Outlines of the Earth's History Part 6

Along the sh.o.r.es of the continents about the North Atlantic, where the tides act in a vigorous manner, we almost everywhere find an underwater shelf extending from the sh.o.r.e with a declivity of only five to ten feet to the mile toward the centre of the sea, until the depth of about five hundred feet is attained; from this point the bottom descends more steeply into the ocean's depth. It is probable that the larger part of the material composing these continental shelves has been brought to its position by tidal action. Each time the tidal wave sweeps in toward the sh.o.r.e it urges the finer particles of sediment along with it. When it moves out it drags them on the return journey toward the depths of the sea. If this shelf were perfectly horizontal, the two journeys of the sand and mud grains would be of the same length; but as the movement takes place up and down a slope, the bits will travel farther under the impulse which leads them downward than under that which impels them up. The result will be that the particles will travel a little farther out from the sh.o.r.e each time it is swung to and fro in the alternating movement of the tide.

The effect of tidal movement in nurturing marine life is very great.

It aids the animals fixed on the bottoms of the deep seas to obtain their provision of food and their share of oxygen by drawing the water by their bodies. All regions which are visited by strong tides commonly have in the shallows near the sh.o.r.es a thick growth of seaweed which furnishes an ample provision of food for the fishes and other forms of animal life.

A peculiar effect arising from tidal action is believed by students of the phenomena to be found in the slowing of the earth's rotation on its axis. The tides rotate around the earth from east to west, or rather, we should say, the solid ma.s.s of the earth rubs against them as it spins from west to east. As they move over the bottom and as they strike against the sh.o.r.es this push of the great waves tends in a slight measure to use up the original spinning impulse which causes the earth's rotation. Computation shows that the amount of this action should be great enough gradually to lengthen the day, or the time occupied by the earth in making a complete revolution on the polar axis. The effect ought to be great enough to be measurable by astronomers in the course of a thousand years. On the other hand, the records of ancient eclipses appear pretty clearly to show that the length of the day has not changed by as much as a second in the course of three thousand years. This evidence does not require us to abandon the supposition that the tides tend to diminish the earth's rate of rotation. It is more likely that the effect of the reduction in the earth's diameter due to the loss of heat which is continually going on counterbalances the influence of the tidal friction. As the diameter of a rotating body diminishes, the tendency is for the ma.s.s to spin more rapidly; if it expands, to turn more slowly, provided in each case the amount of the impulse which leads to the turning remains the same. This can be directly observed by whirling a small weight attached to a string in such a manner that the cord winds around the finger with each revolution; it will be noted that as the line shortens the revolution is more quickly accomplished. We can readily conceive that the earth is made up of weights essentially like that used in the experiment, each being drawn toward the centre by the gravitative stress, which is like that applied to the weight by the cord.

The fact that the days remain of the same length through vast periods of time is probably due to this balance between the effects of tidal action and those arising from the loss of heat--in other words, we have here one of those delicate arrangements in the way of counterpoise which serve to maintain the balanced conditions of the earth's surface amid the great conflicts of diverse energies which are at work in and upon the sphere.

It should be understood that the effects of the attraction which produces tides are much more extensive than they are seen to be in the movements of the sea. So long as the solar and planetary spheres remain fluid, the whole of their ma.s.ses partake of the movement. It is a consequence of this action, as the computations of Prof. George Darwin has shown, that the moon, once nearer the earth than it is at present, has by a curious action of the tidal force been pushed away from the centre of our sphere, or rather the two bodies have repelled each other. An American student of the problem, Mr. T.J.J. See, has shown that the same action has served to give to the double stars the exceeding eccentricity of their orbits.

Although these recent studies of tidal action in the celestial sphere are of the utmost importance to the theory of the universe, for they may lead to changes in the nebular hypotheses, they are as yet too incomplete and are, moreover, too mathematical to be presented in an elementary treatise such as this.

We now turn to another cla.s.s of waves which are of even more importance than those of the tides--to the undulations which are produced by the action of the wind on the surface of the water. While the tide waves are limited to the open ocean, and to the seas and bays which afford them free entrance, wind waves are produced everywhere where water is subjected to the friction of air which flows over it.

