud, but has remained invisible in the air. It is brought to the earth through the radiation of heat which continually takes place, but which is most effective during the darkened half of the day, when the action is not counterbalanced by the sun's rays. While the sun is high and the air is warm there is a constant absorption of moisture in large part from the ground or from the neighbouring water areas, probably in some part from those suspended stores of water, the clouds, if such there be in the neighbourhood. We can readily notice how clouds drifting in from the sea often melt into the dry air which they encounter. Late in the afternoon, even before the sun has sunk, the radiation of heat from the earth, which has been going on all the while, but has been less considerable than the incurrent of temperature, in a way overtakes that influx. The air next the surface becomes cooled from its contact with the refrigerating earth, and parts with its moisture, forming a coating of water over everything it touches. At the same time the moisture escaping from the warmed under earth likewise drops back upon its cooled surface almost as soon as it has escaped. The thin sheet of water precipitated by this method is quickly returned to the air when it becomes warmed by the morning sunshine, but during the night quantities of it are absorbed by the plants; very often, indeed, with the lowlier vegetation it trickles down the leaves and enters the earth about the base of the stem, so that the roots may appropriate it. Our maize, or Indian corn, affords an excellent example of a plant which, having developed in a land of droughts, is well contrived, through its capacities for gathering dew, to protect itself against arid conditions. In an ordinary dew-making night the leaves of a single stem may gather as much as half a pint of water, which flows down their surfaces to the roots. So efficient is this dew supply, this nocturnal cloudless rain, that on the western coast of South America and elsewhere, where the ordinary supply of moisture is almost wanting, many important plants are able to obtain from it much of the water which they need. The effect is particularly striking along seashores, where the air, although it may not have the humidity necessary for the formation of rain, still contains enough to form dew.

It is interesting to note that the quantity of dew which falls upon an area is generally proportioned to the amount of living vegetation which it bears. The surfaces of leaves are very efficient agents of radiation, and the tangle which they make offers an amount of heat-radiating area many times as great as that afforded by a surface of bared earth. Moreover, the ground itself can not well cool down to the point where it will wring the moisture out of the air, while the thin membranes of the plants readily become so cooled. Thus vegetation by its own structure provides itself with means whereby it may be in a measure independent of the accidental rainfall. We should also note the fact that the dewfall is a concomitant of cloudless skies. The quantity which is precipitated in a cloudy night is very small, and this for the reason that when the heavens are covered the heat from the earth can not readily fly off into space. Under these conditions the temperature of the air rarely descends low enough to favour the precipitation of dew.

Having noted the process by which in the rain circuit the water leaves the sea and the conditions of distribution when it returns to the earth, we may now trace in more detail the steps in this great round. First, we should take note of the fact that the water after it enters the air may come back to the surface of the earth in either of two ways—directly in the manner of dewfall, or in a longer circuit which leads it through the state of clouds. As yet we are not very well informed as to the law of the cloud-making, but certain features in this picturesque and most important process have been tolerably well ascertained.

Rising upward from the sea, the vapour of water commonly remains transparent and invisible until it attains a considerable height above the surface, where the cooling tends to make it assume again the visible state of cloud particles. The formation of these cloud particles is now believed to depend on the fact that the air is full of small dust motes, exceedingly small bits of matter derived from the many actions which tend to bring comminuted solid matter into the air, as, for instance, the combustion of meteoric stones, which are greatly heated by friction in their swift course through the air, the ejections of volcanoes, the smoke of forest and other fires, etc. These tiny bits, floating in the air, because of their solid nature radiate their heat, cool the air which lies against them, and thereby precipitate the water in the manner of dew, exactly as do the leaves and other structures on the surface of the earth. In fact, dew formation is essentially like cloud formation, except that in the one case the water is gathered on fixed bodies, and in the other on floating objects. Each little dust raft with its cargo of condensed water tends, of course, to fall downward toward the earth's surface, and, except for the winds which may blow upward, does so fall, though with exceeding slowness. Its rate of descent may be only a few feet a day. It was falling before it took on the load of water; it will fall a little more rapidly with the added burden, but even in a still air it might be months or years before it would come to the ground. The reason for this slow descent may not at first sight be plain, though a little consideration will make it so.

If we take a shot of small size and a feather of the same weight, we readily note that their rate of falling through the air may vary in the proportion of ten to one or more. It is easy to conceive that this difference is due to the very much less friction which the smaller body encounters in its motion by the particles of air. With this point in mind, the student should observe that the surface presented by solid bodies in relation to their solid contents is the greater the smaller the diameter. A rough, though not very satisfactory, instance of this principle may be had by comparing the surface and interior contents of two boxes, one ten feet square and the other one foot square. The larger has six hundred feet of surface to one thousand cubic feet of interior, or about half a square foot of outer surface to the cubic foot of contents; while the smaller box has six feet of surface for the single cubic foot of interior, or about ten times the proportion of exterior to contents. The result is that the smaller particles encounter more friction in moving toward the earth, until, in the case of finely divided matter, such as the particles of carbon in the smoke from an ordinary fire, the rate of down-falling may be so small as to have little effect in the turbulent conditions of atmospheric motion.



