Outlines of the Earth's History Part 5

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 uprus.h.i.+ng current where the air pa.s.ses 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 pa.s.sage. 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 pa.s.ses. 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 aerial 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 pa.s.sage 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 pa.s.sing 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 lat.i.tudes 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 s.h.i.+ps, 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 s.h.i.+p 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 s.h.i.+p can maintain her way, in a score or two of miles of journey toward the centre, generally very quickly, it pa.s.ses into the calm disk, where the winds, blowing upward, cease to be felt. In this area the s.h.i.+p 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. Pa.s.sing through this disk of calm, the s.h.i.+p 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 s.h.i.+pmasters 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 trespa.s.s 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 sh.o.r.e 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 acc.u.mulation 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 lat.i.tudes, 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 pa.s.sage 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 pa.s.sed 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 ma.s.s 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 circ.u.mstances 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 thres.h.i.+ng 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 s.p.a.ce 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 inters.p.a.ces between that covering and the lath and plaster within (see Fig. 9).

[Ill.u.s.tration: Fig. 9.--Showing effect of expansion of air contained in a hollow wall during the pa.s.sage 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 ma.s.ses 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 cla.s.s 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 a.s.sertion 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 s.h.i.+ps, the old-fas.h.i.+oned 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 s.h.i.+ps 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 Camoens 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 Camoens 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 quant.i.ty 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 s.p.a.ce 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 diminis.h.i.+ng 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 superinc.u.mbent 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 diminis.h.i.+ng 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 s.p.a.ce. 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 a.s.sisted 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 lat.i.tudes during the periods of long-continued calm attended with clear skies. Thus on the island of Martha's Vineyard, in Ma.s.sachusetts, 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 sh.o.r.es 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 sh.o.r.e 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 ma.s.ses 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 ma.s.s. 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 ma.s.ses. 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.

[Ill.u.s.tration: _South sh.o.r.e, Martha's Vineyard, Ma.s.sachusetts, 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 Fohn of the Swiss valleys receive that name. Fohns are produced where a body of air blowing against the slope of a continuous mountain range is lifted to a considerable height, and, on pa.s.sing 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 ma.s.s, this heat is not much called for by that heat-storing fluid, and so the air is warmed. So far Fohn 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 quant.i.ties of sand and other fine _debris_, 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 quant.i.ties 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 acc.u.mulate on its surface, forming very thick deposits. Thus in northwestern China there is a wide area where dust acc.u.mulations 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 _debris_ is continually being borne away by the action of the rain waters as they journey toward the sea. Such dust acc.u.mulations 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 sh.o.r.es or cliffs of the same nature, large quant.i.ties 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 sh.o.r.e.

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 quant.i.ty 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 gra.s.ses 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 acc.u.mulations within the limits of the United States.

When blown sands invade a forest and the deposit is rapidly acc.u.mulated, the trees are often buried in an undecayed condition. In this state, with certain chemical reactions which may take place in the ma.s.s, 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 quant.i.ty 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 ma.s.s 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 pa.s.ses 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 quant.i.ty 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 role 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 s.p.a.ce, 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 pa.s.s 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 circ.u.mference. Those produced by the moon would have an alt.i.tude 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 pa.s.s 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 pa.s.s up the narrowing s.p.a.ces the waves become compressed--that is, an equal volume of moving water has less horizontal room for its pa.s.sage, and is forced to rise higher. We see a tolerably good ill.u.s.tration of the same principle when we observe a wind-made wave enter a small recess of the sh.o.r.e, 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 sh.o.r.es 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 alt.i.tude 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 sh.o.r.e the tides do important work in carving channels into the lands.