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183
on spids Vinkel a med Gradienten, skulde man regne mod
Formelen
u = 10.338
«fø Ap’" eos a
S ’ A a.-» ’ *
I dette Tilfælde vil Bevægelsen kunne blive
accelererende. Men det er at merke, at den for en Del er
opstigende, modvirkes altsaa af Tyngden. I det Tilfælde, at
Bevægelsen foregaar paa skraa mod Isobarerne, fra det
lavere mod det højere Tryk, vil den kunne blive retarderende,
men her har den en nedstigende Component og
understøttes saaledes af Tyngden. Endvidere sees, at de forskjellige
Slags Bevægelser langs Isobathen jevnlig afvexle hverandre,
og Vandets Continuitet vil regulere Bevægelsen saaledes, at
den ogsaa her nærmer sig til at blive continuerlig og
overensstemmende med den, der følger af Formelen Side 176
med « = 90°.
Isobarsystemet for 3(H) Favnes Dyb er væsentlig
det samme som Vindfladens. Vi faa saaledes i dette Dyb
den samme Fordeling af Hastighederne, som er beskrevet
under Vindfladens Beskrivelse. Værdien af Gradienten for
Afstanden mellem to Isobarer er den samme i begge
Tilfælder. Den Omstændighed, at Vandet i 300 Favnes Dyb
er noget tungere end i Overfladen, gjør Hastighederne i
Dybet forholdsvis mindre, dog kun i Forholdet 1.027: 1.030
eller 0.9971, det er 0.3 Procent mindre. Man kan derfor
godt benytte Skalaen paa PI. XLIII til at udmaale
Hastighederne.
Ved at sammenligne Karterne PI. XLIII og PI. XLIV
ser man, at Strømhastighederne i Overfladen ere
idethele-taget betydelig større end i 300 Favnes Dyb. Forskjellen
er netop de Hastigheder, der svare til Tæthedsfladen. Thi
Vindfladens Ordinater Pl. XXXIII og XLIV ere
Strømfladens (PI. XLIII) minus Tæthedsfladons (PI. XLII).
Der, hvor Vandet i 300 Favnes Dyb passerer over
AVvville Thomson-Ryggen, angiver Kartet en Hastighed af
0.1 ni. p. S. (5 Kvm. i 24A, 0.21 Kvm. i 1A). D.
Stevenson angiver,1 at en Hastighed af Strømmen i en Elv af
3 Tommer pr. Secund (0.170 Mil pr. Time) vil netop
begynde at virke paa fint Ler, medens en Hastighed af (i
Tommer pr. Secund (0.34 Mil pr. Time) vil løfte fint Sand.
Mellem disse Hastigheder ligger den af mig beregnede
Strømhastighed. Denne skulde saaledes være tilstrækkelig
til at transportere lettere Bundmateriale fra Ryggen ind i
Færø-Shetland-Renden. J. Murray siger2: "At der over
Ryggen stryger saavel sterke Tidevandsstrømme som den
stadige Strøm mod Nordost, fremgaar af den
Omstændighed, at ingen Aflagring af fint Material faar Lov til at lægge
sig paa den, og af de Sandkorn og Smaastene, der ere
spredte over Havbunden imod Nordost. Over Ryggen viser
a direction forming an acute angle, «. with the gradient,
the computation should be made with the formula
«= 10.333^.^.^°.
£ Aa"" k
In this case the motion can become accelerated.
Meanwhile, we must bear in mind that to some extent it
is ascending, and therefore counteracted by gravity. Should
the motion proceed obliquely to the isobars, from the lower
to the higher pressure, it may become retarded, but then
it has a descending component, and is thus assisted by
gravity. Moreover, it appears that the different kinds of
motion proceeding along the isobath continually vary, and
the continuity of the water will regulate the motion, so
that here too it shall tend to become continuous, and to
accord with that resulting from the formula, p. 170, with
a = 90°.
The isobar-system, for a depth of 300 fathoms, is in
all essential particulars the same as that of the
wind-surface. Hence, at this depth we get the same
distribution of velocity as set forth in the description of the
wind-surface. The value of the gradient for the distance
between any two isobars is the same in either case. The
circumstance, that at a depth of 300 fathoms the water is
somewhat heavier than at the surface, entails in the
deep comparatively reduced velocities — but only in the
ratio of 1.027 : 1.030 or 0.9971, i. e., 0.3 per cent less.
We may therefore, without apprehension, make use of the
scale given in PI. XLIII for measuring the velocity.
If we compare the maps PI. XLIII and PI. XLIV, it
will appear that the current-velocities at the surface, taken
on tlu> whole, are considerably greater than at a depth of
300 fathoms. The difference applies exclusively to the
velocities resulting from the surface of density. For the
ordinates of the wind-surface (Pl. XXXIII and PI. XLIV)
are those of the current-surface (PI. XLIII) minus those
of the surface of density (PI. XLII).
Where, at a depth of 300 fathoms, the water passes
over the Wyville-Thomson Ridge, the map gives a velocity
of 0.1 m. per see. (5 naut. miles in 24 hours = 0.21 naut.
mile in 1 hour). Mr. D. Stevenson states1 that a
river-current with a velocity of 3 inches per second (0.170 mile an
hour) will just begin to work on fine clay, whereas a
velocity of 0 inches per second (0.34 mile an hour) will lift
fine sand. Between these two velocities lies the
current-velocity I have computed. The latter should accordingly
be sufficient to transport lighter bottom-materials from off
the ridge into the Færoe-Shetland Channel. Mr. J.
Murray says:2 — "That the Wyville-Thomson Ridge is swept
by strong tidal currents as well as by the steady flow to
the north-east is shown by the fact that no fine deposit is
allowed to accumulate on it, and by the particles of sand
and gravel from the ridge which are spread over the sea-bottom
A. Geikie, Text-Book of Geology, 1882, S. 5ttW. 1 A. Geikie, Text-Book of Geology, 1KS2, p. :i<>8.
Encyclopaedia Britannica S. ö!)4. I 2 Encyelopædia Britannica, p. f>!)4.
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