The little drops of water which gather round dust motes, falling but slowly toward the earth, are free to obey the attractions which they exercise upon each other—impulses which are partly gravitative and partly electrical. We have no precise knowledge concerning these movements, further than that they serve to aggregate the myriad little floats into cloud forms, in which the rafts are brought near together, but do not actually touch each other. They are possibly kept apart by electrical repulsion. In this state of association without union the divided water may undergo the curiously modified aggregations which give us the varied forms of clouds. As yet we know little as to the cause of cloud shapes. We remark the fact that in the higher of these agglomerations of condensed vapour, the clouds which float at an elevation of from twenty to thirty thousand feet or more, the masses are generally thin, and arranged more or less in a leaflike form, though even here a tendency to produce spherical clouds is apparent. In this high realm floating water is probably in the frozen state, answering to the form of dew, which we call hoar frost. The lower clouds, gathering in the still air, show very plainly the tendency to agglomerate into spheres, which appears to be characteristic of all vaporous material which is free to move by its own impulses. It is probable that the spherical shape of clouds is more or less due to the same conditions as gathered the stellar matter from the ancient nebular chaos into the celestial spheres. Upon these spherical aggregations of the clouds the winds act in extremely varied ways. The cloud may be rubbed between opposite currents, and so flattened out into a long streamer; it may take the same form by being carried off by a current in the manner of smoke from a fire; the spheres may be kept together, so as to form the patchwork which we call "mackerel" sky; or they may be actually confounded with each other in a vast common cloud-heap. In general, where the process of aggregation of two cloud bodies occurs, changes of temperature are induced in the masses which are mixed together. If the temperature resulting from this association of cloud masses is an average increase, the cloud may become lighter, and in the manner of a balloon move upward. Each of the motes in the cloud with its charge of vapour may be compared with the ballast of the balloon; if they are warmed, they send forth a part of their load of condensed water again to the state of invisible vapour. Rising to a point where it cools, the vapour gathers back on the rafts and tends again to weight the cloud downward. The ballast of an ordinary balloon has to be thrown away from its car; but if some arrangement for condensing the moisture from the air could be contrived, a balloon might be brought into the adjustable state of a cloud, going up or down according as it was heated or cooled.

When the formation of the drop of water or snowflake begins, the mass is very small. If in descending it encounters great thickness of cloud, the bit may grow by further condensation until it becomes relatively large. Generally in this way we may account for the diversities in the size of raindrops or snowflakes. It often happens that the particles after taking on the form of snowflakes encounter in their descent air so warm that they melt into raindrops, or, if only partly melted, reach the surface as sleet. Or, starting as raindrops, they may freeze, and in this simple state may reach the earth, or after freezing they may gather other frozen water about them, so that the hailstone has a complicated structure which, from the point of view of classification, is between a raindrop and a snowflake.

In the process of condensation—indeed, in the steps which precede the formation of rain and snow—there is often more or less trace of electrical action; in fact, a part of the energy which was involved in the vapourization of water, on its condensation, even on the dust motes appears to be converted into electrical action, which probably operates in part to keep the little aggregates of water asunder. When they coalesce in drops or flakes, this electricity often assumes the form of lightning, which represents the swift passage of the electric store from a region where it is most abundant to one where it is less so. The variations in this process of conveying the electricity are probably great. In general, it probably passes, much as an electric current is conveyed, through a wire from the battery which produces the force. In other cases, where the tension is high, or, in other words, where the discharge has to be hastened, we have the phenomena of lightning in which the current burns its way along its path, as it may traverse a slender wire, vapourizing it as it goes. In general, the lightning flash expends its force on the air conductors, or lines of the moist atmosphere along which it breaks its path, its energy returning into the vapour which it forms or the heat which it produces in the other parts of the air. In some cases, probably not one in the thousand of the flashes, the charge is so heavy that it is not used up in its descent toward the earth, and so electrifies, or, as we say, strikes, some object attached to the earth, through which it passes to the underlying moisture, where it finds a convenient place to take on a quiet form. Almost all these hurried movements of electrical energy which intensely heat and light the air which they traverse fly from one part of a cloud to another, or cross from cloud sphere to cloud sphere; of those which start toward the earth, many are exhausted before they reach its surface, and even those that strike convey but a portion of their original impulse to the ground.

The wearing-out effect of lightning in its journey along the air conductors in its flaming passages is well illustrated by what happens when the charge strikes a wire which is not large enough freely to convey it. The wire is heated, generally made white hot, often melted, and perhaps scattered in the form of vapour. In doing this work the electricity may, and often is, utterly dissipated—that is, changed into heat. It has been proposed to take advantage of this principle in protecting buildings from lightning by placing in them many thin wires, along which the current will try to make its way, being exhausted in melting or vaporizing the metal through which it passes.

There are certain other forms of lightning, or at least of electrical discharges, which produce light and which may best be described in this connection. It occasionally happens that the earth becomes so charged that the current proceeds from its surface to the clouds. More rarely, and under conditions which we do not understand, the electric energy is gathered into a ball-like form, which may move slowly along the surface until it suddenly explodes. It is a common feature of all these forms of lightning which we have noted that they ordinarily make in their movement considerable noise. This is due to the sudden displacement of the air which they traverse—displacement due to the action of heat in separating the particles. It is in all essential regards similar to the sounds made by projectiles, such as meteors or swift cannon shots, as they fly through the air. It is even more comparable to the sound produced by exploding gunpowder. The first sound effect from the lightning stroke is a single rending note, which endures no longer—indeed, not as long—as the explosion of a cannon. Heard near by, this note is very sharp, reminding one of the sound made by the breaking of glass. The rolling, continuous sound which we commonly hear in thunder is, as in the case of the noise produced by cannon, due to echo from the clouds and the earth. Thunder is ordinarily much more prolonged and impressive in a mountainous country than in a region of plains, because the steeps about the hearer reverberate the original single crash.

