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6°2 Fahr. along the western coast of Scotland and its | thermometric observations made in the voyages of D'Urville islands, and rising to 14°•5 at Fruholm near the North Cape. and Sir James Ross. No such precaution was taken, And it is also a very significant fact (ascertained by the care- however, in these observations as that to which Lenz had ful inquiries of Mr Buchan), that while the summer isotherms recourse, to obviate the effects of the tremendous pressure cross the British Islands nearly east and west (the tempera- | (1 ton per square inch for every 800 fathoms of depth) to ture diminishing pretty regularly from south to north), the which deep-sea thermometers are exposed; and it is now winter isotherms traverse them nearly north and south (the certain that the temperatures at great depths recorded by temperature diminishing from west to east); whilst in D'Urville and Russ were several degrees too high. Ireland the isotherms seem to envelope the islands in their It was in entire ignorance of the doctrine of Lenz, and folds, which increase in warmth from the centre of the under the influence of that of D'Urville and Ross, which island to its sea-board. So in Norway the isothermal lines had been stamped with the great weight of Sir John run parallel to the coast-line, and this alike in summer and Herschel's weight of authority, that Dr Carpenter comin winter,—the temperature falling in winter, and rising menced in 1868 (in concert with Professor Wyville Thomin summer, with the increase of distance from the sea.
son) a course of inquiry into the thermal condition of Nothing could prove more conclusively than such facts as the deep sea, which at once convinced him of the fallacy these (taken in connection with the absence of ice in the of the uniform 39° doctrine, and led him to conclusions harbours of Norway, even as far north as Hammerfest, essentially accordant with those of Lenz. For in the through the whole winter) the dependence of the mild channel of from 500 to 600 fathoms' depth between the winter climate of the north-western coasts of Europe north of Scotland and the Faroe Islands, they found the upon the proximity of a sea which is warmer than the deeper half to be occupied by a stratum of glacial water, superincumbent atmosphere ; and we have now to inquire whose temperature ranged downwards from 32° to 29o.5 ; how this great N. E. movement of a stratum of warm water whilst the upper half was occupied by a stratum warmer sufficiently thick to retain a surface-temperature con than the normal temperature of the latitudes. This siderably higher than that of the air above it is to be phenomenon was interpreted by Carpenter as indicataccounted for.
ing a deep glacial flow from N.E. to S.W., and a warm The solution of the problem seems to be afforded by the upper flow from S.W. to N.E.; and finding that to the doctrine of a General anic Circulation, sustained by west of this channel, on the border of the deep Atlantic opposition of temperature only, which was first distinctly basin, the excess of warmth extended to a depth of more propounded in 1845 by Professor Lenz of St Petersburg, than 500 fathoms, he came to the conclusion that the on the basis of observations made by him during the north-moving stratum which brought it could not be an second voyage of Kotzebue (1825-1828). Others had extension of the true Gulf Stream, but must be urged on been previously led to surmise that “Polar Currents” flow by some much more general force. A series of temperaalong the floors of the great oceans, even as far as the ture-soundings taken along the west of Ireland, the Bay of equator, balancing the superficial counter-currents which Biscay, and the coast of Portugal
, confirmed him in this are observable in the opposite direction, But Lenz was view, by showing that the division between an upper warm led to conclude that the whole of the deeper portion of the stratum and a cold under-stratum exists in the North great ocean-basins in communication with the polar areas Atlantic at a depth of from 700 to 900 fathoms, the whole is occupied by polar water, which is constantly, though mass of water below this having either flowed into the slowly, flowing towards the equator ; whilst conversely basin from the polar area, or having had its temperature the whole upper stratum of equatorial water is as con brought down to from 39° to 36°.5 by mixture with the stantly, though slowly, flowing towards one or both of the polar inflow. And this conclusion was confirmed by the poles. And he particularly dwelt on the existence of a result of temperature-soundings taken at corresponding belt of water under the equator, colder than that which depths and under the same parallels of latitude in the lies either north or south of it, as an evidence that polar Mediterranean; for as they showed a uniform temperature water is there continually rising from beneath towards of from 54° to 56°, from beneath the stratum of 100 the surface,-a phenomenon which, he considered, admits fathoms that was superheated by direct insolation, to the of no other explanation. He further adduced the low very bottom, it became clear that depth per se could have salinity of equatorial water (previously noticed by Hum- no effect in reducing the bottom-temperature ; and that boldt, and confirmed by his own observations), compared the cause of the excess of temperature in the mass of water with that of tropical water, as evidence that the equatorial occupying the Mediterranean basin above that of Atlantic water of the surface is derived from the polar underflow. water at the same depths, lies in the seclusion of the former And he attributed the maintenance of this circulation to from the polar undertlow which brings down the deep the continually renewed disturbance of equilibrium between temperature of the latter. This conclusion having received the polar and equatorial columns,—the greater lateral marked confirmation from temperature-soundings taken in (because downward) pressure of the former causing a the Eastern seas, was put forward by Carpenter as justifybottom outflow of polar water in the direction of the ing the doctrine of a vertical oceanic circulation sustained latter, whilst the reduction of level thus occasioned will by opposition of temperature only, quite independent of produce a surface indraught from the warmer towards and distinct from the horizontal circulation produced by the colder areas.
