to much discussion. I again quote Professor Buff for what may
be regarded as the view most generally received among physical
geographers:
“A great part of the warm water is carried partly by its own motion,
but chiefly by the prevailing west and northwest winds, toward the
coast of Europe and even beyond Spitzbergen and Nova Zembla; and
thus a part of the heat of the south reaches far into the Arctic
Ocean. Hence, on the north coast of the Old Continent, we always
find driftwood from the southern regions, and on this side the
Arctic Ocean remains free from ice during a great part of the year,
even as far up as 80° north latitude; while on the opposite coast
(of Greenland) the ice is not quite thawed even in summer.” The two
forces invoked by Professor Buff to perform the work are thus the
_vis à tergo_ of the trade-wind drift and the direct driving power
of the anti-trades, producing what has been called the anti-trade
drift. This is quite in accordance with the views here advocated.
The proportion in which these two forces act, it is undoubtedly
impossible in the present state of our knowledge to determine.
Mr. A. G. Findlay, a high authority on all hydrographic matters, read
a paper on the Gulf Stream before the Royal Geographical Society,
reported in the 13th volume of the Proceedings of the Society. Mr.
Findlay, while admitting that the temperature of Northern Europe is
abnormally ameliorated by a surface-current of the warm water of the
Atlantic which reaches it, contends that the Gulf Stream proper--that
is to say, the water injected, as it were, into the Atlantic through
the Strait of Florida by the impulse of the trade-winds--becomes
entirely thinned out, dissipated, and lost opposite the Newfoundland
banks about lat. 45° N. The warm water of the southern portion of
the North Atlantic basin is still carried northward; but Mr. Findlay
attributes this movement solely to the anti-trades--the southwest
winds--which by their prevalence keep up a balance of progress in a
northeasterly direction in the surface layer of the water.
Dr. Carpenter entertains a very strong opinion that the dispersion of
the Gulf Stream may be affirmed to be complete in about lat. 45° N.
and long. 35° W. Dr. Carpenter admits the accuracy of the projection
of the isotherms on the maps of Berghaus, Dove, Petermann, and Keith
Johnston, and he admits likewise the conclusion that the abnormal
mildness of the climate on the northwestern coast of Europe is due to
a movement of equatorial water in a northeasterly direction. “What I
question is the correctness of the doctrine that the northeast flow
is an extension or prolongation of the Gulf Stream, still driven on
by the _vis à tergo_ of the trade-winds--a doctrine which (greatly to
my surprise) has been adopted and defended by my colleague, Professor
Wyville Thomson. But while these authorities attribute the whole or
nearly the whole of this flow to the true Gulf Stream, _I_ regard a
large part, if not the whole, of that which takes place along our
own western coast, and passes north and northeast between Iceland
and Norway toward Spitzbergen, as quite independent of that agency;
so that it would continue if the North and South American Continents
were so completely disunited that the equatorial currents would be
driven straight onward by the trade-winds into the Pacific Ocean,
instead of being embayed in the Gulf of Mexico and driven out in a
northeast direction through the ‘narrows’ off Cape Florida.” Dr.
Carpenter does not mean by this to indorse Mr. Findlay’s opinion
that the movement beyond the 54th parallel of latitude is due solely
to the drift of the anti-trades; he says, “On the view I advocate,
the northeasterly flow is regarded as due to the _vis à fronte_
originating in the action of cold upon the water of the polar area,
whereby its level is always tending to depression.” The amelioration
of the climate of northwestern Europe is thus caused by a “modified
case” of the general oceanic circulation, and neither by the Gulf
Stream nor by the anti-trade drift.
Although there are, up to the present time, very few trustworthy
observations of deep-sea temperatures, the surface temperature of
the North Atlantic has been investigated with considerable care.
The general character of the isothermal lines, with their singular
loop-like northern deflections, has long been familiar through the
temperature charts of the geographers already quoted, and of late
years a prodigious amount of data have been accumulated.
In 1870, Dr. Petermann, of Gotha, published an extremely valuable
series of temperature charts, embodying the results of the reduction
of upward of 100,000 observations.
