Part 9

Doubtless the cooling influence of the arctic currents is appreciable; but it would be a mistake to suppose that this influence can suffice to deprive the Gulf current of its distinctive warmth. If all the effect of the cold current were operative on the Gulf Stream alone we might suppose that, despite the enormous quantity of comparatively warm water which is continually being carried northwards, the current would be reduced to the temperature of the surrounding water. But this is not so. The arctic current not only cools the Gulf current, but the surrounding water also—possibly to a greater extent, for it is commonly supposed that a bed of ordinary sea-water separates the two main currents from each other. Thus the characteristic difference of temperature remains unaffected. But in reality we may assume that the cooling effect actually exercised by the arctic current upon the neighbouring sea is altogether disproportionate to the immense amount of heat continually being carried northwards by the Gulf Stream. It is astonishing how unreadily two sea-currents exchange their temperatures—to use a somewhat inexact mode of expression. The very fact that the littoral current of the United States is so cold—a fact thoroughly established—shows how little warmth this current has drawn from the neighbouring seas. Another fact, mentioned by Captain Maury, bears in a very interesting manner upon this peculiarity. He says: ‘If any vessel will take up her position a little to the northward of Bermuda, and steering thence for the capes of Virginia, will try the water-thermometer all the way at short intervals, she will find its reading to be now higher, now lower; and the observer will discover that he has been crossing streak after streak of warm and cool water in regular alternations.’ Each portion maintains its own temperature, even in the case of such warm streaks as these, all belonging to one current.

Similar considerations dispose of the arguments which have been founded on the temperature of the sea-bottom. It has been proved that the living creatures which people the lower depths of the sea exist under circumstances which evidence a perfect uniformity of temperature; and arguments on the subject of the Gulf Stream have been derived from the evidence of what is termed a minimum thermometer—that is, a thermometer which will indicate the lowest temperature it has been exposed to—let down into the depths of the sea. All such arguments, whether adduced against or in favour of the Gulf Stream theory, must be held, to be futile, since the thermometer in its descent may pass through several submarine currents of different temperature.

Lastly, an argument has been urged against the warming effects of the Gulf Stream upon our climate which requires to be considered with some attention. It is urged that the warmth derived from so shallow a current as the Gulf Stream must be, by the time it has reached our shores, could not provide an amount of heat sufficient to affect our climate to any appreciable extent. The mere neighbourhood of this water at a temperature slightly higher than that due to the latitude could not, it is urged, affect the temperature of the inland counties at all.

This argument is founded on a misapprehension of the beautiful arrangement by which Nature carries heat from one region to distribute it over another. Over the surface of the whole current the process of evaporation is going on at a greater rate than over the neighbouring seas, because the waters of the current are warmer than those which surround them. The vapour thus rising above the Gulf Stream is presently wafted by the south-westerly winds to our shores and over our whole land. But as it thus reaches a region of comparative cold, the vapour is condensed—that is, turned into fog, or mist, or cloud, according to circumstances. It is during this change that it gives out the heat it has brought with it from the Gulf Stream. For precisely as the evaporation of water is a process requiring heat, the change of vapour into water—whether in the form of fog, mist, cloud, or rain—is a process in which heat is given out. Thus it is that the south-westerly wind, the commonest wind we have, brings clouds and fogs and rain to us from the Gulf Stream, and with them brings the Gulf Stream warmth.

Why the south-westerly winds should be so common, and how it is that over the Gulf Stream there is a sort of air-channel along which winds come to us as if by their natural pathway, are matters inquired into farther on (see p. 164). The subject is full of interest, but need not here detain us.

It would seem that a mechanism involving the motion of such enormous masses of water as the current-system of the Atlantic should depend on the operation of very evident laws. Yet a variety of contradictory hypotheses have been put forward from time to time respecting this system of circulation, and even now the scientific world is divided between two opposing theories.

Of old the Mississippi River was supposed to be the parent of the Gulf Stream. It was noticed that the current flows at about the same rate as the Mississippi, and this fact was considered sufficient to support the strange theory that a river can give birth to an ocean-current.

It was easy, however, to overthrow this theory. Captain Livingston showed that the volume of water which is poured out of the Gulf of Mexico in the form of an ocean stream is more than a thousand times greater than the volume poured into the Gulf by the Mississippi River.

