Discourses - Thomas H. Huxley
Pages:
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23
In this way, it would seem inevitable that the surface waters of the
northern and southern frigid zones must, sooner or later, find their way
to the bottom of the rest of the ocean; and there accumulate to a
thickness dependent on the rate at which they absorb heat from the crust
of the earth below, and from the surface water above.
If this hypothesis be correct, it follows that, if any part of the ocean
in warm latitudes is shut off from the influence of the cold polar
underflow, the temperature of its deeps should be less cold than the
temperature of corresponding depths in the open sea. Now, in the
Mediterranean, Nature offers a remarkable experimental proof of just the
kind needed. It is a landlocked sea which runs nearly east and west,
between the twenty-ninth and forty-fifth parallels of north latitude.
Roughly speaking, the average temperature of the air over it is 75 deg. Fahr.
in July and 48 deg. in January.
This great expanse of water is divided by the peninsula of Italy
(including Sicily), continuous with which is a submarine elevation
carrying less than 1,200 feet of water, which extends from Sicily to Cape
Bon in Africa, into two great pools--an eastern and a western. The
eastern pool rapidly deepens to more than 12,000 feet, and sends off to
the north its comparatively shallow branches, the Adriatic and the Aegean
Seas. The western pool is less deep, though it reaches some 10,000 feet.
And, just as the western end of the eastern pool communicates by a
shallow passage, not a sixth of its greatest depth, with the western
pool, so the western pool is separated from the Atlantic by a ridge which
runs between Capes Trafalgar and Spartel, on which there is hardly 1,000
feet of water. All the water of the Mediterranean which lies deeper than
about 150 fathoms, therefore, is shut off from that of the Atlantic, and
there is no communication between the cold layer of the Atlantic (below
1,000 fathoms) and the Mediterranean. Under these circumstances, what is
the temperature of the Mediterranean? Everywhere below 600 feet it is
about 55 deg. Fahr.; and consequently, at its greatest depths, it is some 20 deg.
warmer than the corresponding depths of the Atlantic.
It seems extremely difficult to account for this difference in any other
way, than by adopting the views so strongly and ably advocated by Dr.
Carpenter, that, in the existing distribution of land and water, such a
circulation of the water of the ocean does actually occur, as
theoretically must occur, in the universal ocean, with which we started.
It is quite another question, however, whether this theoretic
circulation, true cause as it may be, is competent to give rise to such
movements of sea-water, in mass, as those currents, which have commonly
been regarded as northern extensions of the Gulf-stream. I shall not
venture to touch upon this complicated problem; but I may take occasion
to remark that the cause of a much simpler phenomenon--the stream of
Atlantic water which sets through the Straits of Gibraltar, eastward, at
the rate of two or three miles an hour or more, does not seem to be so
clearly made out as is desirable.
The facts appear to be that the water of the Mediterranean is very
slightly denser than that of the Atlantic (1.0278 to 1.0265), and that
the deep water of the Mediterranean is slightly denser than that of the
surface; while the deep water of the Atlantic is, if anything, lighter
than that of the surface. Moreover, while a rapid superficial current is
setting in (always, save in exceptionally violent easterly winds) through
the Straits of Gibraltar, from the Atlantic to the Mediterranean, a deep
undercurrent (together with variable side currents) is setting out
through the Straits, from the Mediterranean to the Atlantic.
Dr. Carpenter adopts, without hesitation, the view that the cause of this
indraught of Atlantic water is to be sought in the much more rapid
evaporation which takes place from the surface of the Mediterranean than
from that of the Atlantic; and thus, by lowering the level of the former,
gives rise to an indraught from the latter.
But is there any sound foundation for the three assumptions involved
here? Firstly, that the evaporation from the Mediterranean, as a whole,
is much greater than that from the Atlantic under corresponding
parallels; secondly, that the rainfall over the Mediterranean makes up
for evaporation less than it does over the Atlantic; and thirdly,
supposing these two questions answered affirmatively: Are not these
sources of loss in the Mediterranean fully covered by the prodigious
quantity of fresh water which is poured into it by great rivers and
submarine springs? Consider that the water of the Ebro, the Rhine, the
Po, the Danube, the Don, the Dnieper, and the Nile, all flow directly or
indirectly into the Mediterranean; that the volume of fresh water which
they pour into it is so enormous that fresh water may sometimes be baled
up from the surface of the sea off the Delta of the Nile, while the land
is not yet in sight; that the water of the Black Sea is half fresh, and
that a current of three or four miles an hour constantly streams from it
Mediterraneanwards through the Bosphorus;--consider, in addition, that no
fewer than ten submarine springs of fresh water are known to burst up in
the Mediterranean, some of them so large that Admiral Smyth calls them
"subterranean rivers of amazing volume and force"; and it would seem, on
the face of the matter, that the sun must have enough to do to keep the
level of the Mediterranean down; and that, possibly, we may have to seek
for the cause of the small superiority in saline contents of the
Mediterranean water in some condition other than solar evaporation.