While tidal waves come upon the sh.o.r.es but twice each day, the wind waves of ordinary size which roll in from the ocean deliver their blows at intervals of from three to ten seconds. Although the tidal waves sometimes, by a packing-up process, attain the height of fifty feet, their average alt.i.tude where they come in contact with the sh.o.r.e probably does not much exceed four feet; usually they come in gently.

It is likely that in a general way the ocean surges which beat against the coast are of greater alt.i.tude.

Wind waves are produced and perform their work in a manner which we shall now describe. When the air blows over any resisting surface, it tends, in a way which we can hardly afford here to describe, to produce motions. If the particle is free to move under the impulse which it communicates, it bears it along; if it is linked together in the manner of large ma.s.ses, which the wind can not transport, it tends to set it in motion in an alternating way. The sounds of our musical instruments which act by wind are due to these alternating vibrations, such as all air currents tend to produce. An aeolian harp ill.u.s.trates the action which we are considering. Moving over matter which has the qualities that we denote by the term fluid, the swayings which the air produces are of a peculiar sort, though they much resemble those of the fiddle string. The surface of the liquid rises and falls in what we term waves, the size of which is determined by the measure of fluidity, and by the energy of the wind. Thus, because fresh water is considerably lighter than salt, a given wind will produce in a given distance for the run of the waves heavier surges in a lake than it will in the sea. For this reason the surges in a great storm which roll on the ocean sh.o.r.e, because of the wide water over which they have gathered their impetus, are in size very much greater than those of the largest lakes, which do not afford room for the development of great undulations.

To the eye, a wave in the water appears to indicate that the fluid is borne on before the wind. Examination, however, shows that the amount of motion in the direction in which the wind is blowing is very slight. We may say, indeed, that the essential feature of a wave is found in the transmission of impulse rather than in the movement of the fluid matter. A strip of carpet when shaken sends through its length undulations which are almost exactly like water waves. If we imagine ourselves placed in a particle of water, moving in the swayings of a wave in the open and deep sea, we may conceive ourselves carried around in an ellipse, in each revolution returning through nearly the same orbit. Now and then, when the particle came to the surface, it would experience the slight drift which the continual friction of the wind imposes on the water. If the wave in which the journey was made lay in the trade winds, where the long-continued, steadfast blowing had set the water in motion to great depths, the orbit traversed would be moving forward with some rapidity; where also the wind was strong enough to blow the tops of the waves over, forming white-caps, the advance of the particle very near the surface would be speedy. Notwithstanding these corrections, waves are to be regarded each as a store of energy, urging the water to sway much in the manner of a carpet strip, and by the swaying conveying the energy in the direction of the wave movement.

The rate of movement of wind waves increases with their height.

Slight undulations go forward at the rate of less than half a mile an hour. The greater surges of the deeps when swept by the strongest winds move with the speed which, though not accurately determined, has been estimated by the present writer as exceeding forty miles an hour.

As these surges often have a length transverse to the wind of a mile or more, a width of about an eighth of a mile, and a height of from thirty-five to forty-five feet, the amount of energy which they transmit is very great. If it could be effectively applied to the sh.o.r.es in the manner in which the energy of exploding gunpowder is applied by cannon shot, it is doubtful whether the lands could have maintained their position against the a.s.saults of the sea. But there are reasons stated below why the ocean waves can use only a very small part of their energy in rending the rocks against which they strike on the coast line.

In the first place, we should note that wind waves have very little influence on the bottom of the deep sea. If an observer could stand on the sea floor at the depth of a mile below a point over which the greatest waves were rolling, he could not with his unaided senses discern that the water was troubled. He would, indeed, require instruments of some delicacy to find out that it moved at all. Making the same observations at the depth of a thousand feet, it is possible that he would note a slight swaying motion in the water, enough sensibly to affect his body. At five hundred feet in depth the movement would probably be sufficient to disturb fine mud. At two hundred feet, the rasping of the surge on the bottom would doubtless be sufficient to push particles of coa.r.s.e sand to and fro. At one hundred feet in depth, the pa.s.sage of the surge would be strong enough to urge considerable pebbles before it. Thence up the slope the driving action would become more and more intense until we attained the point where the wave broke. It should furthermore be noted that, while the movement of the water on the floor of the deep sea as the wave pa.s.ses overhead would be to and fro, with every advance in the shallowing and consequent increased friction on the bottom, the forward element in the movement would rapidly increase. Near the coast line the effect of the waves is continually to shove the detritus up the slopes of the continental shelf. Here we should note the fact that on this shelf the waves play a part exactly the opposite of that effected by the tides. The tides, as we have noted, tend to drag the particles down the slope, while the waves operate to roll them up the declivity.