wind,—which doctrine he expressed in terms closely corThe doctrine of Lenz, so far from meeting with the responding with those that had been used by Lenz. And general acceptance to which it had a fair claim,-alike on the collection of data for the establishment or confutation theoretical grounds and from its accordance with the facts of this doctrine was one of the objects of the “Challenger" ascertained by careful observation,-seems to have been put expedition, which has already made, in the determination aside and forgotten, a preference being given to the doc- of the thermal stratification of the Atlantic between 38° trine of the prevalence of a uniform deep-sea temperature N. lat. and 38° S. lat., what may be fairly characterised as of 39°, which was supposed to be established by the the grandest single contribution ever yet made to terrestrial
physics. 1 It must be borne in mind that sea water does not expand like The following are the most important of the facts thus fresh water in cooling below 39o. 2, but continues to contract down to its freezing point, which lies between 27° and 25° Fahr., according as See his Physical Geography of the Globe, originally published in it is still or agitated.
the eighth edition of this Encyclopædia.
established :-Of the water which fills the deep trough of and St Thomas (lat. 18}° N.), divided by the “Dolphin the North Atlantic (fig. 1) between Teneriffe (lat. 28.1. N.) rise" into an eastern and western basin, by far the larger
Fig. 1.-Section of North Atlantic Ocean between St Thomas and Teneriffe. mass has a temperature ranging from 40° downwards, in 347° S.), this trough also being divided into two basins by the eastern basin, to a bottom-temperature of 35%, whilst the elevation of the bottom which culminates in the island in the western basin-apparently under the influence of of Tristan da Cunha. The temperature of the water that the Antarctic underflow-the bottom-temperature sinks to occupies it, however, is lower through its whole vertical 34°4. A tolerably regular descent is shown in this sec- range than that of the North Atlantic. The stratification tion, from a surface-temperature rising near St Thomas to is nearly uniform from the surface downwards to the 75°, to the bathymetrical isotherm of 45°, which lies be- isotherm of 40°, which lies at from 300 to 450 fathoms' tween 400 and 600 fathoms' depth ; there is then a stratum depth, the isotherms of 39° and 38° also lying within between 45o and 40°, of which the thickness varies from about 500 fathoms; there is then à slower reduction about 250 to 450 fathoms, the isotherm of 40° lying at down to the isotherm of 35°, which lies between 1400 between 750 and 1000 fathoms' depth, while below this, and 1800 fathoms; while the whole sea-bed is covered by down to the bottom at between 2000 and 3000 fathoms, a stratum of about 600 fathoms' thickness, whose temperathe further reduction to 34°4 is very gradual.