Again, if the Gibraltar indraught is the effect of evaporation, why does
it go on in winter as well as in summer?
All these are questions more easily asked than answered; but they must be
answered before we can accept the Gibraltar stream as an example of a
current produced by indraught with any comfort.
The Mediterranean is not included in the _Challenger's_ route, but she
will visit one of the most promising and little explored of
hydrographical regions--the North Pacific, between Polynesia and the
Asiatic and American shores; and doubtless the store of observations upon
the currents of this region, which she will accumulate, when compared
with what we know of the North Atlantic, will throw a powerful light upon
the present obscurity of the Gulf-stream problem.
III
ON SOME OF THE RESULTS OF THE EXPEDITION OF H.M.S. _CHALLLENGER_
[1875]
In May, 1873, I drew attention[1] to the important problems connected
with the physics and natural history of the sea, to the solution of which
there was every reason to hope the cruise of H.M.S. _Challenger_ would
furnish important contributions. The expectation then expressed has not
been disappointed. Reports to the Admiralty, papers communicated to the
Royal Society, and large collections which have already been sent home,
have shown that the _Challenger's_ staff have made admirable use of their
great opportunities; and that, on the return of the expedition in 1874,
their performance will be fully up to the level of their promise. Indeed,
I am disposed to go so far as to say, that if nothing more came of the
_Challengers_ expedition than has hitherto been yielded by her
exploration of the nature of the sea bottom at great depths, a full
scientific equivalent of the trouble and expense of her equipment would
have been obtained.
[Footnote 1: See the preceding Essay.]
In order to justify this assertion, and yet, at the same time, not to
claim more for Professor Wyville Thomson and his colleagues than is their
due, I must give a brief history of the observations which have preceded
their exploration of this recondite field of research, and endeavour to
make clear what was the state of knowledge in December, 1872, and what
new facts have been added by the scientific staff of the _Challenger_. So
far as I have been able to discover, the first successful attempt to
bring up from great depths more of the sea bottom than would adhere to a
sounding-lead, was made by Sir John Ross, in the voyage to the Arctic
regions which he undertook in 1818. In the Appendix to the narrative of
that voyage, there will be found an account of a very ingenious apparatus
called "clams"--a sort of double scoop--of his own contrivance, which Sir
John Ross had made by the ship's armourer; and by which, being in
Baffin's Bay, in 72 deg. 30' N. and 77 deg. 15' W., he succeeded in bringing up
from 1,050 fathoms (or 6,300 feet), "several pounds" of a "fine green
mud," which formed the bottom of the sea in this region. Captain (now Sir
Edward) Sabine, who accompanied Sir John Ross on this cruise, says of
this mud that it was "soft and greenish, and that the lead sunk several
feet into it." A similar "fine green mud" was found to compose the sea
bottom in Davis Straits by Goodsir in 1845. Nothing is certainly known of
the exact nature of the mud thus obtained, but we shall see that the mud
of the bottom of the Antarctic seas is described in curiously similar
terms by Dr. Hooker, and there is no doubt as to the composition of this
deposit.
In 1850, Captain Penny collected in Assistance Bay, in Kingston Bay, and
in Melville Bay, which lie between 73 deg. 45' and 74 deg. 40' N., specimens of
the residuum left by melted surface ice, and of the sea bottom in these
localities. Dr. Dickie, of Aberdeen, sent these materials to Ehrenberg,
who made out[2] that the residuum of the melted ice consisted for the
most part of the silicious cases of diatomaceous plants, and of the
silicious spicula of sponges; while, mixed with these, were a certain
number of the equally silicious skeletons of those low animal organisms,
which were termed _Polycistineoe_ by Ehrenberg, but are now known as
_Radiolaria_.
[Footnote 2: _Ueber neue Anschauungen des kleinsten noerdlichen
Polarlebens_.--Monatsberichte d. K. Akad. Berlin, 1853.]