As the wave in advancing toward the sh.o.r.e ordinarily comes into continually shallowing water, the friction on the bottom is ever-increasing, and serves to diminish the energy the surge contains, and therefore to reduce its proportions. If this action operated alone, the subtraction which the friction makes would cause the surf waves which roll in over a continental shelf to be very low, probably in height less than half that which they now attain. In fact, however, there is an influence at work to increase the height of the waves at the expense of its width. Noting that the friction rapidly increases with the shallowing, it is easy to see that this resistance is greatest on the advancing front of the wave, and least on its seaward side. The result is that the front moves more slowly than the rear, so that the wave is forced to gain in height; but for the fact that the total friction which the wave encounters takes away most of its impetus, we might have combers a hundred feet high rolling upon the shelving sh.o.r.es which almost everywhere face the seas.

As the wave shortens its width and gains in relative height, though not in actual elevation, another action is introduced which has momentous consequences. The water in the bottom of the wave is greatly r.e.t.a.r.ded in its ongoing by its friction over the sea floor, while the upper part of the surge is much less affected in this way. The result is that at a certain point in the advance, the place of which is determined by the depth, the size, and the speed of the undulation, the front swiftly steepens until it is vertical, and the top shoots forward to a point where it is no longer supported by underlying water, when it plunges down in what is called the surf or breaker. In this part of the wave's work the application of the energy which it transmits differs strikingly from the work previously done. Before the wave breaks, the only geological task which it accomplishes is effected by forcing materials up the slope, in which movement they are slightly ground over each other until they come within the battering zone of the sh.o.r.e, where they may be further divided by the action of the mill which is there in operation.

When the wave breaks on the sh.o.r.e it operates in the following manner: First, the overturning of its crest sends a great ma.s.s of water, it may be from the height of ten or more feet, down upon the sh.o.r.e. Thus falling water has not only the force due to its drop from the summit of the wave, but it has a share of the impulse due to the velocity with which the surge moved against the sh.o.r.e. It acts, in a word, like a hammer swung down by a strong arm, where the blow represents not only the force with which the weight would fall of itself, but the impelling power of the man's muscles. Any one who will expose his body to this blow of the surf will recognise how violent it is; he may, if the beach be pebbly, note how it drives the stones about; fragments the size of a man's head may be hurled by the stroke to the distance of twenty feet or more; those as large as the fist may be thrown clear beyond the limits of the wave. So vigorous is this stroke that the sound of it may sometimes be heard ten miles inland from the coast where it is delivered.

Moving forward up the slope of a gently inclined beach, the fragments of the wave are likely to be of sufficient volume to permit them to regather into a secondary surge, which, like the first, though much smaller, again rises into a wall, forming another breaker. Under favourable conditions as many as four or five of these successive diminis.h.i.+ng surf lines may be seen. The present writer has seen in certain cases as many as a dozen in the great procession, the lowest and innermost only a few inches high, the outer of all with a height of perhaps twenty feet.

Along with the direct bearing action of the surf goes a to-and-fro movement, due to the rus.h.i.+ng up and down of the water on the beach.

This swas.h.i.+ng affects not only the broken part of the waves, but all the water between the outer breaker and the sh.o.r.e. These swayings in the surf belt often swing the _debris_ on the inner margin over a range of a hundred feet or more, the movement taking place with great swiftness, affecting the pebbles to the depth of several inches, and grinding the bits together in a violent way. Listening to the turmoil of a storm, we can on a pebbly beach distinctly hear the sound of the downward stroke, a cras.h.i.+ng tone, and the roar of the rolling stones.