ture ranges downwards from 35° to 33°. The whole of The same general condition prevails in the South this deepest stratum is colder than any water that is found Atlantic (fig. 2), between Abrolhos Island (lat. 18° S.) on in the corresponding portion of the North Atlantic, except the coast of Brazil, and the Cape of Good Hope (lat. near St Thomas
It is not a little remarkable that the sub-surface stratum | North Atlantic (fig. 1), and 100 fathoms nearer to the of water, having a temperature above 40°, is thinner under surface than even in the colder South Atlantic (fig. 2); the equator than it is in any other part of the Atlantic whilst the temperature of the bottom is but little above from the Faroe Islands to the Cape of Good Hope. Not- 32°. Thus the influence of the polar underflow is more withstanding the rise of the surface-temperature to 760-80°, pronounced under the equator than it is elsewhere; as is the thermometer descends in the first 300 fathoms more distinctly seen in the section shown in fig. 4, which is rapidly than anywhere else; so that polar water is met taken in a north and south direction so as to exhibit the with, as shown in fig. 3, at a much less depth than in the relation of the thermal stratification of the North to that
of the South Atlantic, and of both to that of the equatorial depth of about 700 fathoms, gradually rises as the equator
is approached ; and it is between the equator and 7° S., Tamandarone SECTION OF EQUATORIAL ATLANTIC.
where the surface-temperature rises to nearly 80°, that cold water is soonest reached,—the isotherm of 40° rising to within 300 fathoms of the surface, while that of 55°, which in lat. 38° N. lies at nearly 400 fathoms' depth, and in lat. 22° N. at about 250 fathoms, actually comes up under the equator within 100 fathoms of the surface. At the same time, while the bottom-temperature under the equator is the lowest anywhere met with, namely, 32°:4,1 the thickness of the stratum beneath the isotherm of 35° is not less than 600 fathoms. In passing southwards, the superficial isotherms are observed to separate again from each other, partly by the reduction of the surface-temperature, and partly by the descent of the isotherm of 40° to a depth of something less than 400 fathoms, which it keeps with little reduction as far south as the Cape of Good Hope. The
significance of these facts becomes more remarkable, when FIG. 3.-Section of Equatorial Atlantic.
we consider that if a portion of the oceanic area under the belt. The isotherm of 40°, which in lat. 22° N. lies at a equator were to be secluded, like the Mediterranean or the
2800 3000 1200
Red Sea, from all but local influences, the temperature of continual ascent of glacial water under the line, showing itself by its water from the sub-surface stratum downwards to the
a nearer approach of cold water to the surface in the inter-tropical
than in the extra-tropical zone. bottom-whatever its depth-would be its isocheimal or mean winter-temperature, which, in the equatorial zone, It was further pointed out by Lenz, and more recently would be certainly not below 75°.
(in ignorance of his doctrine) hy Carpenter, that additional Nothing, Dr Carpenter contends, could more conclu evidence of such ascent is furnished by the low salinity sively support the general doctrine of a Vertical Oceanic of the surface-water of the equatorial belt corresponding Circulation sustained by opposition of temperature, than with that of polar water. For, as was originally observed the precise conformity of the facts thus determined by by Humboldt, then by Lenz himself, and subsequently by observation to the predictions which his confidence in the many other voyagers, the specific gravity of the surfacetheory had led him to put forth. These predictions were water of the Atlantic gradually increases as either tropic is essentially as follows
approached from the polar side of its own hemisphere, “1. That instead of the local depressions of bottom-temperature reaches its maximum a little nearer the equator, and then imputed by previous writers to polar currents, the temperature of rapidly diminishes, coming down under the equator to the every part of the deep sea-bed in communication with either of the standard of polar water. Thus a mean of eight observations polar areas would be not many degrees above that of the polar taken in the “ Challenger” expedition between Bermuda areas themselves.
“ 2. That this general depression of bottom-temperature would (32° N.) and St Thomas (184° N.) gave 1027-2 as the be found to depend, not upon such a shallow glacial stream might sp. gr. of surface-water, whilst a mean of seventeen observabe maintained to be a return from the polar areas of water propelled tions between the Cape Verd Islands (161° N.) and Bahia towards them by wind-currents, but upon a creeping flow of the (13° S.) gave a sp. gr. of only 1026-3. Now, since between whole under-stratum, having a thickness of from 1000 to 2000 fathoms.