In 1856, a very remarkable addition to our knowledge of the nature of the
sea bottom in high northern latitudes was made by Professor Bailey of
West Point. Lieutenant Brooke, of the United States Navy, who was
employed in surveying the Sea of Kamschatka, had succeeded in obtaining
specimens of the sea bottom from greater depths than any hitherto
reached, namely from 2,700 fathoms (16,200 feet) in 56 deg. 46' N., and 168 deg.
18' E.; and from 1,700 fathoms (10,200 feet) in 60 deg. 15' N. and 170 deg. 53'
E. On examining these microscopically, Professor Bailey found, as
Ehrenberg had done in the case of mud obtained on the opposite side of
the Arctic region, that the fine mud was made up of shells of
_Diatomacoe_, of spicula of sponges, and of _Radiolaria_, with a small
admixture of mineral matters, but without a trace of any calcareous
organisms.
Still more complete information has been obtained concerning the nature
of the sea bottom in the cold zone around the south pole. Between the
years 1839 and 1843, Sir James Clark Ross executed his famous Antarctic
expedition, in the course of which he penetrated, at two widely distant
points of the Antarctic zone, into the high latitudes of the shores of
Victoria Land and of Graham's Land, and reached the parallel of 80 deg. S.
Sir James Ross was himself a naturalist of no mean acquirements, and Dr.
Hooker,[3] the present President of the Royal Society, accompanied him as
naturalist to the expedition, so that the observations upon the fauna and
flora of the Antarctic regions made during this cruise were sure to have
a peculiar value and importance, even had not the attention of the
voyagers been particularly directed to the importance of noting the
occurrence of the minutest forms of animal and vegetable life in the
ocean.
[Footnote 3: Now Sir Joseph Hooker. 1894.]
Among the scientific instructions for the voyage drawn up by a committee
of the Royal Society, however, there is a remarkable letter from Von
Humboldt to Lord Minto, then First Lord of the Admiralty, in which, among
other things, he dwells upon the significance of the researches into the
microscopic composition of rocks, and the discovery of the great share
which microscopic organisms take in the formation of the crust of the
earth at the present day, made by Ehrenberg in the years 1836-39.
Ehrenberg, in fact, had shown that the extensive beds of "rotten-stone"
or "Tripoli" which occur in various parts of the world, and notably at
Bilin in Bohemia, consisted of accumulations of the silicious cases and
skeletons of _Diatomaceoe_, sponges, and _Radiolaria_; he had proved that
similar deposits were being formed by _Diatomaceoe_, in the pools of the
Thiergarten in Berlin and elsewhere, and had pointed out that, if it were
commercially worth while, rotten-stone might be manufactured by a process
of diatom-culture. Observations conducted at Cuxhaven in 1839, had
revealed the existence, at the surface of the waters of the Baltic, of
living Diatoms and _Radiolaria_ of the same species as those which, in a
fossil state, constitute extensive rocks of tertiary age at Caltanisetta,
Zante, and Oran, on the shores of the Mediterranean.
Moreover, in the fresh-water rotten-stone beds of Bilin, Ehrenberg had
traced out the metamorphosis, effected apparently by the action of
percolating water, of the primitively loose and friable deposit of
organized particles, in which the silex exists in the hydrated or soluble
condition. The silex, in fact, undergoes solution and slow redeposition,
until, in ultimate result, the excessively fine-grained sand, each
particle of which is a skeleton, becomes converted into a dense opaline
stone, with only here and there an indication of an organism.
From the consideration of these facts, Ehrenberg, as early as the year
1839, had arrived at the conclusion that rocks, altogether similar to
those which constitute a large part of the crust of the earth, must be
forming, at the present day, at the bottom of the sea; and he threw out
the suggestion that even where no trace of organic structure is to be
found in the older rocks, it may have been lost by metamorphosis.[4]
[Footnote 4: _Ueber die noch jetzt zahlreich lebende Thierarten der
Kreidebildung und den Organismus der Polythalamien. Abhandlungen der Koen.
Akad. der Wissenchaften._ 1839. _Berlin_. 1841. I am afraid that this
remarkable paper has been somewhat overlooked in the recent discussions
of the relation of ancient rocks to modern deposits.]
The results of the Antarctic exploration, as stated by Dr. Hooker in the
"Botany of the Antarctic Voyage," and in a paper which he read before
the British Association in 1847, are of the greatest importance in
connection with these views, and they are so clearly stated in the former
work, which is somewhat inaccessible, that I make no apology for quoting
them at length--
"The waters and the ice of the South Polar Ocean were alike found to
abound with microscopic vegetables belonging to the order _Diatomaceoe_.