As waves are among the interesting things in the world, partly on account of their living quality and partly because of their immediate and often exceeding interest to man, we may here note one or two peculiar features in their action. In the first place, as the reader who has gained a sense of the changes in form of action may readily perceive, the beating of waves on the sh.o.r.e converts the energy which they possess into heat. This probably warms the water during great storms, so that by the hand we may note the difference in temperature next the coast line and in the open waters. This relative warmth of the surf water is perhaps a matter of some importance in limiting the development of ice along the sh.o.r.e line; it may also favour the protection of the coast life against the severe cold of the winter season.

The waves which successively come against the sh.o.r.e in any given time, particularly if a violent wind is blowing on to the coast, are usually of about the same size. When, however, in times of calm an old sea, as it is called, is rolling in, the surges may occasionally undergo very great variations in magnitude. Not infrequently these occasional waves are great enough to overwhelm persons who are upon the rocks next the sh.o.r.e. These greater surges are probably to be accounted for by the fact that in the open sea waves produced by winds blowing in different directions may run on with their diverse courses and varied intervals until they come near the sh.o.r.e. Running in together, it very well happens that two of the surges belonging to different sets may combine their forces, thus doubling the swell. The danger which these conjoined waves bring is obviously greatest on cliff sh.o.r.es, where, on account of the depth of water, the waves do not break until they strike the steep.

Having considered in a general way the action of waves as they roll in to the sh.o.r.e, bearing with them the solar energy which was contributed to them by the winds, we shall now take up in some detail the work which goes on along the coast line--work which is mainly accomplished by wave action.

On most coast lines the observer readily notes that the sh.o.r.e is divided into two different kinds of faces--those where the inner margin of the wave-swept belt comes against rocky steeps, and those bordered by a strand altogether composed of materials which the surges have thrown up. These may be termed for convenience cliff sh.o.r.es and wall-beach sh.o.r.es. We shall begin our inquiry with cliff sh.o.r.es, for in those sections of the coast line the sea is doing its most characteristic and important work of a.s.saulting the land. If the student has an opportunity to approach a set of cliffs of hard rock in time of heavy storm, when the waves have somewhere their maximum height, he should seek some headland which may offer him safe foothold whence he can behold the movements which are taking place. If he is so fortunate as to have in view, as well may be the case, cliffs which extend down into deep water, and others which are bordered by rude and generally steeply sloping beaches covered with large stones, he may perceive that the waves come in against the cliffs which plunge into deep water without taking on the breaker form. In this case the undulation strikes but a moderate blow; the wave is not greatly broken. The part next the rock may shoot up as a thin sheet to a considerable height; it is evident that while the ongoing wave applies a good deal of pressure to the steep, it does not deliver its energy in the effective form of a blow as when the wave overturns, or in the consequent rush of the water up a beach slope. It is easy to perceive that firm-set rock cliffs, with no beaches at their bases, can almost indefinitely withstand the a.s.saults. On the steep and stony beach, because of its relatively great declivity, the breaker or surf forms far in, and even in its first plunge often attains the base of the precipice. The blow of the overfalling as well as that of the inrush moves about stones of great size; those three feet or more in diameter are often hurled by the action against the base of the steep, striking blows, the sharp note of which can often be heard above the general roar which the commotion produces. The needlelike crags forming isles standing at a distance from the sh.o.r.e, such as are often found along hard rock coasts, are singularly protected from the action of effective waves. The surges which strike against them are unarmed with stones, and the water at their bases is so deep that it does not sway with the motion with sufficient energy to move them on the bottom.

Where a cliff is in this condition, it may endure until an elevation of the coast line brings its base near the level of the sea, or until the process of decay has detached a sufficient quant.i.ty of stone to form a talus or inclined plane reaching near to the water level.

As before noted, it is the presence of a sloping beach reaching to about the base of the cliff which makes it possible for the waves to strike at with a hammer instead of with a soft hand. Battering at the base of the cliff, the surges cut a crease along the strip on which they strike, which gradually enters so far that the overhanging rock falls of its own weight. The fragments thus delivered to the sea are in turn broken up and used as battering instruments until they are worn to pieces. We may note that in a few months of heavy weather the stones of such a fall have all been reduced to rudely spherical forms.

