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.
The most destructive floods which have occurred in Switzerland have
usually been those which take place in early summer. The floods which
inundated the plains of Martigny in 1818 were a remarkable instance of
the effects which result from the natural damming up of large volumes
of water in the upper parts of the Alpine hill-country. The whole of
the valley of Bagnes, one of the largest of the lateral branches of
the main valley of the Rhone above Geneva, was converted into a lake,
in the spring of 1818, by the damming up of a narrow pass into which
avalanches of snow and ice had been precipitated from a lofty glacier
overhanging the bed of the river Dranse. The ice barrier enclosed a
lake no less than half a league in length and an eighth of a mile
wide, and in places two hundred feet deep. The inhabitants of the
neighbouring villages were terrified by the danger which was to be
apprehended from the bursting of the barrier. They cut a gallery seven
hundred feet long through the ice, while the waters had as yet risen
to but a moderate height; and when the waters began to flow through
this channel, its course was deepened by the melting of the ice, and
at length nearly half the contents of the lake were safely carried
off. It was hoped that the process would continue, and the country be
saved from the danger which had been so long impending over it. But
as the heat of the weather increased, the central part of the barrier
slowly melted away, until it became too weak to bear the enormous
weight of water which was pressing against it. At length it gave way,
so suddenly and completely that all the water which remained in the
lake rushed out in half an hour. The downward passage of the water
illustrated, in a very remarkable way, the fact that the chief mischief
of floods is occasioned where water is checked in its outflow. For it
is related that, ‘in the course of their descent the waters encountered
several narrow gorges, and at each of these they rose to a great
height, and then burst with new violence into the next basin, sweeping
along forests, houses, bridges, and cultivated land.’ Along the greater
part of its course the flood resembled rather a moving mass of rock and
mud than a stream of water. Enormous masses of granite were torn out
of the sides of the valleys and whirled for hundreds of yards along
the course of the flood. M. Escher relates that one of the fragments
thus swept along was no less than sixty yards in circumference. At
first the water rushed onwards at a rate of more than a mile in three
minutes, and the whole distance (forty-five miles) which separates the
valley of Bagnes from the Lake of Geneva was traversed in little more
than six hours. The bodies of persons who had been drowned in Martigny
were found floating on the farther side of the lake of Geneva, near
Vevey. Thousands of trees were torn up by the roots, and the ruins of
buildings which had been overthrown by the flood were carried down
beyond Martigny. In fact, the flood at this point was so high that
some of the houses in Martigny were filled with mud up to the second
storey.‘ Beyond Martigny the flood did but little damage, as it here
expanded over the plain, and was reduced both in depth and velocity.
(From the _Daily News_ for October 20, 1868.)
FOOTNOTES:
[9] See ‘Light Science’ (second series) for a discussion of later
researches.
_A GREAT TIDAL WAVE._
During the last few days anxious questionings have been heard
respecting the next spring tides. A certain naval officer, who
conceives that he can trace in the relative positions of the sun and
moon the secret of every important change of weather, has described
in the columns of a contemporary the threatening significance of the
approaching conjunction of the sun and moon. He predicts violent
atmospheric disturbances; though in another place he tells us merely
that the conjunction is to cause ‘unsettled weather,’ a state of
matters to which we in England have become tolerably well accustomed.
But people are asking what is the actual relation which is to bring
about such terrible events. The matter is very simple. On October 5,
the moon will be new—in other words, if it were not for the brightness
of the sun, we should see the moon close by that luminary on the
heavens. Thus the sun and moon will pull with combined effect upon
the waters of the earth, and so cause what are called spring tides.
This, of course, happens at the time of every new moon, but sometimes
the moon exerts a more effective pull than at other times; and the
same happens also in the case of the sun; and on October 5, it happens
that both the sun and the moon will give a particularly vigorous
haul upon the earth’s waters. As regards the sun, there is nothing
unusual. Every October his pull on the ocean is much the same as in
preceding Octobers. But October is a month of high solar tides—and for
these reasons:—In September, as everyone knows, the sun crosses the
equinoctial; and, other things being equal, it would be when on the
equinoctial that his power to raise a tidal wave would be greatest.
But other things are not equal; for the sun is not always at the same
distance from the earth. He is nearest in January; so that he would
exert more power in that month than in any other, if his force depended
solely on distance. As matters actually stand, it will be obvious that
at some time between September and January the sun’s tidal power would
have a maximum value. Thus it is that October is a month of high solar
tidal waves.
But it is the lunar wave which will be most effectively strengthened
at the next spring tide. If we could watch the lunar tidal wave alone
(instead of always finding it combined with the solar wave) we should
find it gradually increasing, and then gradually diminishing, in a
period of about a lunar month. And we should find that it was always
largest when the moon looked largest, and _vice versâ_. In other words,
when the moon is in perigee the lunar wave is largest. But then there
is another consideration. The lunar wave would vary according to the
moon’s proximity to the equinoctial; and (other things being equal)
would be largest when the moon is exactly opposite the earth’s equator.
If the two effects are combined, that is, if the moon happens to be in
perigee and on the equinoctial at the same time, then of course we get
the largest lunar tidal wave we can possibly have.