Dr. Petermann has devoted the special attention of a great part of
his life to the distribution of heat on the surface of the ocean, and
the accuracy and conscientiousness of his work in every detail are
beyond the shadow of a doubt.
In the North Atlantic every curve of equal temperature, whether for
the summer, for the winter, for a single month, or for the whole
year, instantly declares itself as one of a system of curves which
are referred to the Strait of Florida as a source of heat, and the
flow of warm water may be traced in a continuous stream--indicated
when its movement can no longer be observed by its form--fanning out
from the neighborhood of the Strait across the Atlantic, skirting
the coasts of France, Britain, and Scandinavia, rounding the North
Cape, and passing the White Sea and the Sea of Kari, bathing the
western shores of Nova Zembla and Spitzbergen, and finally coursing
round the coast of Siberia, a trace of it still remaining to find its
way through the narrow and shallow Behring’s Strait into the North
Pacific.
Now, it seems to me that if we had only these curves upon the
chart, deduced from an almost infinite number of observations which
are themselves merely laboriously multiplied corroborations of
many previous ones, without having any clew to their rationale,
we should be compelled to admit that whatever might be the
amount and distribution of heat derived from a general oceanic
circulation--whether produced by the prevailing winds of the region,
by convection, by unequal barometric pressure, by tropical heat, or
by arctic cold--the Gulf Stream, the majestic stream of warm water
whose course is indicated by the deflections of the isothermal lines,
is sufficiently powerful to mask all the rest, and, broadly speaking,
to produce of itself all the abnormal thermal phenomena.
The deep-sea temperatures taken in the _Porcupine_ have an important
bearing upon this question, since they give us the depth and volume
of the mass of water which is heated above its normal temperature,
and which we must regard as the softener of the winds blowing on
the coasts of Europe. In the Bay of Biscay, after passing through a
shallow band superheated by direct radiation, a zone of warm water
extends to the depth of 800 fathoms, succeeded by cold water to a
depth of nearly two miles. In the Rockall channel the warm layer
has nearly the same thickness, and the cold underlying water is 500
fathoms deep. Off the Butt of Lewis the bottom temperature is 5°.2
C. at 767 fathoms, so that there the warm layer evidently reaches
to the bottom. In the Faroe channel the warm water forms a surface
layer, and the cold water underlies it, commencing at a depth of 200
fathoms--567 fathoms above the level of the bottom of the warm water
off the Butt of Lewis. The cold water abuts against the warm--there
is no barrier between them. Part of the warm water flows over the
cold indraught, and forms the upper layer in the Faroe channel. What
prevents the cold water from slipping, by virtue of its greater
weight, under the warm water of the Butt of Lewis? It is quite
evident that there must be some force at work keeping the warm water
in that particular position, or, if it be moving, compelling it to
follow that particular course. The comparatively high temperature
from 100 fathoms to 900 fathoms I have always attributed to the
northern accumulation of the water of the Gulf Stream. The amount of
heat derived directly from the sun by the water as it passes through
any particular region, must be regarded, as I have already said, as
depending almost entirely upon latitude. Taking this into account,
the surface temperatures in what we were in the habit of calling the
“warm area” coincided precisely with Petermann’s curves indicating
the northward path of the Gulf Stream.