Observations made on the eastern face of Cape Ann, Ma.s.s., where the seas are only moderately heavy, show that the storms of a single winter reduce the fragments thrown into the sea from the granite quarries to spheroidal shapes, more than half of their weight commonly being removed in the form of sand and small pebbles which have been worn from their surfaces.

We can best perceive the effect of battering action which the sea applies to the cliffs by noting the points where, owing to some chance features in the structure in the rock, it has proved most effective.

Where a joint or a dike, or perhaps a softer layer, if the rocks be bedded, causes the wear to go on more rapidly, the waves soon excavate a recess in which the pebbles are retained, except in stormy weather, in an unmoved condition. When the surges are heavy, these stones are kept in continuous motion, receding as the wave goes back, and rus.h.i.+ng forward with its impulse until they strike against the firm-set rock at the end of the chasm. In this way they may drive in a cut having the length of a hundred feet or more from the face of the precipice.

In most cases the roofs over these sea caves fall in, so that the structure is known as a chasm. Occasionally these roofs remain, in which case, for the reason that the floor of the cutting inclines upward, an opening is made to the surface at their upper end, forming what is called in New England a "spouting horn"; from the inland end of the tunnel the spray may be thrown far into the air. As long as the cave is closed at this inner end, and is not so high but that it may be buried beneath a heavy wave, the inrus.h.i.+ng water compresses the air in the rear parts of the opening. When the wave begins to retreat this air blows out, sending a gust of spray before it, the action resembling the discharge of a great gun from the face of a fortification. It often happens that two chasms converging separate a rock from the cliff. Then a lowering of the coast may bring the ma.s.s to the state of a columnar island, such as abound in the Hebrides and along various other sh.o.r.es.

If a cliff sh.o.r.e retreats rapidly, it may be driven back into the sh.o.r.e, and its face a.s.sumes the curve of a small bay. With every step in this change the bottom is sure to become shallower, so that the waves lose more and more of their energy in friction over the bottom.

Moreover, in entering a bay the friction which the waves encounter in running along the sides is greater than that which they meet in coming in upon a headland or a straight sh.o.r.e. The result is, with the inward retreat of the steep it enters on conditions which diminish the effectiveness of the wave stroke. The embayment also is apt to hold detritus, and so forms in time a beach at the foot of the cliff, over which the waves rarely are able to mount with such energy as will enable them to strike the wall in an effective manner. With this sketch of the conditions of a cliff sh.o.r.e, we will now consider the fate of the broken-tip rock which the waves have produced on that section of the coast land.

By observation of sea-beaten cliffs the student readily perceives that a great amount of rocky matter has been removed from most cliff-faced sh.o.r.es. Not uncommonly it can be shown that such sea faces have retreated for several miles. The question now arises, What becomes of the matter which has been broken up by the wave action? In some part the rock, when pulverized by the pounding to which it is subjected, has dissolved in the water. Probably ninety per cent of it, however, retains the visible state, and has a fate determined by the size of the fragments of which it is composed. If these be as fine as mud, so that they may float in the water, they are readily borne away by the currents which are always created along a storm-swept sh.o.r.e, particularly by the undertow or bottom outcurrent--the "sea-puss," as it is sometimes called--that sweeps along the bottom from every sh.o.r.e, against which the waves form a surf. If as coa.r.s.e as sand grains, or even very small pebbles, they are likely to be drawn out, rolling over the bottom to an indefinite distance from the sea margin. The coa.r.s.er stones, however, either remain at the foot of the cliff until they are beaten to pieces, or are driven along the sh.o.r.e until they find some embayment into which they enter. The journey of such fragments may, when the wind strikes obliquely to the sh.o.r.e, continue for many miles; the waves, running with the wind, drive the fragments in oscillating journeys up and down the beach, sometimes at the rate of a mile or more a day. The effect of this action can often be seen where a vessel loaded with brick or coal is wrecked on the coast. In a month fragments of the materials may be stretched along for the distance of many miles on either side of the point where the cargo came ash.o.r.e.

Entering an embayment deep enough to restrain their further journey, the fragments of rock form a boulder beach, where the bits roll to and fro whenever they are struck by heavy surges. The greater portion of them remain in this mill until they are ground to the state of sand and mud. Now and then one of the fragments is tossed up beyond the reach of the waves, and is contributed to the wall of the beach. In very heavy storms these pebbles which are thrown inland may amount in weight to many tons for each mile of sh.o.r.e.