St Thomas and Bermuda the eight “ Challenger' observa“3. That as the depression of bottom-temperature in any part tions of bottom (polar) water gave a mean sp. gr. of 1026-3, of the general oceanic basin would be proportional to the freedom whilst between Cape Verd and Bahia the mean sp. gr. oil of communication between its deeper portion and that of one or
the bottom-water was even slightly lower (the results being other of the polar areas, the bottom-temperature of the South Atlantic would probably range downwards to 32°, while that of the North Atlantic would not be below 35°, except where it first receives the 1 That the bottom-temperature beneath the equator was lower than Arctic flow, or comes under the influence of the Antarctic underflow, any that was met with in the South Atlantic, is attributable to the cirwhich would very probably extend itself to the north of the equator. cumstance that, in consequence of unfavourable weather, the tempera
“4. That as the Arctic and Antarctic underflows must meet at or ture-soundings were taken at intervals too wide to detect the deep near the equator, whilst the surface-stratum is there continually channel through which the coldest Antarctic water doubtless flowed tobeing draughted off thence towards either pole, there would be a wards the equator.
in all cases expressed according to a common standard of seen that, although the surface-temperature is reduced temperature), such a close conformity subsists between the by the thinning-out of the superficial stratum, there is salinity of the equatorial water of the surface and that of but a slight change in the position of the bathymetrical the polar waters of the bottom, as can scarcely be accounted isotherms of 45° and 40°; so that there is an obvious for in any other way than by the continual and tolerably continuity of a stratum of many hundred fathoms' thickrapid ascent of the latter.
ness between these two points, notwithstanding their sepaAnother indication of this ascent is given by the moder ration by 251° of latitude. The contrast between the ation of the surface-temperature of oceanic water, even position of the isotherm of 40° at 800 fathoms' depth off under the equator. If there were no ascent of colder the Faroes, and its position at less than 300 fathoms' water from beneath, there seems no reason why the constant depth under the equator, is most remarkable. We have powerful insolation to which equatorial water is subjected seen that the isocheimal in the latter area would not be should not raise the temperature of its surface to the below 75°, and yet we find water colder than 40° lying highest possible elevation. The limit to that elevation, at within 300 fathoms of the surface; whilst, on the which is obviously set by the cooling influence of evapora- other hand, the normal isocheimal at 59° N. would tion, is probably that which is met with in the Red Sea, certainly be below 40° (probably no more than 35°), and where the monthly average for August rises to 864° and yet we find water above 43° extending downwards to 600 for September to 88°, whilst the maxima rise much higher, fathoms, and water above 40° to 800 fathoms. Thus the temperatures of 100°, 106', 100°, and 96° having been vertical oceanic circulation carries a vast mass of water noted on four consecutive days. Moreover, along the Guinea which is below the normal off the coast of Portugal, into a Coast, and especially in the Bight of Biafra, the surface- region where it is above the normal, with very little loss of temperature is stated to range as high as 90°. But in heat by the way, except in its surface-film; and a little these cases there is no reduction of surface-temperature by consideration will show that such a movement must be the upward movement of polar water; for this is altogether much more effectual as a heater than a corresponding moveexcluded from the Red Sea by the shallowness of the ment of a thin stratum of much warmer water. For the Strait of Babelmandeb, whilst the depth of the bottom latter, when it passes beneath an atmosphere much colder along the Guinea Coast is too small to allow of its being than itself, will soon be brought down to a like standard, overflowed by the glacial stratum. Now, over the deeper not having warmer water from below to take its place when parts of the equatorial Atlantic the surface-temperature it has been cooled down; whilst in the former, each surusually ranges between 75° and 80°; and this is its ordi- face-layer, when cooled below the temperature of the warmer nary range in the Mediterranean during the months of stratum beneath, will sink and be replaced by it. Now August and September. That the temperature of an equa- since the true Gulf Stream, when we last know it, has been torial ocean should be thus kept down to that of a sea of so thinned out that it could not long retain any excess of which the greater part lies between the parallels of 40° and temperature, it seems inconceivable that it should exert any 35°, can scarcely be accounted for in any other way than decided effect on the temperature of the Faroes and the by the continual uprising of polar waters from beneath. coast of Norway, unless (as supposed by Dr Petermann and
