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71
meste opveies af den Forøgelse af samme, der skyldes de
der herskende lavere Temperaturer.
Det fremgaar altsaa, at Differentserne mellem
Salt-gehalten i de atlantiske Overfladelag og de paa Bunden
hvilende iskolde Vandmasser gjennemsnitlig kun ere meget
smaa, om de end paa de Puncter, hvor Vandet i de dybere
Lag besidder en overveiende polar Karakter, turde være
adskilligt mere fremtrædende. Disse Differentser af
henimod 0.02 % ere dog mere end tilstrækkelig store til i de
nederste mere afkjølede Lag af det atlantiske Vand at
fremkalde et, som det vil sees af Tabellen, meget tydeligt
om end svagt Maximum af den specifiske Vægt, hvad der
nærmest bevirkes derved, at Søvandet ved Afkjøling under
0° nærmer sig sit Tæthedsmaximum og derfor i Nærheden
af dette for mindre Temperaturvariationer kun forandrer
sit Volum meel næsten umærkelig smaa’Værdier, saaledes
at en selv meget ringe Forøgelse af Saltgehalten under
disse Omstændigheder faar en. overveiende Indflydelse
lige-overfor en Grads Foraudring af Temperaturen.
Det er saaledes saa langt fra Tilfælde, at der i de
specifiske Vægter af de forskjellige Vandlag ligger nogen
Hindring for Antagelsen af, at det atlantiske Vand skulde
synke gjennem det koldere Polar vand, at man tvertom af
disse maa slutte, at saa maa være Tilfælde, dersom ikke
andre i Havet herskende Strømme virke hemmende paa en
saadan Bevægelse. Man tænke sig f. Ex. ved Siclen af
hinanden i Havet to Vandsøiler af 2000 Favnes (3658
Meters) Dybde, hvori Temperaturens Variation med Dybden
for Simpelheds Skyld kan antages at være den samme,
hvorimod Saltgehalten i den 1ste helt igjennem sættes til
3.52 °/o, medens den i den 2den paa Strøget fra Overfladen,
til 500 Favnes (914 Meters) Dyb gives Værdien 3.52°/0
og fra 500 til 2000 (914 til 3658) Værdien 3.5Ö°/0,
saaledes som Forholdet ifølge Observationerne virkelig synes
at stille sig paa enkelte Steder i det undersøgte Hav. Det
er da umiddelbart indlysende, "at en saadan Fordeling af
Saltgehalten vil have en Synkning i den 1ste Søile til Følge,
saaledes at Vandet i denne vil søge at udbrede sig langs
Bunden og fordrive det omliggende specifisk lettere Vand.
Den Hastighed, hvormed en saadan Bevægelse foregaar,
vil naturligvis rette sig efter Differentsen mellem Trykkene
i samme Niveau i begge Søiler, en Differents, som ved
Bunden i 2000 Favnes (3658 Meters) Dyb efter Beregning
beløber sig til henimod 32mm Kviksølvsøile.
For nærmere at begrunde clen forhen fremsatte
Hypothese, om at det paa Bunden hvilende iskolde Vand paa
de i Kartet med rød Farve betegnede Steder skulde have
atlantisk Oprindelse, vil jeg benytte mig af de i en tidligere
Afhandling1 beskrevne Observationer over de i Søvandet
indeholdte Kvælstofmængder, hvis Anvendelse i saadant
Øiemed allerede paa det Sted løselig er bleven antydet.
Som bekjendt herskede der i ældre Tider den
Anskuelse, at de i Søvandet i de større Dyb indeholdte
Luftmængder paa Grund af det der herskende Tryk maatte
by a decrease in the amount of salt, is almost compensated
by the rise resulting from the low temperatures prevailing
there.
It appears, therefore, that the differences between the
amount of salt in the warm upper strata and that in the
cold water at the bottom, are, on an average, exceedingly
small, though more striking perhaps in localities where the
water of the deeper strata to a very great extent is Polar
in origin. These differences — about 0.02 per cent — are,
however, as will be seen from the Table, more than
sufficient in the deepest and coldest strata of Atlantic water
to occasion an appreciable, though a low, maximum of
specific gravity, which is explained by the fact, that
sea-water below 0" has very nearly reached its maximum of
density, and the increase in volume then resulting from
trifling variations in temperature, is a, well nigh
inappreciable magnitude: hence, under such circumstances, the
influence of a very slight addition to the amount of salt
with but one degree’s difference in temperature will be
exceedingly great.
Such being the case, there is nothing in the specific
gravities observed in the different strata of water to
disfavour the assumption that the comparatively warm
Atlantic water should sink through the cold water of Polar
origin; nav, from these specific gravities we may infer its
correctness, provided only that such descending motion
be not counteracted by the effect of ocean currents. To
give an illustration. Let us imagine two columns of
water. 2000 fathoms (3658 metres) deep, in both of
which, for convenience’ sake, the variation in temperature
with the depth is assumed to be equal; the amount of salt
on the other hand, being put at 3.52 per cent throughout
the whole of the first, but in the second, at 3.52 per cent
from the surface to a depth of 500 fathoms (914 metres),
at 3.50 per cent from 500 to 2000 fathoms (914—3658
metres), — a ratio of distribution actually observed in some
localities. This given, it is obvious that such a distribution
must cause the water in the first column to sink, and spread
itself over the bottom, displacing as it does so the
specifically lighter. The rapidity of this downward motion will
of course be proportionate with the difference in pressure
at the same level in the two columns, a difference which,
at the depth of 2000 fathoms (3658 metres), has been
computed equal to that of a column of mercury 32""" in
height.
With a view to furnish additional confirmation of
the hypothesis brought forward, above, which assumes the
cold bottom-water in the red-coloured sections of the
Plate to be of Atlantic origin. I shall have recourse to
my observations on the amount of nitrogen in sea-water,
published in a former paper,1 where their application to
such a purpose was briefly alluded to.
The opinio11 formerly entertained, that the quantity
of air contained in sea-water at great depths must be
exceedingly great, by reason of the immense pressure pre-
1 f On the Air in Sea-Water.’
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