I B er et Maximum at Tryk og i B' et Minimum. Tryk- 
kets Fordeling i Bundens Niveauflade er modsat den, der 
finder Sted i Overfladen. I Overfladen er Ligetryksfladen 
fremdeles concay, ved Bunden er den convex. I en vis Ni- 
veautflade mellem Overfladen og Bunden maa Overgangen 
mellem disse to Tilstande finde Sted. I denne maa Lige- 
trykstladen 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, NN. 
| Mellem Overfladen TT og Grændsefladen MN 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 Greendsefladen 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 1 det øvre Lag mod Mid- 
ten og Udstromningen 1 det nedre Lag fra Midten frem- 
kalder en nedstigende Bevægelse i det midtre Parti. Ved 
Renderne af Bassinet fremkalder Vandets Bevægelse fra 
disse 1 det øvre Lag og mod disse i det nedre Lag en op- 
opadstigende Bevægelse 
adstigende Bevægelse. Den ned- og 
den af de horizontale 
Stigning af Overfladen 
I Grændsetladen er 
Bevægelsen nedadstigende eller opadstigende. 
virker i Retning af at formindske 
Bevægelser 1 de øvre Lag flydende 
1 Midten og Sænkning ved Renderne. 
Saaledes maatte man tænke sig Vandets Bevægelse, 
dersom Tyngden var den eneste virkende Kraft. Men for- 
uden Tyngden vilde andre Kræfter komme 1 Virksomhed, 
nemlig Jordrotationens Aftbøjningskratt, Centrifugalkraften, 
Trægheden og Frictionen. Trægheden kunne vi sætte ud 
af Betragtning, naar Bevægelsen er jevn 
Afbøjningen ved Jordrotationen og Centrifugalkraften 
ville i vort Tilfælde virke i samme Retning 1 de øvre Lag. 
Naar en Vandpartikkel var kommet i Bevægelse ned over 
Ligetryksfladens Skraaning ad den korteste Vej til det la- 
veste 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 Bevægelsen er af- 
bøjet, har den faaet en Component lodret paa Gradientens 
Retning. Centrifugalkraften kommer til, forsaavidt Banen 
15 
| 
Il 
7 
( 
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 placé. At the latter, the 
surface of equal pressure must coincide with the sur- 
face of level, or the pressure be constant. This sur- 
face — the linuting 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, MN. 
Between the upper surface, TT, and the limiting sur- 
face, NN, 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 di- 
minish with the depth, and are nil at the limiting surface. 
Between the limiting surface and the greatest depth, the 
the — the 
middle towards the borders, and the surface of equal pres- 
ditference im pressure, or gradient, acts from 
sure inclines outward in the same direction. Both are 
nil at the limiting surface, and increase with the depth. 
The motion which this occasions will be found 
The influx of water in the upper 
middle its efflux the 
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 
represented in fig. 2. 
stratum towards the and in lower 
stratum and towards 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 
foree. 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 pres- 
sure by the shortest way towards the lowest point, 1. 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 com- 
ponent perpendicular to the direction of the gradient. 