Though much too small to be discernible by the naked eye, they occurred
in such countless myriads as to stain the berg and the pack ice wherever
they were washed by the swell of the sea; and, when enclosed in the
congealing surface of the water, they imparted to the brash and pancake
ice a pale ochreous colour. In the open ocean, northward of the frozen
zone, this order, though no doubt almost universally present, generally
eludes the search of the naturalist; except when its species are
congregated amongst that mucous scum which is sometimes seen floating on
the waves, and of whose real nature we are ignorant; or when the coloured
contents of the marine animals who feed on these Algae are examined. To
the south, however, of the belt of ice which encircles the globe, between
the parallels of 50 deg. and 70 deg. S., and in the waters comprised between that
belt and the highest latitude ever attained by man, this vegetation is
very conspicuous, from the contrast between its colour and the white snow
and ice in which it is imbedded. Insomuch, that in the eightieth degree,
all the surface ice carried along by the currents, the sides of every
berg and the base of the great Victoria Barrier itself, within reach of
the swell, were tinged brown, as if the polar waters were charged with
oxide of iron.
"As the majority of these plants consist of very simple vegetable cells,
enclosed in indestructible silex (as other Algae are in carbonate of
lime), it is obvious that the death and decomposition of such multitudes
must form sedimentary deposits, proportionate in their extent to the
length and exposure of the coast against which they are washed, in
thickness to the power of such agents as the winds, currents, and sea,
which sweep them more energetically to certain positions, and in purity,
to the depth of the water and nature of the bottom. Hence we detected
their remains along every icebound shore, in the depths of the adjacent
ocean, between 80 and 400 fathoms. Off Victoria Barrier (a perpendicular
wall of ice between one and two hundred feet above the level of the sea)
the bottom of the ocean was covered with a stratum of pure white or green
mud, composed principally of the silicious shells of the _Diatomaceoe_.
These, on being put into water, rendered it cloudy like milk, and took
many hours to subside. In the very deep water off Victoria and Graham's
Land, this mud was particularly pure and fine; but towards the shallow
shores there existed a greater or less admixture of disintegrated rock
and sand; so that the organic compounds of the bottom frequently bore but
a small proportion to the inorganic." ...
"The universal existence of such an invisible vegetation as that of the
Antarctic Ocean, is a truly wonderful fact, and the more from its not
being accompanied by plants of a high order. During the years we spent
there, I had been accustomed to regard the phenomena of life as differing
totally from what obtains throughout all other latitudes, for everything
living appeared to be of animal origin. The ocean swarmed with
_Mollusca_, and particularly entomostracous _Crustacea_, small whales,
and porpoises; the sea abounded with penguins and seals, and the air with
birds; the animal kingdom was ever present, the larger creatures preying
on the smaller, and these again on smaller still; all seemed carnivorous.
The herbivorous were not recognised, because feeding on a microscopic
herbage, of whose true nature I had formed an erroneous impression. It
is, therefore, with no little satisfaction that I now class the
_Diatomaceoe_ with plants, probably maintaining in the South Polar Ocean
that balance between the vegetable and the animal kingdoms which prevails
over the surface of our globe. Nor is the sustenance and nutrition of the
animal kingdom the only function these minute productions may perform;
they may also be the purifiers of the vitiated atmosphere, and thus
execute in the Antarctic latitudes the office of our trees and grass turf
in the temperate regions, and the broad leaves of the palm, &c., in the
tropics." ...
With respect to the distribution of the _Diatomaceoe_, Dr. Hooker
remarks:--
"There is probably no latitude between that of Spitzbergen and Victoria
Land, where some of the species of either country do not exist: Iceland,
Britain, the Mediterranean Sea, North and South America, and the South
Sea Islands, all possess Antarctic _Diatomaceoe_. The silicious coats of
species only known living in the waters of the South Polar Ocean, have,
during past ages, contributed to the formation of rocks; and thus they
outlive several successive creations of organized beings. The phonolite
stones of the Rhine, and the Tripoli stone, contain species identical
with what are now contributing to form a sedimentary deposit (and
perhaps, at some future period, a bed of rock) extending in one
continuous stratum for 400 measured miles. I allude to the shores of the
Victoria Barrier, along whose coast the soundings examined were
invariably charged with diatomaceous remains, constituting a bank which
stretches 200 miles north from the base of Victoria Barrier, while the
average depth of water above it is 300 fathoms, or 1,800 feet. Again,
some of the Antarctic species have been detected floating in the
atmosphere which overhangs the wide ocean between Africa and America. The
knowledge of this marvellous fact we owe to Mr. Darwin, who, when he was
at sea off the Cape de Verd Islands, collected an impalpable powder which
fell on Captain Fitzroy's ship. He transmitted this dust to Ehrenberg,
who ascertained it to consist of the silicious coats, chiefly of American
_Diatomaceoe_, which were being wafted through the upper region of the
air, when some meteorological phenomena checked them in their course and
deposited them on the ship and surface of the ocean.