The study of a pebbly beach, drawn from crest to the deep water outside, will give an idea as to the history of its work. On either horn of the crescent by which the pebbles are imported into the pocket we find the largest fragments. If the sh.o.r.e of the bay be long, the innermost part of the recess may show even only very small pebbles, or perhaps only fine sand, the coa.r.s.er material having been worn out in the journey. On the bottom of the bay, near low tide, we begin to find some sand produced by the grinding action. Yet farther out, below high-tide mark, there is commonly a layer of mud which represents the finer products of the mill.

Boulder beaches are so quick in answering to every slight change in the conditions which affect them that they seem almost alive. If by any chance the supply of detritus is increased, they fill in between the horns, diminish the incurve of the bay, and so cause its beach to be more exposed to heavy waves. If, on the other hand, the supply of grist to the mill is diminished, the beach becomes more deeply incurved, and the wave action is proportionately reduced. We may say, in general, that the curve of these beaches represents a balance between the consumption and supply of the pebbles which they grind up.

The supply of pebbles brought along the sh.o.r.e by the waves is in many cases greatly added to by a curious action of seaweeds. If the bottom of the water off the coast is covered by these fragments, as is the case along many coast lines within the old glaciated districts, the spores of algae are p.r.o.ne to take root upon them. Fastening themselves in those positions, and growing upward, the seaweeds may attain considerable size. Being provided with floats, the plant exercises a certain lifting power on the stone, and finally the tugging action of the waves on the fronds may detach the fragments from the bottom, making them free to journey toward the sh.o.r.e. Observing from near at hand the straight wall of the wave in times of heavy storm, the present writer has seen in one view as many as a dozen of these plant-borne stones, sometimes six inches in diameter, hanging in the walls of water as it was about to topple over. As soon as they strike the wave-beaten part of the sh.o.r.e these stones are apt to become separated from the plants, though we can often notice the remains or prints of the attachments adhering to the surface of the rock. Where the pebbles off the sh.o.r.e are plenty, a rocky beach may be produced by this process of importation through the agency of seaweeds without any supply being brought by the waves along the coast line.

Returning to sand beaches, we enter the most interesting field of contact between seas and lands. Probably nine tenths of all the coast lines of the open ocean are formed of arenaceous material. In general, sand consists of finely broken crystals of silica or quartz. These bits are commonly distinctly faceted; they rarely have a spherical form. Not only do acc.u.mulations of sand border most of the sh.o.r.e line, but they protect the land against the a.s.saults of the sea, and this in the following curious manner: When sh.o.r.e 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 m.u.f.fled 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 gla.s.s 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 gla.s.s 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 seash.o.r.e may wear more in going the distance of a mile in the dry sand dune than in travelling for hundreds along the wet sh.o.r.e.

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 sh.o.r.es 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 dest.i.tute of shelters which can serve the use of s.h.i.+ps, 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 pa.s.ses 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 inst.i.tution 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 lat.i.tude, 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 sh.o.r.es, 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 lat.i.tudes, conveying their store of heat to the circ.u.mpolar 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 anch.o.r.ed s.h.i.+p, a part--rather more than one half--of the stream turning to the northward, the remainder pa.s.sing 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 inters.p.a.ces between these isles and enters the Gulf of Mexico. Another portion, failing to find sufficient room through these pa.s.sages, skirts the Antilles on their eastern and northern sides, pa.s.ses 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, diminis.h.i.+ng 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 pa.s.ses 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 princ.i.p.al timber supply of the inhabitants of the first-named isle.

Attaining this circ.u.mpolar 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 circ.u.mpolar 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 sh.o.r.es for a certain distance toward the poles, soon diverge from them, just as the Gulf Stream pa.s.ses 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 pa.s.sage 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 lat.i.tudes 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 ma.s.sive drift which we have now to note.

The tropical waters when they attain high lat.i.tudes 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 Fahr. The effect of this action is gradually to press down the surface cold water until it attains the very bottom in all the circ.u.mpolar 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 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.