The same principle, once admitted, fully accounts for Professor Wyville Thomson) its thickness undergoes an that amelioration of the cold of north-western Europe, increase from less than 100 fathoms to 600. But since the which (as already shown) cannot be fairly attributed to course of Dr Petermann's isotherms shows that the norththe Florida Current or true Gulf Stream. For it is obvious ward flow extends across the whole breadth of the Atlantic that a continual efflux of the lower stratum from the polar between Newfoundland and the British Isles—a distance areas towards the equatorial must involve a continual of about 2000 miles—we are required to believe that a indraught of the upper stratum towards the polar areas; rivulet (for such it is by comparison) of 60 miles' breadth and this indraught will be much more marked in the and 100 fathoms' depth (see section, fig. 5), of which the Northern than in the Southern Atlantic, on account of the greater part turns southwards round the Azores, and of progressive narrowing of the former, whilst the latter which the remainder is flowing due east when we last progressively widens out. Of such a slow northerly set of recognise it, is able to impart a northerly movement to a a stratum of water, extending downwards to a depth of at stratum of 2000 miles in breadth, and at least 600 fathoms' least 600 fathoms, we have evidence in a comparison of the depth. On the other hand, the eastward set of this temperature-soundings taken in the “Porcupine” expedi- stratum, considered as a northward indraught into the
polar area, is readily accounted for by the excess of easterly momentum which it derives from the earth's rotation, this being only half as rapid in lat. 60° as it is under the equator; and since there is a still more rapid reduction in the rate of this rotation in yet higher latitudes, the continually increasing excess of easterly momentum will give to the northward flow a progressively stronger eastward set.
On the other hand, the deficiency of easterly momentum in the cold underflow coming from the pole towards the equator will tend to produce a lagging-behind, or westward set of that underflow; and this has been shown by the “ Challenger” temperature-soundings to be the case, the cold deep strata of the Western Atlantic surging upwards along the slope of the North American coast-line, as is shown in fig. 6, where we see not only the bathymetrical isotherms of 60°, 55°, and 50°, but the yet deeper isotherms
of 45° and 40°, successively rising to the surface as we F10, 5.-Section of North Atlantic, taken nearly north and south.
approach the land; while at a depth of only 83 fathoms, tions of 1869 and 1870, between the coast of Portugal a temperature of 35° was encountered, which, at no great (34° N.) and the Faroe Islands (591° N.), from which distance to the south, would only be found at a depth of the section fig. 5, has been worked out. For it is there 2000 fathoms. That the cold water should thus run up
Deltasol the Gulf Stream
FROM 40 TO 45*
FROM 35'14 40
are never seen.
300 400 500 600 Horizontal Scale of Nautical Miles.
hill is quite conformable to what we see in other cases, in than that of anchorage), the enormous mass of Gulf-weed which a heavier under-stratum has a definite set towards a found in the Sargasso Sea seems quite independent of any slope; and whilst
such attachment. It was at one time supposed that this the existence of
originally grew on the Bahama and Florida shores, and such a westerly
was torn thence by the powerful current of the Gulf set is, ex hypo
Stream ; but it seems certain that if such was its original thesi, a necessary
source, the “Gulf-weed” now lives and propagates whilst consequence of
freely floating on the ocean-surface, having become adapted the southerly
by various modifications to its present mode of existence. movement of the
The distribution of the animals that habitually live in that Arctic under
upper stratum of the ocean whose degree of warmth varies flow, no other
with the latitude, seems mainly determined by temperature. explanation of
Thus the “right whale" of Arctic seas, and its representative it has been sug
in the Antarctic, seems never to enter the inter-tropical area, gested. We now $ 1400
generally keeping away from even the temperate seas, whilst, see that the cold
on the other hand, the sperm-whale ranges through the parts Labrador Cur
of the ocean where the “ right whales rent overlies a
The distribution of fishes seems generally to follow the band of water as
same rule ; as does also that of floating mollusks. Thus cold as itself ;
the little Clio (a Pteropod mollusk), which is a principal and the south
article of the food of the "right whales" in polar seas, is ward extension
rarely met with in the Atlantic, where, however, other of this cold band,
pteropods, as Hyalæa, present themselves in abundance. far beyond that
On the other hand, the warmer parts of its area swarm with of any definite
Salpa-chains, which are not frequent in higher latitudes; current
and the few representatives of the Nautiloid Cephalopods, Fig. 6.--Section from Bermuda to Halifax. ment, and its
that were so abundant in Cretaceous seas, are now restricted entrance into the Gulf of Mexico, through the Florida to tropical or sub-tropical areas. And the distribution of the Channel, at the side of and beneath the outflowing Gulf mollusks, echinoderms, and corals, which habitually live on Stream, are thus accounted for.