The distribution of thunderstorms is as yet not well understood, but it appears in many cases that they are attendants on the advancing face of cyclones and hurricanes, the area in front of these great whirlstorms being subjected to the condensation and irregular air movements which lead to the development of much electrical energy. There are, however, certain parts of the earth which are particularly subjected to lightning flashes. They are common in the region near the equator, where the ascending currents bring about heavy rains, which mean a rapid condensation and consequent liberation of electrical energy. They diminish in frequency toward the arctic regions. An observer at the pole would probably fail ever to perceive strong flashes. For the same reason thunderstorms are more frequent in summer, the time when the difference in temperature between the surface and the upper air is greatest, when, therefore, the uprushes of air are likely to be most violent. They appear to be more common in the night than in the daytime, for the reason that condensation is favoured by the cooling which occurs in the dark half of the day. It is rare, indeed, that a thunderstorm occurs near midday, a period when the air is in most cases taking up moisture on account of the swiftly increasing heat.

There are other forms of electrical discharges not distinctly connected with the then existing condensation of moisture. What the sailors call St. Elmo's fire—a brush of electric light from the mast tops and other projections of the ship—indicates the passage of electrical energy between the vessel and the atmosphere. Similar lights are said sometimes to be seen rising from the surface of the water. Such phenomena are at present not satisfactorily explained. Perhaps in the same group of actions comes the so-called "Jack-o'-lantern" or "Will-o'-the-wisp" fires flashing from the earth in marshy places, which are often described by the common people, but have never been observed by a naturalist. If this class of illuminations really exists, we have to afford them some other explanation than that they are emanations of self-inflamed phosphoretted hydrogen, a method of accounting for them which illogically finds a place in many treatises on atmospheric phenomena. A gas of any kind would disperse itself in the air; it could not dance about as these lights are said to do, and there is no chemical means known whereby it could be produced in sufficient purity and quantity from the earth to produce the effects which are described.[3]

[Footnote 3: The present writer has made an extended and careful study of marsh and swamp phenomena, and is very familiar with the aspect of these fields in the nighttime. He has never been able to see any sign of the Jack-o'-lantern light. Looking fixedly into any darkness, such as is afforded by the depths of a wood, the eye is apt to imagine the appearance of faint lights. Those who have had to do with outpost duty in an army know how the anxious sentry, particularly if he is new to the soldier's trade, will often imagine that he sees lights before him. Sometimes the pickets will be so convinced of the fact that they see lights that they will fire upon the fiction of the imaginations. These facts make it seem probable that the Jack-o'-lantern and his companion, the Will-o'-the-wisp, are stories of the overcredulous.]

In the upper air, or perhaps even beyond the limits of the field which deserves the name, in the regions extending from the poles to near the tropics, there occur electric glowings commonly known as the aurora borealis. This phenomenon occurs in both hemispheres. These illuminations, though in some way akin to those of lightning, and though doubtless due to some form of electrical action, are peculiar in that they are often attended by glows as if from clouds, and by pulsations which indicate movements not at electric speed. As yet but little is known as to the precise nature of these curious storms. It has been claimed, however, that they are related to the sun spots; those periods when the solar spots are plenty, at intervals of about eleven years, are the times of auroral discharges. Still further, it seems probable that the magnetic currents of the earth, that circling energy which encompasses the sphere, moving round in a general way parallel to the equator, are intensified during these illuminations of the circumpolar skies.

GEOLOGICAL WORK OF WATER.

We turn now to the geological work which is performed by falling water. Where the rain or snow returns from the clouds to the sea, the energy of position given to the water by its elevation above the earth through the heat which it acquired from the sun is returned to the air through which it falls or to the ocean surface on which it strikes. In this case the circuit of the rain is short and without geological consequence which it is worth while to consider, except to note that the heat thus returned is likely to be delivered in another realm than that in which the falling water acquired the store, thus in a small way modifying the climate. When, however, the precipitation occurs on the surface of the land, the drops of frozen or fluid water apply a part of their energy in important geological work, the like of which is not done where they return at once to the sea.



We shall first consider what takes place when the water in the form of drops of rain comes to the surface of the land. Descending as they do with a considerable speed, these raindrops apply a certain amount of energy to the surface on which they fall. Although the beat of a raindrop is proverbially light, the stroke is not ineffective. Observing what happens where the action takes place on the surface of bare rock, we may notice that the grains of sand or small pebbles which generally abound on such surfaces, if they be not too steeply inclined, dance about under the blows which they receive. If we could cover hard plate glass, a much firmer material than ordinary stone, with such bits, we should soon find that its surface would become scratched all over by the friction. Moreover, the raindrops perceptibly urge the small detached bits of stone down the slopes toward the streams.