[Illustration: Typical Forms of Snowflakes
Showing the Tendency to take the Form of Six-Pointed Figures]
The North Atlantic and Arctic seas form together a _cul de sac_
closed to the northward, for there is practically no passage for
a body of water through Behring’s Strait. While, therefore, a large
portion of the water, finding no free outlet toward the northeast,
turns southward at the Azores, the remainder, instead of thinning
off, has rather a tendency to accumulate against the coasts bounding
the northern portions of the trough. We accordingly find that it
has a depth off the west coast of Iceland of at least 4,800 feet,
with an unknown lateral extension. Dr. Carpenter, discussing this
opinion, says: “It is to me physically inconceivable that this
surface film of _lighter_ (because warmer) water should collect
itself together again--even supposing it still to retain any excess
of temperature--and should burrow downward into the ‘trough,’
_displacing colder and heavier water_, to a depth much greater than
that which it possesses at the point of its greatest ‘glory’--its
passage through the Florida Narrows. The upholders of this hypothesis
have to explain how such a recollection and dipping-down of the
Gulf Stream water is to be accounted for on physical principles.” I
believe that, as a rule, experimental imitations on a small scale are
of little use in the illustration of natural phenomena; a very simple
experiment will, however, show that such a process is possible. If we
put a tablespoonful of cochineal into a can of hot water, so as to
give it a red tint, and then run it through a piece of India-rubber
tube with a considerable impulse along the surface of a quantity of
cold water in a bath, we see the red stream widening out and becoming
paler over the general surface of the water till it reaches the
opposite edge, and very shortly the rapidly heightening color of a
band along the opposite wall indicates an accumulation of the colored
water where its current is arrested. If we now dip the hand into the
water of the centre of the bath, a warm bracelet merely encircles the
wrist; while at the end of the bath opposite the warm influx, the hot
water, though considerably mixed, envelops the whole hand.
The North Atlantic forms a basin closed to the northward. Into the
corner of this basin, as into a bath--with a northeasterly direction
given to it by its initial velocity, as if the supply pipe of the
bath were turned so as to give the hot water a definite impulse--this
enormous flood is poured, day and night, winter and summer. When the
basin is full--and not till then--overcoming its northern impulse,
the surplus water turns southward in a southern eddy, so that there
is a certain tendency for the hot water to accumulate in the northern
basin, to “bank down” along the northeastern coasts.
It is scarcely necessary to say that for every unit of water which
enters the basin of the North Atlantic, and which is not evaporated,
an equivalent must return. As cold water can gravitate into the
deeper parts of the ocean from all directions, it is only under
peculiar circumstances that any movement having the character of
a current is induced; these circumstances occur, however, in the
confined and contracted communication between the North Atlantic and
the Arctic Sea. Between Cape Farewell and North Cape there are only
two channels of any considerable depth, the one very narrow along
the east coast of Iceland, and the other along the east coast of
Greenland. The shallow part of the sea is entirely occupied, at all
events during summer, by the warm water of the Gulf Stream, except
at one point, where a rapid current of cold water, very restricted
and very shallow, sweeps round the south of Spitzbergen and then dips
under the Gulf Stream water at the northern entrance of the German
Ocean.
This cold flow, at first a current, finally a mere indraught, affects
greatly the temperature of the German Ocean; but it is entirely
lost, for the slight current which is again produced by the great
contraction at the Strait of Dover has a summer temperature of 7°.5
C. The path of the cold indraught from Spitzbergen may be readily
traced by the depressions in the surface isothermal lines, and in
dredging by the abundance of gigantic amphipodous and isopodous
crustaceans, and other well-known Arctic animal forms.
From its low initial velocity the Arctic return current, or
indraught, must doubtless tend slightly in a westerly direction, and
the higher specific gravity of the cold water may probably even more
powerfully lead it into the deepest channels; or possibly the two
causes may combine, and in the course of ages the currents may hollow
out deep southwesterly grooves. The most marked is the Labrador
current, which passes down inside the Gulf Stream along the coasts
of Carolina and New Jersey, meeting it in the strange abrupt “cold
wall,” dipping under it as it issues from the Gulf, coming to the
surface again on the other side, and a portion of it actually passing
under the Gulf Stream, as a cold counter-current, into the Gulf of
Mexico.
Fifty or sixty miles out from the west coast of Scotland, I believe
the Gulf Stream forms another, though a very mitigated, “cold wall.”