the bottom, seems to be determined, within certain limits The remarkable accordance of so many facts of actual at least, by temperature rather than by depth. observation, in the Atlantic area, with the probabilities The bathymetrical range to which animal life of any deducible from a theory whose soundness can scarcely be higher type than the Rhizopodal might extend, was until disputed, seems now to justify the admission of the general recently quite unknown; but the researches initiated by (vertical) oceanic circulation sustained by opposition of Prof. Wyville Thomson and Dr Carpenter in 1868, and since temperature as an accepted doctrine of terrestrial physics. prosecuted by the “Challenger" expedition, have fully
Distribution of Organic Life.—All that will be attempted established the existence of a varied and abundant fauna in under this head will be to indicate the general conditions ocean-depths ranging downwards to 2000 fathoms. And that seem, from recent researches, to have the greatest these researches have further established that the distribuinfluence on the distribution of plants and animals through tion of this fauna is mainly determined by the temperathis great oceanic basin.
ture of the sea-bed ; so that whilst in the channel between The distribution of marine plants seems mainly deter- the north of Scotland and the Faroes there were found at mined by light, temperature, and depth,-a further influence the same depths, and within a few miles of each other, two being exerted by the character of the shores. The diminu- faunæ almost entirely distinct-one a boreal and the other tion of light in its passage through sea-water is so rapid, a warmer-temperate-on sea-beds having respectively the that the quantity which penetrates to a depth of 250 or temperatures of 30° and 43°, various types to which a low 300 fathoms may be regarded as almost infinitesimal ; and temperature is congenial are traceable continuously along in conformity with this we find a very rapid diminution the whole abyssal sea-bed that intervenes between those of Algal life below the depth of 100 fathoms. The upper northern and southern polar areas within which they stratum is occupied for the most part by the larger and present themselves at or near the surface. And hence it coarser forms of the Fucaceæ, or olive-green sea-weeds, becomes clear that, since glacial types are even now being whilst the more delicate Ceramiaceæ, or red sea-weeds, fre- embedded in the strata which are in process of formation quent deeper waters ; and, as it appears from experiments beneath the equator, no inferences as to terrestrial climate made in aquaria that the latter do not flourish in full light, can be drawn from the character of marine deposits. but grow well in shadow, it may be concluded that their One very remarkable feature which presents itself over preference for a moderate depth is rather for reduced light a large proportion of the Atlantic basin is the abundand stillness than for depth per se. At a depth of 150 ance of the minute Globigerinæ and other Foraminifera, fathoms very few ordinary sea-weeds maintain their ground; the accumulation of whose shells, and of their disintegrated and below this we seldom find any Algæ, save the Coral remains, is giving rise to a calcareous deposit of unknown lines and Nullipores consolidated by calcareous deposit. thickness, that corresponds in all essential particulars to The distribution of particular types over different parts of Chalk. This deposit, in some parts of the North Atlantic, the Atlantic area appears to be mainly regulated by tem- is replaced by an Arctic drift of fine sand, whilst in other perature; and this would seem to be remarkably the case parts there is a mixture of arenaceous and of calcareous with the floating Diatomaceæ, which, though they form components, such as is found in certain beds of the green bands in the surface-water of polar seas, have not Cretaceous formation. Now on the surface of this deposit been encountered in like abundance in the Atlantic, and do there have been found so many living types, especially not contribute largely, by the subsidence of their siliceous belonging to the groups of Echinoderms, Corals, Siliceous loricæ, to the composition of its bottom-deposit. Although Sponges, and Foraminifera, which closely correspond with it is the habit of the larger Algæ to grow from a base of types hitherto regarded as characteristic of the Cretaceous attachment (their roots serving no other purpose however, I epoch, that the question naturally suggests itself whether