If all the earth's surface were bare rocks, the blow of the raindrops would deserve to be reckoned among the important influences which lead to the wearing of land. As it is, when a country is in a state of Nature, only a small part of its surface is exposed to this kind of wearing. Where there is rain enough to effect any damage, there is sure to be sufficient vegetation to interpose a living and self-renewed covering between the rocks and the rain. Even the lichens which coat what at first sight often seems to be bare rock afford an ample covering for this purpose. It is only where man bares the field by stripping away and overturning this protecting vegetation that the raindrops cut away the earth. The effect of their action can often be noted by observing how on ploughed ground a flat stone or a potsherd comes after a rain to cap a little column. The geologist sometimes finds in soft sandstones that the same action is repeated in a larger way where a thin fragment of hard rock has protected a column many feet in height against the rain work which has shorn down the surrounding rock.

When water strikes the moistened surface it at once loses the droplike form which all fluids assume when they fall through the air.[4]

[Footnote 4: This principle of the spheroidal form in falling fluids is used in making ordinary bird shot. The melted lead drops through sievelike openings, the resulting spheres of the metal being allowed to fall into water which chills them. Iron shot, used in cutting stone, where they are placed between the saw and the surface of the rock, are also made in the same manner. The descending fluid divides into drops because it is drawn out by the ever-increasing speed of the falling particles, which soon make the stream so thin that it can not hold together.]

When the raindrops coalesce on the surface of the earth, the role of what we may call land water begins. Thenceforward until the fluid arrives at the surface of the sea it is continually at work in effecting a great range of geological changes, only a few of which can well be traced by the general student. The work of land water is due to three classes of properties—to the energy with which it is endowed by virtue of its height above the sea, a power due to the heat of the sun; to the capacity it has for taking substances into solution; and to its property of giving some part of its own substance to other materials with which it comes in contact. The first of these groups of properties may be called dynamical; the others, chemical.

The dynamic value of water when it falls upon the land is the amount of energy it can apply in going down the slope which separates it from the sea. A ton of the fluid, such as may gather in an ordinary rain on a thousand square feet of ground in the highlands of a country—say at an elevation of a thousand feet above the sea—expends before it comes to rest in the great reservoir as much energy as would be required to lift that weight from the ocean's surface to the same height. The ways in which this energy may be expended we shall now proceed in a general way to trace.

As soon as the water has been gathered, from its drop to its sheet state—a process which takes place as soon as it falls—the fluid begins its downward journey. On this way it is at once parted into two distinct divisions, the surface water and the ground water: the former courses more or less swiftly, generally at the rate of a mile or more an hour, in the light of day; the latter enters the interstices of the earth, slowly descends therein to a greater or less depth, and finally, journeying perhaps at the rate of a mile a year, rejoins the surface water, escaping through the springs. The proportion of these two classes, the surface and the ground water, varies greatly, and an intermixture of them is continually going on. Thus on the surface of bare rock or frozen earth all the rain may go away without entering the ground. On very sandy fields the heaviest rainfall may be taken up by the porous earth, so that no streams are found. On such surfaces the present writer has observed that a rainfall amounting to six inches in depth in two hours produced no streams whatever. We shall first follow the history of the surface water, afterward considering the work which the underground movements effect.

If the student will observe what takes place on a level ploughed field—which, after all, will not be perfectly level, for all fields are more or less undulating—he will note that, though the surface may have been smoothed by a roller until it appears like a floor, the first rain, where the fall takes place rapidly enough to produce surface streams, will create a series of little channels which grow larger as they conjoin, the whole appearing to the eye like a very detailed map, or rather model, of a river system; it is, indeed, such a system in miniature. If he will watch the process by which these streamlet beds are carved, he will obtain a tolerably clear idea as to that most important work which the greater streams do in carving the face of the lands. The water is no sooner gathered into a sheet than, guided by the slightest irregularities which it encounters, it begins to flow. At first the motion is so slow that it does not disturb its bed, but at some points in the bottom of the sheet the movement soon becomes swift enough to drag the grains of sand and clay from their adhesions, bearing them onward. As soon as this beginning of a channel is formed the water moves more swiftly in the clearer way; it therefore cuts more rapidly, deepening and enlarging its channel, and making its motion yet more free. The tiny rills join the greater, all their channels sway to and fro as directed this way and that by chance irregularities, until something like river basins are carved out, those gentle slopes which form broad valleys where the carving has been due to the wanderings of many streams. If the field be large, considerable though temporary brooks may be created, which cut channels perhaps a foot in depth. At the end of this miniature stream system we always find some part of the waste which has been carved out. If the streamlet discharges into a pool, we find the tiny representative of deltas, which form such an important feature on the coast line where large rivers enter seas or lakes. Along the lines of the stream we may observe here and there little benches, which are the equivalent in all save size of the terraces that are generally to be observed along the greater streams. In fact, these accidents of an acre help in a most effective way the student to understand the greater and more complicated processes of continental erosion.