In 1868, after our first investigation of the very remarkable cold
indraught into the channel between Shetland and Faroe, I stated my
belief that the current was entirely banked up in the Faroe Channel
by the Gulf Stream passing its gorge. Since that time I have been led
to suspect that a part of the Arctic water oozes down the Scottish
coast, much mixed, and sufficiently shallow to be affected throughout
by solar radiation. About sixty or seventy miles from shore the
isothermal lines have a slight but uniform deflection. Within that
line types characteristic of the Scandinavian fauna are numerous in
shallow water, and in the course of many years’ use of the towing net
I have never met with any of the Gulf Stream pteropods, or of the
lovely Polycystina and Acanthometrina which absolutely swarm beyond
that limit. The difference in mean temperature between the east and
west coasts of Scotland, amounting to about 1° C., is almost somewhat
less than might be expected if the Gulf Stream came close to the
western shore.
While the communication between the North Atlantic and the Arctic
Sea--itself a second _cul de sac_--is thus restricted, limiting the
interchange of warm and cold water in the normal direction of the
flow of the Gulf Stream, and causing the diversion of a large part
of the stream to the southward, the communication with the Antarctic
basin is as open as the day; a continuous and wide valley upward of
2,000 fathoms in depth stretching northward along the western coasts
of Africa and Europe.
That the southern water wells up into this valley there could be
little doubt from the form of the ground; but here again we have
curious corroborative evidence in the remarkable reversal of the
curves of the isotherms. The temperature of the bottom water at 1,230
fathoms off Rockall is 3°.22 C., exactly the same as that of water
at the same depth in the serial sounding, lat. 47° 38′ N., long.
12° 08′ W. in the Bay of Biscay, which affords a strong presumption
that the water in both cases is derived from the same source; and
the bottom water off Rockall is warmer than the bottom water in the
Bay of Biscay (2°.5 C.), while a cordon of temperature soundings
drawn from the northwest of Scotland to a point on the Iceland
shallow gives no temperature lower than 6°.5 C. This makes it very
improbable that the low temperature of the Bay of Biscay is due to
any considerable portion of the Spitzbergen current passing down
the west coast of Scotland; and as the cold current to the east of
Iceland passes southward considerably to the westward, as indicated
by the successive depressions in the surface isotherms, the balance
of probability seems to be in favor of the view that the conditions
of temperature and the slow movement of this vast mass of moderately
cold water, nearly two statute miles in depth, are to be referred to
an Antarctic rather than to an Arctic origin.
The North Atlantic Ocean seems to consist first of a great sheet of
warm water, the general northerly reflux of the equatorial current.
Of this the greater part passes through the Strait of Florida, and
its northeasterly flow is aided and maintained by the anti-trades,
the whole being generally called the Gulf Stream. This layer is of
varying depths, apparently from the observations of Captain Chimmo
and others, thinning to a hundred fathoms or so in the mid-Atlantic,
but attaining a depth of 700 to 800 fathoms off the west coasts of
Ireland and Spain. Secondly, of a “stratum of intermixture” which
extends to about 200 fathoms in the Bay of Biscay, through which the
temperature falls rather rapidly; and, thirdly, of an underlying mass
of cold water, in the Bay of Biscay 1,500 fathoms deep, derived as
an indraught falling in by gravitation from the deepest available
source, whether Arctic or Antarctic. It seems at first sight a
startling suggestion, that the cold water filling deep ocean valleys
in the Northern Hemisphere may be partly derived from the southern;
but this difficulty, I believe, arises from the idea that there is a
kind of diaphragm at the equator between the northern and southern
ocean basins, one of the many misconceptions which follow in the
train of a notion of a convective circulation in the sea similar to
that in the atmosphere. There is undoubtedly a gradual elevation
of an intertropical belt of the underlying cold water, which is
being raised by the subsiding of still colder water into its bed
to supply the place of the water removed by the equatorial current
and by excessive evaporation; but such a movement must be widely
and irregularly diffused and excessively slow, not in any sense
comparable with the diaphragm produced in the atmosphere by the
rushing upward of the northeast and southeast trade-winds in the zone
of calms. Perhaps one of the most conclusive proofs of the extreme
slowness of the movement of the deep indraught is the nature of the
bottom. Over a great part of the floor of the Atlantic a deposit
is being formed of microscopic shells. These with their living
inhabitants differ little in specific weight from the water itself,
and form a creamy flocculent layer, which must be at once removed
wherever there is a perceptible movement. In water of moderate depth,
in the course of any of the currents, this deposit is entirely
absent, and is replaced by coarser or finer gravel.
