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hour og en Hastighed af 4 til 23 miles. Heraf beregnes,
at Vindstyrken 3.9 svarer til 22.5 miles an hour. Da 1
mile an hour svarer til 0.447 Meter pr, Secund, faar man
Vindstyrke 3.9 = 10.0 Meter per Secund,
altsaa: til en Vindhastighed af 10.0 m. p. S. svarer en
Strømhastighed af 15 Kvartmil i 24 Timer eller, idet
vi sætte Strømhastigheden proportional med
Vindhastigheden,
Vindhastighed 1 m. p. S. giver Strømhastighed 1.5
Kvart-mil i 24 Timer eller 0.032206 Meter pr. Secund jlog
0.032206 = 8.507941.
Man har i den senere Tid gjort Indvendinger mod
Rigtigheden af Scotts Reductionstabel, der hovedsagelig gaa
i den Retning, at den til en vis Vind-Styrke svarende
Hastighed skulde være mindre end den, Scotts Tabel angiver.
Saalænge Discussionen om denne Sag ikke har faaet nogen
bestemt Afslutning, har jeg fundet at burde holde mig til
Scotts Tabel, saameget mere, som de samme Data, der ere
benyttede til at udlede Strøinhastighedsfactoren, tidligere1 have
ledet til en særdeles god Overensstemmelse mellem den efter
Scotts Tabel beregnede Vindhastighed og den af
Barometerhøjderne paa dynamisk Vej beregnede. Den af
Gradienten beregnede Vindhastighed var nemlig 9.15 m. p. S.,
medens den af den observerede Vindstyrke efter Scotts Tabel
udledede var 9.38 m. p. S. Forskjellen, 0.23 m., peger i
samme Retning som ovenfor bemerket og antyder en
Formindskelse, dog kun af 2.5 Procent. En Formindskelse af
de respective Vindhastigheder vilde forøvrigt give Vinden
en forholdvis større Evne til at fremkalde Strøm.
Den Maade, hvorpaa Vinden virker paa den med Is
dækkede Del af Havet, er en anden end den, hvorpaa den
virker paa det aabne Hav. For det første svækkes selve
Vindens Styrke, som vi ovenfor have seet, ved at den blæser
over Isen, og dernæst har Vinden at sætte i Bevægelse
directe selve Isen og gjennem dens Bevægelse middelbart
Vandet. Da Havisen frembyder en yderst ujevn saavel
Overflade som Underflade og kan paa sine Steder stikke
temmelig dybt, maa man vente, at Vindens Virkning til at
sætte et isfyldt Hav i Bevægelse resulterer i en
langsommere Fart, end naar Havet er isfrit. For at faa et Maal
for denne Virkning, med andre Ord, for at tinde
Strønifac-toren for det isfyldte Hav har jeg benyttet Sir Leopold
M’Clintock’s Observationer fra "Fox"s Drift i Baffins-Bugt
og Davis-Strædet Vinteren 1857—58. Efter disse har
man følgende Tabel, i hvilken Vindstyrken (den
observerede, altsaa af Isen paavirkede) er angivet efter Beaufort
Skala, Driftens Retning efter den Compasstreg, henimod
hvilken den fandt Sted, og Driftens Hastighed i Kvartmil
)
corresponds to a velocity of 18 miles an hour, and a, velocity
of 4 to 23 miles. From these figures was computed, that a
wind-force of 3.9 corresponds to 22.5 miles an hour. As
1 mile an hour corresponds to 0.447 metre per second,
we get —
Force of wind 3.9 = 10,0 metres pr. second;
hence, to a wind-velocity of 10.0 in. per second
corresponds a current-velocity of 15 nautical miles in 24
hours, or, putting the current-velocity proportional to
the wind-velocity,
a wind-velocity of 1 in. per second gives a current-velocity
of 1.5 nautical miles in 24 hours, or 0.032206 metre
per second |log 0.032206 = 8.507941.
Of late objections have been made to Scott’s Table
of Reduction, which conclude in assuming the velocity
corresponding to a given force of wind as less than given in
Scott’s Table. So long as the discussion on this subject
has not attained a definite conclusion, I have seen fit to
abide by Scott’s Table, more especially since the same data
that have been used for educing the factor for current
velocity, on a former occasion led to excellent agreement between
the velocity of wind computed according to Scott’s Table
and that computed dynamically. 1 The wind-velocity
computed from the gradient was namely found to be
9.15 m. per see., whereas that deduced from the observed
force of the wind according to Scott’s Table was 9.38 m.
per see. The difference, 0.23 m., points in the same
direction, as remarked above, and indicates a decrease, but of
only 2.5 per cent. Besides, a diminution of the respective
wind-velocities would give the wind a proportionally greater
power to produce currents.
The way in which the wind acts on the ice-covered
part of the sea, is another compared to its action on the
open water. To begin with, the force of the wind itself
is materially weakened, as shown above, by blowing over
ice; and in the next place, the wind has to impart motion
directly to the ice, and through that motion indirectly to
the water beneath it. Sea-ice presenting an exceedingly
rough surface, both the upper and the under, and reaching
in places a considerable depth, we cannot but expect that the
• power of the wind to set in motion an ice-encumbered sea
should result in a slower rate than with a sea free of ice.
To obtain a standard for this influence, or, in other words,
to find the current-factor for an ice-encumbered sea, I made
use of Sir Leopold M’Clintock’s observations from the drift
of the "Fox’’ in Baffin’s Bay and Davis Strait during the
winter 1857—58. From these we have the following Table,
in which the force of the wind (viz., the observed, or that
influenced by the ice) is given according to Beaufort
Scale, the direction of the drift by the point of the com-
1 Ueber «lie Bewegung tier horizontalen Luftstriime in tier
Niihe des Æquators. Von C. M. Guldberg unci H. Mohn. Zeitsclirift
tier østerreichisclien Gesellschaft fiir Meteorologie 1877, 8. 182.
’Pallet 9.1.’t er beregnet efter de lii Normaltyngden reducerede
Baro-liieterhøjder.
1 Ueber die Bewegung tier horizontalen Luftströme in der Nälie
des Æquators. Von 0. M. Guldberg und H. Mohn. Zeitsclirift der
österreichischen Gesellschaft für Meteorologie, 1877, p. 182. The
figures 9.1.’i have been computed from the heights of the barometer
reduced to normal gravity.
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