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157
1 B er et Maximum af Tryk og i B’ et Minimum.
Trykkets Fordeling i Bundens Niveauflade er modsat den, der
finder Sted i Overfladen. I Overfladen er Ligetryksfladen
fremdeles concav, ved Bunden er den convex. I en vis
Niveauflade mellem Overfladen og Bunden maa Overgangen
mellem disse to Tilstande finde Sted. I denne maa
Ligetryksfladen falde sammen med Niveaufladen, eller Trykket
være constant. Denne Flade — Grændsefladen — er
altsaa ved Vandets Bevægelse fra Renderne mod Midten
rykket op fra det dybeste Punkt B til et højere Niveau, N’N’.
Mellem Overfladen TT og Grændsefladen N’N’ virke
Trykforskjellerne eller Gradienterne i Retning mod Midten.
I Overfladen ere de størst, eller Ligetryksfladens Heldning
mod Niveaufladen størst; begge blive mindre med Dybet
og blive lig Nul i Grændsefladen. Imellem Grændsefladen og
det største Dyb virke Trykforskjellerne eller Gradienterne
fra Midten mod Renderne, og Ligetryksfladen helder udad
i samme Retning. Begge ere Nul i Grændsefladen og voxe
mod Dybet.
Den heraf resulterende Bevægelse er fremstillet i
Fig. 2. Tilstrømningen af Vand i det øvre Lag mod
Midten og Udstrømningen i det nedre Lag fra Midten
fremkalder en nedstigende Bevægelse i det midtre Parti. Ved
Renderne af Bassinet fremkalder Vandets Bevægelse" fra
disse i det øvre Lag og mod disse i det nedre Lag en
opadstigende Bevægelse. Den ned- og opadstigende Bevægelse
virker i Retning af at formindske den af de horizontale
Bevægelser i de øvre Lag flydende Stigning af Overfladen
i Midten og Samkning ved Renderne. I Grændsefladen er
Bevægelsen nedadstigende eller opadstigende.
Saaledes maatte man tænke sig Vandets Bevægelse,
dersom Tyngden var den eneste virkende Kraft. Men
foruden Tyngden vilde andre Kræfter komme i Virksomhed,
nemlig .1 ordrotationens Afbøjningskraft, Centrifugalkraften,
Trægheden og Frietionen. Trægheden kunne vi sætte ud
af Betragtning, naar Bevægelsen er jevn.
Afbøjningen ved .lordrotationen og Centrifugalkraften
ville i vort Tilfælde virke i samme Retning i de øvre Lag.
Naar en Vandpartikkel var kommet i Bevægelse ned over
Ligetryksfladens Skraaning ad den korteste Vej til det
laveste Punkt, det er langs den Linie, hvor Skraaningen —
Gradienten — er størst, vil Jordrotationen drive den til
højre og det desto sterkere, jo større Hastigheden, der i
Begyndelsen er voxende, bliver. Naar Bevarelsen er
afbøjet, har den faaet en Component lodret paa Gradientens
Retning. Centrifugalkraften kommer til, forsaavidt Banen
which the mass of water has augmented, and has diminished
at the point B’, above which the height of water has
experienced a decrease. At B there is a maximum of
pressure, and at. B’ a minimum. The distribution of
pressure over the surface of level of the bottom is the
reverse of that at the surface. At the top-surface, the
surface of equal pressure still remains concave; at the
bottom it is convex. At a certain level between the
surface and the bottom, the transition between these
two conditions must take place. At the latter, the
surface of equal pressure must coincide with the
surface of level, or tin- pressure be constant. This
surface the limiting surface — has, therefore, owing to
the water having moved from the border towards the
middle, risen from the deepest point, B, to a higher
level, N’N’.
Between the upper surface, TT, and the limiting
surface, N’N’, the differences of pressure, or the gradients, act
in the direction, of the centre. At the sea-surface they
are steepest, or the inclination of the surface of equal
pressure to the surface of level is greatest; both
diminish with the depth, and are nil at the limiting surface.
Between the limiting surface and the greatest depth, the
difference in pressure, or the gradient, acts from the
middle towards the borders, and the surface of equal
pressure inclines outward iu the same direction. Both are
nil at the limiting surface, and increase with the depth.
The motion which this occasions will be found
represented in fig. 2. The influx of water in the upper
stratum towards the middle and its efflux in the lower
stratum from the middle, produce a descending motion
throughout the central part. Along the margins of the
basin, the motion of the water — from them in the upper
stratum and toivards them in the lower — gives rise to an
ascending motion. The descending and ascending motions
tend to diminish the rise of the surface in the middle and
its depression at the margins, occasioned by the horizontal
motion in the upper strata. In the limiting surface the
motion is either descending or ascending.
Such is the system we should reasonably ascribe to
the motion of the water were gravity the sole operating
force. But exclusive of gravity, other agencies will come
into play, viz., the deviating force arising from the
earth’s rotation, centrifugal force, inertia, and friction.
Inertia we may disregard altogether when the motion is
uniform.
The deflection arising from the rotation of the earth
and centrifugal force, will act in this case in the same
direction throughout the upper strata. On a particle of
water moving down the slope of the surface of equal
pressure by the shortest way towards the lowest point, i. e. along
the line where the inclination — the gradient — is steepest,
the rotation of the earth will force it to the right, and the
faster the greater the velocity, which at first is increasing.
On the motion becoming deflected, it has acquired a
component perpendicular to the direction of the gradient.
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