It is only on the surface of the sea that a line is drawn between the
two hemispheres by the equatorial current, whose effect in shedding a
vast intertropical drift of water on either side as it breaks against
the eastern shores of equatorial land may be seen at a glance on the
most elementary physical chart.
The Gulf Stream loses an enormous amount of heat in its northern
tour. At a point 200 miles west of Ushant, where observations at the
greatest depths were made on board the _Porcupine_, a section of
the water of the Atlantic shows three surfaces at which interchange
of temperature is taking place. First, the surface of the sea--that
is to say, the upper surface of the Gulf Stream layer--is losing
heat rapidly by radiation, by contact with a layer of air which is
in constant motion and being perpetually cooled by convection, and
by the conversion of water into vapor. As this cooling of the Gulf
Stream layer takes place principally at the surface, the temperature
of the mass is kept pretty uniform by convection. Secondly, the
band of contact of the lower surface of the Gulf Stream water with
the upper surface of the cold indraught. Here the interchange of
temperature must be very slow, though that it does take place is
shown by the slight depression of the surface isotherms over the
principal paths of the indraught. But there is a good deal of
intermixture extending through a considerable layer. The cold water
being beneath, convection in the ordinary sense can not occur,
and interchange of temperature must depend mainly upon conduction
and diffusion, causes which in the case of masses of water must
be almost secular in their action, and probably to a much greater
extent upon mixture produced by local currents and by the tides. The
third surface is that of contact between the cold indraught and the
bottom of the sea. The temperature of the crust of the earth has been
variously calculated at from 4° to 11° C., but it must be completely
cooled down by anything like a movement and constant renewal of cold
water. All we can say, therefore, is that contact with the bottom can
never be a source of depression of temperature. As a general result
the Gulf Stream water is nearly uniform in temperature throughout the
greater part of its depth; there is a marked zone of intermixture at
the junction between the warm water and the cold, and the water of
the cold indraught is regularly stratified by gravitation; so that
in deep water the contour lines of the sea-bottom are, speaking
generally, lines of equal temperature. Keeping in view the enormous
influence which ocean currents exercise in the distribution of
climates at the present time, I think it is scarcely going too far to
suppose that such currents--movements communicated to the water by
constant winds--existed at all geological periods as the great means,
I had almost said the sole means, of producing a general oceanic
circulation, and thus distributing heat in the ocean. They must have
existed, in fact, wherever equatorial land interrupted the path of
the drift of the trade-winds. Wherever a warm current was deflected
to north or south from the equatorial belt a polar indraught crept in
beneath to supply its place; and the ocean consequently consisted,
as in the Atlantic and doubtless in the Pacific at the present day,
of an upper warm stratum and a lower layer of cold water becoming
gradually colder with increasing depth.
I must repeat that I have seen as yet no reason to modify the opinion
which I have consistently held from the first, that the remarkable
conditions of climate on the coasts of Northern Europe are due in a
broad sense solely to the Gulf Stream. That is to say that, although
movements, some of them possibly of considerable importance, must be
produced by differences of specific gravity, yet the influence of the
great current which we call the Gulf Stream, the reflux of the great
equatorial current, is so paramount as to reduce all other causes to
utter insignificance.
THE PHOSPHORESCENCE OF THE SEA
--G. HARTWIG
He who still lingers on the shore after the shades of evening have
descended not seldom enjoys a most magnificent spectacle; for lucid
flashes burst from the bosom of the waters, as if the sea were
anxious to restore to the darkened heavens the light it had received
from them during the day. On approaching the margin of the rising
flood to examine more closely the sparkling of the breaking wave, the
spreading waters seem to cover the beach with a sheet of fire.
Each footstep over the moist sands elicits luminous star-like points
and a splash in the water resembles the awakening of slumbering
flames. The same wonderful and beauteous aspect frequently gladdens
the eye of the navigator who plows his way through the wide deserts
of ocean, particularly if his course leads him through the tropical
seas.