A normal river—in fact, all the greater streams of the earth—originates in high country, generally in a region of mountains. Here, because of the elevation of the region, the streams have cut deep gorges or extensive valleys, all of which have slopes leading steeply downward to torrent beds. Down these inclined surfaces the particles worn off from the hard rock by frost and by chemical decay gradually work their way until they attain the bed of the stream. The agents which assist gravitation in bearing this detritus downward are many, but they all work together for the same end. The stroke of the raindrop accomplishes something, though but little; the direct washing action of the brooklets which form during times of heavy rain, but dry out at the close of the storm, do a good deal of the work; thawing and freezing of the water contained in the mass of detritus help the movement, for, although the thrust is in both directions, it is most effective downhill; the wedges of tree roots, which often penetrate between and under the stones, and there expand in their process of growth, likewise assist the downward motion. The result is that on ordinary mountain slopes the layer of fragments constituting the rude soil is often creeping at the rate of from some inches to some feet a year toward the torrent bed. If there be cliffs at the top of the slope, as is often the case, very extensive falls of rock may take place from it, the masses descending with such speed that they directly attain the stream. If the steeps be low and the rock divided into vertical joints, especially where there is a soft layer at the base of the steep, detached masses from the precipice may move slowly and steadfastly down the slope, so little disturbed in their journey that trees growing upon their summits may continue to develop for the thousands of years before the mass enters the stream bed.

Although the fall of rocks from precipices does not often take place in a conspicuously large way, all great mountain regions which have long been inhabited by man abound in traditions and histories of such accidents. Within a century or two there have been a dozen or more catastrophes of this nature in the inhabited valleys of the Alps. As these accidents are at once instructive and picturesque, it is well to note certain of them in some detail. At Yvorgne, a little parish on the north shore of the Rhone, just above the lake of Geneva, tradition tells that an ancient village of the name was overwhelmed by the fall of a great cliff. The vast debris forming the steep slope which was thus produced now bears famous vineyards, but the vintners fancy that they from time to time hear deep in the earth the ringing of the bells which belonged to the overwhelmed church. In 1806 the district of Goldau, just north of Lake Lucerne, was buried beneath the ruins of a peak which, resting upon a layer of clay, slipped away like a launching ship on the surface of the soft material. The debris overwhelmed a village and many detached houses, and partly filled a considerable lake. The wind produced by this vast rush of falling rock was so great that people were blown away by it; some, indeed, were killed in this singular manner.

The most interesting field of these Swiss mountain falls is a high mountain valley of amphitheatrical form, known as the Diablerets, or the devil's own district. This great circus, which lies at the height of about four thousand feet above the sea, is walled around on its northern side by a precipice, above which rest, or rather once rested, a number of mountain peaks of great bulk. The region has long been valued for the excellent pasturage which the head of the valley affords. Two costly roads, indeed, have been built into it to afford footpaths for the flocks and herds and their keepers in the summer season. Through this human experience with the valley, we have a record of what has gone on in this part of the mountain wilderness. Within the period of history and tradition, three very great mountain falls have occurred in this field, each having made its memory good by widespread disaster which it brought to the people of the chalets. The last of these was brought about by the fall of a great peak which spread itself out in a vast field of ruins in the valley below. The belt of destruction was about half a mile wide and three miles long. When the present writer last saw it, a quarter of a century ago, it was still a wilderness of great rocks, but here and there the process of their decay was giving a foothold for herbage, and in a few centuries the field will doubtless be so verdure-clad that its story will not be told on its face. It is likely, however, to be preserved in the memory of the people, and this through a singular and pathetic tradition which has grown up about the place, one which, if not true, comes at least among the legends which we should like to believe.

As told the present writer by a native of the district, it happened when, in the nighttime the mountain came down, the herdsmen and their cows gathered in the chalets—stout buildings which are prepared to resist avalanches of snow. In one of these, which was protected from crushing by the position of the stones which covered it, a solitary herdsman found himself alive in his unharmed dwelling. With him in the darkness were the cows, a store of food and water, and his provisions for the long summer season. With nothing but hope to animate him, he set to work burrowing upward among the rocks, storing the debris in the room of the chalet. He toiled for some months, but finally emerged to the light of day, blanched by his long imprisonment in the darkness, but with the strength to bear him to his home. In place of the expected warm welcome, the unhappy man found himself received as a ghost. He was exorcised by the priest and driven away to the distance. It was only when long afterward his path of escape was discovered that his history became known.

Returning to the account of the debris which descends at varied speed into the torrents, we find that when the detritus encounters the action of these vigorous streams it is rapidly ground to pieces while it is pushed down the steep channels to the lower country. Where the stones are of such size that the stream can urge them on, they move rapidly; at least in times when the torrent is raging. They beat over each other and against the firm-set rocks; the more they wear, the smaller they become, and the more readily they are urged forward. Where the masses are too large to be stirred by the violent current, they lie unmoved until the pounding of the rolling stones reduces them to the proportions where they may join the great procession. Ordinarily those who visit mountains behold their torrents only in their shrunken state, when the waters stir no stones, and fail even to bear a charge of mud, all detachable materials having been swept away when the streams course with more vigour. In storm seasons the conditions are quite otherwise; then the swollen torrents, their waters filled with clay and sand, bear with them great quantities of boulders, the collisions of which are audible above the muffled roar of the waters, attesting the very great energy of the action.

When the waste on a mountain slope lies at a steep angle, particularly where the accumulation is due to the action of ancient glaciers, it not infrequently happens that when the ground is softened with frost great masses of the material rush down the slope in the manner of landslides. The observer readily notes that in many mountain regions, as, for instance, in the White Mountains of New Hampshire, the steep slopes are often seamed by the paths of these great landslides. Their movement, indeed, is often begun by sliding snow, which gives an impulse to the rocks and earth which it encounters in its descent. At a place known as the Wylie Notch, in the White Mountains, in the early part of this century, a family of that name was buried beneath a mass of glacial waste which had hung on the mountain slope from the ancient days until a heavy rain, following on a period of thaw, impelled the mass down the slope. Although there have been few such catastrophes noted in this country, it is because our mountains have not been much dwelt in. As they become thickly inhabited as the Alps are, men are sure to suffer from these accidents.