Having overthrown this old theory of the Gulf Stream, Captain Livingston attempted to set up one which is equally unfounded. He ascribed the current to the sun’s apparent yearly motion and the influence thus exerted on the waters of the Atlantic. A sort of yearly tide is conceived, according to this theory, to be the true parent of the Gulf current. It need hardly be said, however, that a phenomenon which remains without change through the winter and summer seasons cannot possibly be referred to the operation of such a cause as a yearly tide.

It is to Dr. Franklin that we owe the first theory of the Gulf Stream which has met with general acceptance. He held that the Gulf Stream is formed by the outflow of waters which have been forced into the Caribbean Sea by the trade-winds; so that the pressure of these winds on the Atlantic Ocean forms, according to Dr. Franklin, the true motive power of the Gulf Stream machinery. According to Maury, this theory has ‘come to be the most generally received opinion in the mind of seafaring people.’ It supplies a moving force of undoubted efficiency. We know that as the trade-winds travel towards the equator they lose their westerly motion. It is reasonable to suppose that this is caused by friction against the surface of the ocean, to which, therefore, a corresponding westerly motion must have been imparted.

There is a simplicity about Franklin’s theory which commends it favourably to consideration. But when we examine it somewhat more closely, several very decided flaws present themselves to our attention.

Consider, in the first place, the enormous mass of water moved by the supposed agency of the winds. Air has a weight—volume for volume—which is less than one eight-hundredth part of that of water. So that, to create a water-current, an air-current more than eight hundred times as large and of equal velocity must expend the whole of its motion. Now the trade-winds are gentle winds, their velocity scarcely exceeding in general that of the more swiftly-moving portions of the Gulf Stream. But even assigning to them a velocity four times as great, we still want an air-current two hundred times as large as the water-current. And the former must give up the whole of its motion, which, in the case of so elastic a substance as air, would hardly happen, the upper air being unlikely to be much affected by the motion of the lower.

But this is far from being all. If the trade-winds blew throughout the year, we might be disposed to recognise their influence upon the Gulf Stream as a paramount, if not the sole one. But this is not the case. Captain Maury states that, ‘With the view of ascertaining the average number of days during the year that the north-east trade-winds of the Atlantic operate upon the currents between twenty-five degrees north latitude and the equator, log-books containing no less than 380,284 observations on the force and direction of the wind in that ocean were examined. The data thus afforded were carefully compared and discussed. The results show that within these latitudes—and on the average—the wind from the north-east is in excess of the winds from the south-west only 111 days out of the 365. Now, can the north-east trades,‘ he pertinently asks, ‘by blowing for less than one-third of the time, cause the Gulf Stream to run all the time, and without varying its velocity either to their force or to their prevalence?’

And besides this, we have to consider that no part of the Gulf Stream flows strictly before the trade-winds. Where the current flows most rapidly, namely, in the Narrows of Bemini, it sets against the wind, and for hundreds of miles after it enters the Atlantic ‘it runs,’ says Maury, ‘right in the “wind’s eye.”‘ It must be remembered that a current of air directed with considerable force against the surface of still water has not the power of generating a current which can force its way far through the resisting fluid. If this were so, we might understand how the current, originating in sub-tropical regions, could force its way onward after the moving force had ceased to act upon it, and even carry its waters right against the wind, after leaving the Gulf of Mexico. But experience is wholly opposed to this view. The most energetic currents are quickly dispersed when they reach a wide expanse of still water. For example, the Niagara below the falls is an immense and rapid river. Yet when it reaches Lake Ontario, ‘instead of preserving its character as a distinct and well-defined stream for several hundred miles, it spreads itself out, and its waters are immediately lost in those of the lake.’ Here, again, the question asked by Maury bears pertinently on the subject we are considering. ‘Why,’ he says, ‘should not the Gulf Stream do the same? It gradually enlarges itself, it is true; but, instead of mingling with the ocean by broad spreading, as the immense rivers descending into the northern lakes do, its waters, like a stream of oil in the ocean, preserve a distinctive character for more than three thousand miles.’

