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51
Axen til et Punkt, der ligger nærmere Randen, og med at
Tæthedsforskjellerne ere mindre i Dybet og som Følge
deraf ogsaa Gradienterne i de nedre Lag, vil man komme
til den Slutning, at Gjenneinsnitshastiglieden i de nedre
Lag er betydelig mindre end i de øvre Lag. Vi føres
saaledes snart op paa mindre Dybder for Grændsefladen.
Allerede et Forhold mellem Hastighederne som 2 til 1
rykker den op til 474 Favne og som 3 til 1 til 349 Favne.
Et sterkere Forhold mellem Hastighederne har forholdsvis
mindre Indflydelse paa Grændsefladens Dybde, naar vi komme
op til 300 Favne, der svarer til et Forhold af V:v = 3.022 : 1.
Nærmere kunne vi ikke komme til Løsningen ad
denne Vej. Vi maa derfor se os om efter andre
Kjende-merker og undersøge, om der i Bevægelsens Retning kunde
være saadanne at finde. Her møder den Vanskelighed, at
Bevægelsen i Havet hovedsagelig er bestemt af Vindene,
hvis Virkning overgaar og overdækker Tæthedens. At
udskille den sidste bliver vistnok i de fleste Tilfælder umuligt.
Dens Spor lade sig dog nok paavise, navnlig turde her
henpeges paa Isothermemes Sænkning i Havets Midte og
Opstigning ved Renderne, der er saa fremtrædende i
Snittene Pl. X til XIII. T Projectionssnittet PI. XXVI se
vi ogsaa, paa Norges Kystbanker, den sterkeste
Sammentrængning af Isothermerne ved noget over 300 Favnes Dyb,
et Fænomen, der vidner om, at her lider det varme Vand
i de øvre Lag, der drives frem af Vinden, en sterk
Afkjøling fra koldt Vand, der stiger op fra Dybet langs
Bundens Skraaning. Her turde altsaa være et Parti,
delsvarer til Punktet N’ i Fig. 2.
I Færø-Shetland-Renden, navnlig i dens nordvestlige
Halvdel (Snittene I. II. III, IV og VI, Pl. IX), ligger
iskoldt Vand under varmt Vand. Det varme Vand føres
af de sydvestlige Vinde ind fra Atlanterhavet mod
Nordost, Det iskolde Vand har sin Rod i Nordhavets Dyb
østenfor Island og Færøerne; det maa komme ind langs
Rendens Nordside fra Nordost mod Sydvest. Her have vi
altsaa modsatte Bevægelser i de øvre og i de nedre Lag.
Grændsen mellem disse ligger omtrent paa 300 Favnes Dyb;
thi indtil dette Dyb rækker det varme Atlanterhavsvand
paa Wyville Thomsoii-Ryggen.
Jo højere Grændsefladen lægges, desto mindre Vægt
tilhugges Tæthedernes Ulighed som strømfrembringende
Kraft. Thi jo mindre mægtigt det øvre Lag bliver, desto
mindre vil Forskjellen i de verticale Vandsøjlers Vægt
blive, desto mindre de deraf flydende Uligheder i Trykket.
Man er saaledes muligens paa den sikkre Side, naar man
lægger Grændsefladen noget højt, fremfor i en større Dybde.
Under disse Omstændigheder har jeg maattet gjøre et
Valg indenfor de af Sandsynlighedshensyn optrukne Grændser.
Jeg sætter Grændsefladens Dybde til 300 Favne.
Den norske Nordhavsexpedition. H. Mohn: Nordhavets Dybder, T«
surface they increase from the axis to a point nearer
the margin, and that the differences of density are less
in the deep, and consequently also the gradients in the
lower strata, we shall arrive at the result that the
average velocity in the lower strata is considerably less
than in the upper. We are thus soon brought up to less
depths for the limiting surface. Even a ratio of the
velocities of 2 to 1 will raise it to 474 fathoms, and of 3 to 1
to 349 fathoms. A greater ratio of the velocities has
comparatively less influence on the depth of the limiting
surface when raised to 300 fathoms, which corresponds
to a proportion of V: v = 3.622 : 1.
A nearer solution we cannot arrive at in this way.
We must, therefore, seek other criterions, and investigate
if such are to be found in the direction of the motion.
Here, however, we meet with the difficulty that in the
sea the motion is chiefly determined by the winds, the
effect of which exceeds and conceals that of the density.
To separate the latter will no doubt in most cases prove
impossible. Still, its traces will possibly admit of being
detected; here, more especially, we may call attention to
the dipping down of the isotherms in the middle of the
sea and their rise at the margins, so conspicuous in the
sections Pl. X to Pl. XIII. In the projected section,
Pl. XXVI, Ave also see, on the Norwegian coast banks,
the greatest crowding of the isotherms in a depth of
somewhat over 300 fathoms, a phenomenon clearly proving
that here the warm water in the upper strata, driven
forward by the wind, undergoes a considerable cooling from cold
water, which ascends from the deep along the slope of
the bottom. Hfere, accordingly, may be a part of the
seabed corresponding to the point N’ in fig. 2.
In the Færoe-Shetland Channel, more especially
throughout its north-western half (Sections I, II, III. IV, and VI,
Pl. IX), ice-cold water extends under warm water. The
warm water is carried by the south-westerly winds from
the Atlantic Ocean towards the north-east. The ice-cold
water has its source in the deep of the North Ocean, east of
Iceland and the Færoes; it must find an entrance along
the north side of the channel, from the north-east to the
south-west. Here, we have accordingly opposite motions
in the upper and in the lower strata. The dividing plane
between the two lies at a depth of about 300 fathoms; for
to that depth the warm Atlantic water reaches down on
the Wyville-Thomson Ridge.
The higher we place the limiting surface, the less
importance we attach to the differences of density as a
current-producing force. For the less deep the upper stratum,
the less will be the difference of weight of the vertical
columns of water, and the less the differences of pressure
arising from them. Hence, we are possibly on the safe
side in placing the limiting surface somewhat high, rather
than at a comparatively great depth.
Under these circumstances, I was compelled to choose
within the limits of probability. 1 take the depth of the
limiting surface at 300 fathoms.
srntur og Strømninger. ’21
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