As the volume of a mountain torrent increases through the junction of many tributaries, the energy of its moving waters becomes sufficient to sweep away the fragments which come to its bed. Before this stage is attained the stream rarely touches the solid under rock of the mountain, the base of the current resting upon the larger loose stones which it was unable to stir. In this pebble-paved section, because the stream could not attack the foundation rock, we find no gorges—in fact, the whole of this upper section of the torrent system is peculiarly conditioned by the fact that the streams are dealing not with bed-rock, but with boulders or smaller loose fragments. If they cut a little channel, the materials from either side slip the faster, and soon repave the bed. But when the streams have by a junction gained strength, and can keep their beds clear, they soon carve down a gorge through which they descend from the upper mountain realm to the larger valleys, where their conjoined waters take on a riverlike aspect. It should be noted here that the cutting power of the water moving in the torrent or in the wave, the capacity it has for abrading rock, resides altogether in the bits of stone or cutting tools with which it is armed. Pure water, because of its fluidity, may move over or against firm-set stones for ages without wearing them; but in proportion as it moves rocky particles of any size, the larger they are, the more effective the work, it wears the rock over which it flows. A capital instance of this may be found where a stream from a hose is used in washing windows. If the water be pure, there is no effect upon the glass; but if it be turbid, containing bits of sand, in a little while the surface will appear cloudy from the multitude of line scratches which the hard bits impelled by the water have inflicted upon it. A somewhat similar case occurs where the wind bears sand against window panes or a bottle which has long lain on the shore. The glass will soon be deeply carved by the action, assuming the appearance which we term "ground." This principle is made use of in the arts. Glass vessels or sheets are prepared for carving by pasting paper cut into figures on their surfaces. The material is then exposed to a jet of air or steam-impelling sand grains; in a short time all the surface which has not been protected by paper has its polish destroyed and is no longer translucent.

The passage from the torrent to the river, though not in a geographical way distinct, is indicated to the observant eye by a simple feature—namely, the appearance of alluvial terraces, those more or less level heaps of water-borne debris which accumulate along the banks of rivers, which, indeed, constitute the difference between those streams and torrents. Where the mountain waters move swiftly, they manage to bear onward the waste which they receive. Even where the blocks of stone cling in the bed, it is only a short time before they are again set in motion or ground to pieces. If by chance the detritus accumulates rapidly, the slope is steepened and the work of the torrent made more efficient. As the torrent comes toward the base of the mountains, where it neither finds nor can create steep slopes over which to flow, its speed necessarily diminishes. With each reduction in this feature its carrying power very rapidly diminishes. Thus water flowing at the rate of ten miles an hour can urge stones four times the mass that it can move when its speed is reduced to half that rate. The result is that on the lowlands, with their relatively gentle slopes, the combined torrents, despite the increase in the volume of the stream arising from their confluence, have to lay down a large part of their load of detritus.

If we watch where a torrent enters a mountain river, we observe that the main stream in a way sorts over the waste contributed to it, bearing on only those portions which its rate of flow will permit it to carry, leaving the remainder to be built into the bank in the form of a rude terrace. This accumulation may not extend far below the point where the torrent which imported the debris joins the main stream; a little farther down, however, we are sure to find another such junction and a second accumulation of terrace material. As these contributions increase, the terrace accumulations soon become continuous, lying on one side or the other of the river, sometimes bordering both banks of the stream. In general, it can be said that so long as the rate of fall of the torrent exceeds one hundred feet to the mile it does not usually exhibit these shelves of detritus. Below that rate of descent they are apt to be formed. Much, however, depends upon the amount of detritus which the stream bears and the coarseness of it; moreover, where the water goes through a gorge in the manner of a flume with steep rocky sides, it can urge a larger amount before it than when it traverses a wide valley, through which it passes, it may be, in a winding way.

At first sight it may seem rather a fine distinction to separate torrents from rivers by the presence or absence of terraces. As we follow down the stream, however, and study its action in relation to these terraces, and the peculiar history of the detritus of which they are composed, we perceive that these latter accumulations are very important features. Beginning at first with small and imperfect alluvial plains, the river, as it descends toward the sea, gaining in store of water and in the amount of debris which comes with that water from the hills, while the rate of fall and consequent speed of the current are diminished, soon comes to a stage where it is engaged in an endless struggle with the terrace materials. In times of flood, the walls of the terraces compel the tide to flow over the tops of these accumulations. Owing to the relative thinness of the water beyond the bed, and to the growth of vegetation there, the current moves more slowly, and therefore lays down a considerable deposit of the silt and sand which it contains. This may result during a single flood in lifting the level of the terrace by some inches in height, still further serving to restrict the channel. Along the banks of the Mississippi and other large rivers the most of this detritus falls near the stream; a little of it penetrates to the farther side of the plains, which often have a width of ten miles or more. The result is that a broad elevation is constructed, a sort of natural mole or levee, in a measure damming the flood waters, which can now only enter the "back swamps" through the channels of the tributary streams. Each of these back swamps normally discharges into the main stream through a little river of its own, along the banks of which the natural levees do not develop.