The only other theory which has been considered in recent times to account satisfactorily for all the features of the Gulf Stream mechanism was put forward, we believe, by Captain Maury. In this theory, the motive power of the whole system of oceanic circulation is held to be the action of the sun’s heat upon the waters of the sea. We recognise two contrary effects as the immediate results of the sun’s action. In the first place, by warming the equatorial waters, it tends to make them lighter; in the second place, by causing evaporation, it renders them salter, and so tends to make them heavier. We have to inquire which form of action is most effective. The inquiry would be somewhat difficult, if we had not the evidence of the sea itself to supply an answer. For it is an inquiry to which ordinary experimental processes would not be applicable. We must accept the fact that the heated water from the equatorial seas actually does float upon the cooler portions of the Atlantic, as evidence that the action of the sun results in making the water lighter.

Now, Maury says that the water thus lightened must flow over and form a surface-current towards the Poles; while the cold and heavy water from the polar seas, as soon as it reaches the temperate zone, must sink and form a submarine current. He recognises in these facts the mainspring of the whole system of oceanic circulation. If a long trough be divided into two compartments, and we fill one with oil and the other with water, and then remove the dividing plate, we shall see the oil rushing over the water at one end of the trough, and the water rushing under the oil at the other. And if we further conceive that oil is continually being added at that end of the trough originally filled with oil, while water is continually added at the other, it is clear that the system of currents would continue in action: that is, there would be a continual flow of oil in one direction along the surface of the water, and of water in the contrary direction underneath the oil.

But Sir John Herschel maintains that no such effects as Maury describes could follow the action of the sun’s heat upon the equatorial waters. He argues thus: Granting that these waters become lighter and expand in volume, yet they can only move upwards, downwards, or sideways. There can be nothing to cause either of the two first forms of motion; and as for motion sideways, it can only result from the gradual slope caused by the bulging of the equatorial waters. He proceeds to show that this slope is so slight that we cannot look upon it as competent to form any sensible current from the equatorial towards the polar seas. And even if it could, he says, the water thus flowing off would have an eastward instead of a westward motion, precisely as the counter-trade-winds, blowing from equatorial to polar regions, have an eastward motion.

It is singular how completely the supporter of each rival view has succeeded in overthrowing the arguments of his opponent. Certainly Maury has shown with complete success that the inconstant trade-winds cannot account for the constant Gulf current, which does not even flow before them, but, in places, exactly against their force. And the reasoning of Sir John Herschel seems equally cogent, for certainly the flow of water from equatorial towards polar regions ought from the first to have an eastward, instead of a westward motion; whereas the equatorial current, of which the Gulf Stream is but the continuation, flows from east to west, right across the Atlantic.

Equally strange is it to find that each of these eminent men, having read the arguments of the other, reasserts, but does not effectually defend, his own theory, and repeats with even more damaging effect his arguments against the rival view.

Yet one or other theory must at least point to the true view, for the Atlantic is subject to no other agencies which can for a moment be held to account for a phenomenon of such magnitude as the Gulf Stream.

It appears to me, that on a close examination of the Gulf Stream mechanism, the true mainspring of its motion can be recognised. Compelled to reject the theory that the trade-winds generate the equatorial current westward, let us consider whether Herschel’s arguments against the ‘heat theory’ may not suggest a hint for our guidance. He points out that an overflow from the equator polewards would result in an eastward, and not in a westward, current. This is true. It is equally true that a flow of water towards the equator would result in a westward current. But no such flow is observed. Is it possible that there may be such a flow, but that it takes place in a hidden manner? Clearly there may be. Submarine currents towards the equator would have precisely the kind of motion we require, and if any cause drew them to the surface near the equator, they would account in full for the great equatorial westward current.

At this point we begin to see that an important circumstance has been lost sight of in dealing with the heat theory. The action of the sun on the surface-water of the equatorial Atlantic has only been considered with reference to its warming effects. But we must not forget that this action has drying effects also. It evaporates enormous quantities of water, and we have to inquire whence the water comes by which the sea-level is maintained. A surface flow from the sub-tropical seas would suffice for this purpose, but no such flow is observed. Whence, then, can the water come but from below? Thus we recognise the fact that a process resembling suction is continually taking place over the whole area of the equatorial Atlantic, the agent being the intense heat of the tropical sun. No one can doubt that this agent is one of adequate power. Indeed, the winds, conceived by Franklin to be the primary cause of the Atlantic currents, are in reality due to the merest fraction of the energy inherent in the sun’s heat.