"The existence of the remains of many species of this order (and amongst
them some Antarctic ones) in the volcanic ashes, pumice, and scoriae of
active and extinct volcanoes (those of the Mediterranean Sea and
Ascension Island, for instance) is a fact bearing immediately upon the
present subject. Mount Erebus, a volcano 12,400 feet high, of the first
class in dimensions and energetic action, rises at once from the ocean in
the seventy-eighth degree of south latitude, and abreast of the
_Diatomaceoe_ bank, which reposes in part on its base. Hence it may not
appear preposterous to conclude that, as Vesuvius receives the waters of
the Mediterranean, with its fish, to eject them by its crater, so the
subterranean and subaqueous forces which maintain Mount Erebus in
activity may occasionally receive organic matter from the bank, and
disgorge it, together with those volcanic products, ashes and pumice.
"Along the shores of Graham's Land and the South Shetland Islands, we
have a parallel combination of igneous and aqueous action, accompanied
with an equally copious supply of _Diatomaceoe_. In the Gulf of Erebus
and Terror, fifteen degrees north of Victoria Land, and placed on the
opposite side of the globe, the soundings were of a similar nature with
those of the Victoria Land and Barrier, and the sea and ice as full of
_Diatomaceoe_. This was not only proved by the deep sea lead, but by the
examination of bergs which, once stranded, had floated off and become
reversed, exposing an accumulation of white friable mud frozen to their
bases, which abounded with these vegetable remains."
The _Challenger_ has explored the Antarctic seas in a region intermediate
between those examined by Sir James Ross's expedition; and the
observations made by Dr. Wyville Thomson and his colleagues in every
respect confirm those of Dr. Hooker:--
"On the 11th of February, lat. 60 deg. 52' S., long. 80 deg. 20' E., and March 3,
lat. 53 deg. 55' S., long. 108 deg. 35' E., the sounding instrument came up
filled with a very fine cream-coloured paste, which scarcely effervesced
with acid, and dried into a very light, impalpable, white powder. This,
when examined under the microscope, was found to consist almost entirely
of the frustules of Diatoms, some of them wonderfully perfect in all the
details of their ornament, and many of them broken up. The species of
Diatoms entering into this deposit have not yet been worked up, but they
appear to be referable chiefly to the genera _Fragillaria, Coscinodiscus,
Choetoceros, Asteromphalus_, and _Dictyocha_, with fragments of the
separated rods of a singular silicious organism, with which we were
unacquainted, and which made up a large proportion of the finer matter of
this deposit. Mixed with the Diatoms there were a few small
_Globigerinoe_, some of the tests and spicules of Radiolarians, and some
sand particles; but these foreign bodies were in too small proportion to
affect the formation as consisting practically of Diatoms alone. On the
4th of February, in lat. 52 deg., 29' S., long., 71 deg. 36" E., a little to the
north of the Heard Islands, the tow-net, dragging a few fathoms below the
surface, came up nearly filled with a pale yellow gelatinous mass. This
was found to consist entirely of Diatoms of the same species as those
found at the bottom. By far the most abundant was the little bundle of
silicious rods, fastened together loosely at one end, separating from one
another at the other end, and the whole bundle loosely twisted into a
spindle. The rods are hollow, and contain the characteristic endochrome
of the _Diatomaceoe_. Like the _Globigerina_ ooze, then, which it
succeeds to the southward in a band apparently of no great width, the
materials of this silicious deposit are derived entirely from the surface
and intermediate depths. It is somewhat singular that Diatoms did not
appear to be in such large numbers on the surface over the Diatom ooze as
they were a little further north. This may perhaps be accounted for by
our not having struck their belt of depth with the tow-net; or it is
possible that when we found it on the 11th of February the bottom deposit
was really shifted a little to the south by the warm current, the
excessively fine flocculent _debris_ of the Diatoms taking a certain time
to sink. The belt of Diatom ooze is certainly a little further to the
southward in long. 83 deg. E., in the path of the reflux of the Agulhas
current, than in long. 108 deg. E.