We have now to note a curious swinging movement of rivers which was first well observed by the skilful engineers of British India. This movement can best be illustrated by its effects. If on any river which winds through alluvial plains a jetty is so constructed as to deflect the stream at any point, the course which it follows will be altered during its subsequent flow, it may be, for the distance of hundreds of miles. It will be perceived that in its movements a river normally strikes first against one shore and then against the other. Its water in a general way moves as does a billiard ball when it flies from one cushion to another. It is true that in a torrent we have the same conditions of motion; but there the banks are either of hard rock or, if of detritus, they are continually moving into the stream in the manner before described. In the case of the river, however, its points of collision are often on soft banks, which are readily undermined by the washing action of the stream. In the ordinary course of events, the river beginning, we may imagine, with a straight channel, had its current deflected by some obstacle, it may be even by the slight pressure of a tributary stream, is driven against one bank; thence it rebounds and strikes the other. At each point of impinge it cuts the alluvium away. It can bear on only a small portion of that which it thus obtains; the greater part of the material is deposited on the opposite side of the stream, but a little lower down, where it makes a shallow. On these shallows water-loving plants and even certain trees, such as the willows and poplars, find a foothold. When the stream rises, the sediment settles in this tangle, and soon extends the alluvial plain from the neighbouring bank, or in rarer cases the river comes to flow on either side of an island of its own construction. The natural result of this billiard-ball movement of the waters is that the path of the stream is sinuous. The less its rate of fall and the greater the amount of silt it obtains from its tributaries, the more winding its course becomes. This gain in those parts of the river's curvings where deposition tends to take place may be accelerated by tree-planting. Thus a skilful owner of a tract of land on the south bank of the Ohio River, by assiduously planting willow trees on the front of his property, gained in the course of thirty years more than an acre in the width of his arable land. When told by the present writer that he was robbing his neighbours on the other side of the stream, he claimed that their ignorance of the laws of river motion was sufficient evidence that they did not deserve to own land.

In the primitive state of a country the water-loving plants, particularly the trees which flourish in excessively humid conditions, generally make a certain defence against these incursions of the streams. But when a river has gained an opening in the bank it can, during a flood, extend its width often to the distance of hundreds of feet. During the inundations of the Mississippi the river may at times be seen to eat away acres of land in a single day along one of the outcurves of its banks. The undermined forests falling into the flood join the great procession of drift timber, composed of trees which have been similarly uprooted, which occupies the middle part of the stream. This driftwood belt often has a width of three or four hundred feet, the entangled stems and branches making it difficult for a boat to pass from one side of the river to the other.



When the curves of a river have been developed to a certain point (see Fig. 11), when they have attained what is called the "oxbow" form, it often happens that the stream breaks through the isthmus which connects one of the peninsulas with the mainland. Where, as is not infrequently the case, the bend has a length of ten miles or more, the water just above and below the new-made opening is apt to differ in height by some feet. Plunging down the declivity, the stream, flowing with great velocity, soon enlarges the channel so that its whole tide may take the easier way. When this result is accomplished, the old curve is deserted, sand bars are formed across their mouths, which may gradually grow to broad alluvial plains, so that the long-surviving, crescent-shaped lake, the remnant of the river bed, may be seen far from the present course of the ever-changing stream. Gradually the accumulations of vegetable matter and the silt brought in by floods efface this moat or oxbow cut-off, as it is so commonly termed.

As soon as the river breaks through the neck of a peninsula in the manner above described, the current of the stream becomes much swifter for many miles below and above the opening. Slowly, however, the slopes are rearranged throughout its whole course, yet for a time the stream near the seat of the change becomes straighter than before, and this for the reason that its swifter current is better able to dispose of the debris which is supplied to it. The effect of a change in the current produced by such new channels as we have described as forming across the isthmuses of bends is to perturb the course of the stream in all its subsequent downward length. Thus an oxbow cut-off formed near the junction of the Ohio and Mississippi may tend more or less to alter the swings of the Mississippi all the way to the Gulf of Mexico.

Although the swayings of the streams to and fro in their alluvial plains will give the reader some idea as to the struggle which the greater rivers have with the debris which is committed to them, the full measure of the work and its consequences can only be appreciated by those who have studied the phenomena on the ground. A river such as the Mississippi is endlessly endeavouring to bear its burden to the sea. If its slope were a uniform inclined plane, the task might readily be accomplished; but in this, as in almost all other large water ways, the slope of the bed is ever diminishing with its onward course. The same water which in the mountain torrent of the Appalachians or Cordilleras rolled along stones several feet in diameter down slopes of a hundred feet or more to the mile can in the lower reaches of the stream move no pebbles which are more than one fourth of an inch in diameter over slopes which descend on the average about half a foot in a mile. Thus at every stage from the torrent to the sea the detritus has from time to time to rest within the alluvial banks, there awaiting the decay which slowly comes, and which may bring it to the state where it may be dissolved in the water, or divided into fragments so small that the stream may bear them on. A computation which the present writer has made shows that, on the average, it requires about forty thousand years for a particle of stone to make its way down the Mississippi to the sea after it has been detached from its original bed. Of course, some bits may make the journey straightforwardly; others may require a far greater time to accomplish the course which the water itself makes at most in a few weeks. This long delay in the journey of the detritus—a delay caused by its frequent rests in the alluvial plain—brings about important consequences which we will now consider.

