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nedad og langs Bassinets Render opad. Den første
foregaar med mindre Friction end den sidste.
Et Bevægelsessystem som det her beskrevne
forudsætter en vis Fordeling af Vandets Tæthed, der resulterer
i en Overflade, som afviger fra Niveaufladen og en
Grændse-flade i et vist Dyb mellem denne og det dybeste Punkt i
Havet. Den Fordeling af Havvandets Tæthed, som var den
oprindelige, vilde imidlertid ved de beskrevne Bevægelser
forrykkes. Det lettere Vand fra Bredderne vilde
efterhaanden af de øvre Strømninger føres til Midten og Midtens
tungere af de nedre Strømninger mod Renderne.
Dersom altsaa en oprindelig ulige Fordeling af
Tæthederne var den eneste eller hovedsageligste Aarsag til
Havets Strømninger, vilde en Udjevning af disse Uligheder
være Følgen af Bevægelsen, og denne vilde efterhaanden
tabe sig for at give Plads for Ligevægt.
Men i Naturen er Forholdet det, at der er andre og
sterkere Kræfter, som stadig vedligeholde den stedfindende
Ulighed i Tætheden i de forskjellige Punkter af Havet.
Disse Kræfter ere de af Vindene fremkaldte Strømninger
i Forbindelse med Elvevand og Issmeltning paa Havet,
Opvarmning og Afkjøling, Nedbør og Fordunstning.
Vindene føre salt Atlanterhavsvand ind i vort Nordhav, og
sprede, langs Kysterne Elvevand, fra de isfyldte Partier
smeltet Isvand udover det salte. Indførselsvejene ere andre
end Udførselsvejene, saa at Strømningerne, for en stor Del.
ikke vende tilbage i sine gamle Baner. Men Tilførslerne
og Udførslerne foregaa paa bestemte Steder, og saaledes
opretholdes et constant System af Fordelingen äf de ulige
Tætheder (Saltholdighed saavel som Temperatur) ved de
normale Vinde. Disses Virkning er meget sterkere end
den, der flyder af Tæthedernes Ulighed, og den gaar, i de
ovre Lag, i samme Retning.
Den Udjevning af Tæthedernes Ulighed, som de af
denne fremkaldte Strømninger vilde hidføre, opvejes
saaledes mere end fuldstændig af Vindstrømmene. Det kommer
ikke længere end til en Tendents til Udjevning. Tæthedens
ulige Fordeling bliver et constant System, og de af samme
resulterende Strøm 11 inger ville linde Sted, uden at
Tæthedernes Ulighed udjevnes.
Overensstemmende med disse Betragtninger, hvis
Rigtighed vil fremgaa endnu klarere i det Følgende, naar vi
beskrive det hele resulterende Strømsystem, sætter jeg
Resultatet af Tæthedernes ulige Fordeling i Havvandet deri,
at der dannes en Overflade, som afviger fra Niveaufladeii.
Denne kalder jeg Tætheds-Fladen. Dens Fonn kan
tindes, naar man kjender Dybden af den Flade, jeg kalder
Grændsefladen. Thi i Grændsefladen, der er en Niveauflade,
ere Trykkene overalt lige store, og Tæthedsfladens Højde
over denne bliver omvendt proportional med de, af
Obser-tionerne givne, midlere Tætheder i de verticale Vandsøjler
mellem begge Flader.
Dybden af Grændsefladen er bestemt derved, at denne
Flade ligger imellem Bevægelser, der gaa i modsatte Ret-
)
minimum and upward along the margins of the basin.
The former has less friction to overcome than the latter.
A system of motion such as here set forth, is
based upon a certain distribution of the density of the water
resulting in a top-surface that deviates from the surface of
level and a limiting surface at a certain depth between the
latter and the deepest point of the sea. The original
distribution of the density of the sea-water, would however be
disturbed by the motions described. The lighter water from
the shores would gradually be carried by the upper currents
towards the middle, and the heavier water of the middle
by the lower currents towards the margins.
Hence, therefore, assuming that originally an unequal
distribution of density were the solo or the chief cause of
ocean-currents, an equalisation of these differences would
be a necessary consequence of the motion, which must
gradually yield and give way to equilibrium.
But in nature the condition is such, that other and
stronger forces are found to maintain the inequality of the
density in the various parts of the sea. These forces are
the currents produced by the winds in conjunction with
river-water and the melting of ice in the sea, also heating
and cooling, precipitation, and evaporation. The winds carry
salt Atlantic water into our North Ocean; they spread
along the coasts river-water, and from the ice-covered parts
melted ice-water above the salt-water. The passages of
inlet differ from those of outlet; and hence the currents,
to a great extent, do not return by their original courses.
But the inflow and outflow take place in definite
localities; and thus a constant system is maintained of the
distribution of unequal densities (amount of salt as well
as temperature) by means of the normal winds. The
effect of the winds greatly exceeds • that produced by
unequal density, and it goes throughout the upper strata
in the same direction.
The equalisation of unequal density, to which the
currents occasioned by it would give rise, is accordingly
more than counterbalanced by the wind-currents. It does not
go farther than a tendency to equalisation. The unequal
distribution of density becomes a constant system, and the
currents resulting from it will be produced without
equalising the differences of density.
In accordance with these views, the correctness of
which will appear still more clearly in the sequel, on giving
a description of the whole resulting system of currents, I put
the issue of the unequal distribution of density throughout
the water of the sea in the forming of a top-surface
differing from the surface of level. This surface I call the
Surface oj Density. Its form may be found 011 knowing the depth
of the surface which I call the limiting surface. For at
the limiting surface, which is a surface of level, the
pressure is everywhere; the same, and the height of the surface
of density above it will be inversely proportional to the
mean density of the vertical columns of water between
both surfaces given by the observations.
The depth of the limiting surface is determined by
its lying between motions proceeding in opposite directions.
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