We have other evidence that the indraught is from below in the comparative coldness of the equatorial current. The Gulf Stream is warm by comparison with the surrounding waters, but the equatorial current is cooler than the tropical seas. According to Professor Ansted, the southern portion of the equatorial current, as it flows past Brazil, ‘is everywhere a cold current, generally from four to six degrees below the adjacent ocean.’

If we here recognise the mainspring of the Gulf Stream mechanism, or rather of the whole system of oceanic circulation-for the movements observed in the Atlantic have their exact counterpart in the Pacific—we shall have no difficulty in accounting for all the motions which that mechanism exhibits. We need no longer look upon the Gulf Stream as the rebound of the equatorial current from the shores of North America. Knowing that there is an underflow towards the equator, we see that there must be a surface-flow towards the Poles. And this flow must as inevitably result in an easterly motion as the underflow towards the equator results in a westerly motion. We have, indeed, the phenomena of the trades and counter-trades exhibited in water-currents instead of air-currents.

(From the _St. Paul’s Magazine_, September 1869.)[9]

_FLOODS IN SWITZERLAND._

Recently (see date of essay) we have witnessed a succession of remarkable evidences of Nature’s destructive powers. The fires of Vesuvius, the earth-throes of the sub-equatorial Andes, and the submarine disturbance which has shaken Hawaii, have presented to us the various forms of destructive action which the earth’s, subterranean forces can assume. In the disastrous floods which have recently visited the Alpine cantons of Switzerland, we have evidence of the fact that natural forces which we are in the habit of regarding as beneficent and restorative may exhibit themselves as agents of the most widespread destruction. I have pointed out elsewhere (see p. 226) how enormous is the amount of power of which the rain-cloud is the representative; and in doing so I have endeavoured to exhibit the contrast between the steady action of the falling shower and the energy of the processes of which rain is in reality the equivalent. But in the floods which have lately ravaged Switzerland we see the same facts illustrated, not by numerical calculations or by the results of philosophical experiments, but in action, and that action taking place on the most widely extended scale. The whole of the south-eastern, or, as it may be termed, the Alpine half of Switzerland, has suffered from these floods. If a line be drawn from the Lake of Constance, in the north-east of Switzerland, to the Col de Balme, in the south-west, it will divide Switzerland into two nearly equal portions, and scarcely a canton within the eastern of these divisions has escaped without great damage.

The cantons which have suffered most terribly are those of Tessin, Grisons, and St. Gall. The St. Gothard, Splugen, and St. Bernhardin routes have been rendered impassable. Twenty-seven lives were lost in the St. Gothard Pass, besides horses and waggons full of merchandise. It is stated that on the three routes upwards of eighty persons perished. In the village of Loderio alone, no less than fifty deaths occurred. So terrible a flood has not taken place since the year 1834. Nor have the cantons of Uri and Valais escaped. From Unterwalden we hear that the heavy rains which took place a fortnight ago have carried away several large bridges, and many of the rivers continue still very swollen. I have already described how enormous the material losses are which have been caused by these floods. Many places are under water; others in ruins or absolutely destroyed. In Tessin alone the damage is estimated at forty thousand pounds sterling.

A country like Switzerland must always be liable to the occurrence, from time to time, of catastrophes of this sort. Or rather, perhaps, we should draw a distinction between the two divisions of Switzerland referred to above. Of these the one may be termed the mountain half, and the other the lake half of the country. It is the former portion of the country which is principally subject to the dynamical action of water. A long-continued and heavy rainfall over the higher lands cannot fail to produce a variety of remarkable effects, where the arrangement of mountains and passes, hills, valleys, and ravines is so complicated. There are places where a large volume of water can accumulate until the barriers which have opposed its passage to the plains burst under its increasing weight; and then follow those destructive rushes of water which sweep away whole villages at once. It is, in fact, the capacity of the Swiss mountain region for damming up water, far more than any other circumstance, which renders the Swiss floods so destructive.

And then it must be remembered that there are at all times suspended over the plains and valleys which lie beneath the Alpine ranges enormous masses of water in the form of snow and ice. Although in general these suffer no changes but those due to the partial melting which takes place in summer, and the renewed accumulation which takes place in winter, yet when heavy rains fall upon the less elevated portions of the Alpine snow, they not only melt that snow much more rapidly than the summer sun would do, but they wash down large masses, which add largely to the destructive power of the descending waters.