As an alluvial plain is constructed, we generally find at the base pebbly material which fell to the bottom in the current of the main stream as the shores grew outward. Above this level we find the deposits laid down by the flood waters containing no pebbles, and this for the reason that those weightier bits remained in the stream bed when the tide flowed over the plain. As the alluvial deposit is laid down, a good deal of vegetable matter was built into it. Generally this has decayed and disappeared. On the surface of the plain there has always been growing abundant vegetation, the remains of which decayed on the surface in the manner which we may observe at the present day. This decomposing vegetable matter within and upon the porous alluvial material produces large quantities of carbonic acid, a gas which readily enters the rain water, and gives it a peculiar power of breaking up rock matter. Acting on the debris, this gas-charged water rapidly brings about a decay of the fragments. Much of the material passes at once into solution in this water, and drains away through the multitudinous springs which border the river. As this matter is completely dissolved, as is sugar in water, it goes straight away to the sea without ever again entering the alluvium. In many, if not most, cases this dissolving work which is going on in alluvial terraces is sufficient to render a large part of the materials which they contain into the state where it disappears in an unseen manner; thus while the annual floods are constantly laying down accumulations on the surface of these plains, the springs are bearing it away from below.

In this way, through the decomposition which takes place in them, all those river terraces where much vegetable matter is mingled with the mineral substances, become laboratories in which substances are brought into solution and committed to the seas. We find in the water of the ocean a great array of dissolved mineral substances; it, indeed, seems probable that the sea water contains some share, though usually small, of all the materials which rivers encounter in their journey over and under the lands. As the waters of the sea obtain but little of this dissolved matter along the coast, it seems likely that the greater share of it is brought into the state of solution in the natural laboratories of the alluvial plains.

Here and there along the sides of the valleys in which the rivers flow we commonly find the remains of ancient plains lying at more or less considerable heights above the level of the streams. Generally these deposits, which from their form are called terraces, represent the stages of down-wearing by which the stream has carved out its way through the rocks. The greater part of these ancient alluvial plains has been removed through the ceaseless swinging of the stream to and fro in the valley which it has excavated.

In all the states of alluvial plains, whether they be the fertile deposits near the level of the streams which built them, or the poorer and ruder surfaced higher terraces, they have a great value to mankind. Men early learned that these lands were of singularly uniform goodness for agricultural use. They are so light that they were easily delved with the ancient pointed sticks or stone hoes, or turned by the olden, wooden plough. They not only give a rich return when first subjugated, but, owing to the depth of the soil and the frequency with which they are visited by fertilizing inundations, they yield rich harvests without fertilizing for thousands of years. It is therefore not surprising that we find the peoples who depended upon tillage for subsistence first developed on the great river plains. There, indeed, were laid the foundations of our higher civilization; there alone could the state which demands of its citizens fixed abodes and continuous labour take rise. In the conditions which these fields of abundance afforded, dense populations were possible, and all the arts which lead toward culture were greatly favoured. Thus it is that the civilization of China, India, Persia, and Egypt, the beginnings of man's higher development, began near the mouths of the great river valleys. These fields were, moreover, most favourably placed for the institution of commerce, in that the arts of navigation, originating in the sheltered reaches of the streams, readily found its way through the estuaries to the open sea.

Passing down the reaches of a great river as it approaches the sea, we find that the alluvial plains usually widen and become lower. At length we attain a point where the flood waters cover the surface for so large a part of the year that the ground is swampy and untillable unless it is artificially and at great expense of labour won to agriculture in the manner in which this task has been effected in the lower portion of the Rhine Valley. Still farther toward the sea, the plain gradually dips downward until it passes below the level of the waters. Through this mud-flat section the stream continues to cut channels, but with the ever-progressive slowing of its motion the burden of fine mud which it carries drops to the bottom, and constantly closes the paths through which the water escapes. Every few years they tend to break a new way on one side or the other of their former path. Some of the greatest engineering work done in modern times has been accomplished by the engineers engaged in controlling the exits of large rivers to the sea. The outbreak of the Yellow River in 1887, in which the stream, hindered by its own accumulations, forced a new path across its alluvial plains, destroyed a vast deal of life and property, and made the new exit seventy miles from the path which it abandoned.

Below the surface of the open water the alluvial deposits spread out into a broad fan, which slopes gradually to a point where, in the manner of the continental shelf, the bottom descends steeply into deep water.

It is the custom of naturalists to divide the lower section of river deposits—that part of the accumulation which is near the sea—from the other alluvial plains, terming the lower portion the delta. The word originally came into use to describe that part of the alluvium accumulated by the Nile near its mouth, which forms a fertile territory shaped somewhat like the fourth letter of the Greek alphabet. Although the definition is good in the Egyptian instance, and has a certain use elsewhere, we best regard all the detritus in a river valley which is in the state of repose along the stream to its utmost branches as forming one great whole. It is, indeed, one of the most united of the large features which the earth exhibits. The student should consider it as a continuous inclined plane of diminishing slope, extending from the base of the torrents to the sea, and of course ramifying into the several branches of the river system. He should further bear in mind the fact that it is a vast laboratory where rock material is brought into the soluble state for delivery to the seas.