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Siile
eller 37 Procent ora Natten. Luftens Temperatur var
c. 25°. Vindhastigheden liden — 3 til 4 Meter pr. Sekund.
Fordunstningens Hurtighed afhænger af det
fordunstende Vands Temperatur, dets Saltholdighed, Luftens
Fugtighed, Lufttrykket og Vindens Hastighed. I
Fordunstningsmaaleren kan Temperaturen vanskelig holdes saa lav,
som den er i Havoverfladen, og Luftens Temperatur er
ogsaa i Regelen højere ved Apparatet end over Havet.
Vandets Saltholdighed er. praktisk taget, den samme paa
begge Steder, da Søvandet skiftes i Fordunstningsmaaleren
liver Dag. Luftens absolute Fugtighed og Lufttrykket er
ogsaa ligestort paa begge Steder. Men Vindens Hastighed
er forskjellig, dels fordi Skibet, paa hvilket Apparatet staar,
bevæger sig gjennem Vandet, dels fordi Vinden paa
Dækket altid er svagere end paa Søen. Denne Omstændighed
gjør, at den ombord maalte Fordunstning i Regelen vil
være mindre end Fordunstningen over selve Havfladen.
Og hertil kommer endvidere den Omstændighed, at
Havfladen med sine Bølger og end mere med Sprøjtet af disse
frembyder en i Forhold til den rolige horizontale
Vandflade. saaledes som den er i Fordunstningsmaaleren. i høj
Grad forøget Fordunstnings-Overflade. Den med vore
Apparater maalte Størrelse af Havvandets Fordunstning kan
derfor ikke repræsentere den fulde Fordunstning paa
Havets Overflade, men har i det højeste kun en relativ Værdi.
De Forsøg, som vi have gjort paa at beregne de
ovenfor meddelte Fordunstningsforsøg efter Weilenmann’s
Formler1), have ikke givet noget gyldigt Resultat. For
1877, i hvilket Aar vi havde den roligste Sø. stemmer
Formelen bedst med Maalingerne, men der bliver en
Middelafvigelse mellem de (for 12 Timer) beregnede og
observerede Værdier af Fordunstningshøjden af + 0.m’"0. Som
ovenfor omtalt, ansloges, efter den Nøiagtighed, hvormed
Apparatet kunde indstilles paa Merke, denne Størrelse til
+ 0.""" 1. og de to forskjellige Apparater stemme i sine
Resultater paa + 0."""37. For 1878 kom Coefficienten for
det Led. der indeholder Vindens Hastighed, ud med
negativt Fortegn. For Observationerne fra Vestindien fandtes
Constanterne ganske forskjellige fra de for» 1877 fundne og
en Middelafvigelse mellem observeret og beregnet
Fordunst-ningshøjde for 12 Timer af + l.mmö. De Betingelser,
under hvilke Apparatet har virket og de Elementer, med
hvilke Beregningen er gjort, synes saaledes ikke at stemme
med Formelens Forudsætninger. Navnlig skal jeg i den
Anledning bemerke, at ombord i „Vøringen" maaltes den
i Formelen benyttede Vindhastighed oppe paa Hyttedækket,
medens Fordunstningsmaaleren stod meget mindre udsat for
Vindens fulde Styrke. De paa begge disse Steder
stedfindende Vindhastigheder staa heller neppe altid i samme
Forhold til hverandre. I 1878 var Vejret ofte meget
uroligt, og Apparatet bevægede sig ikke rigtig godt i sine
’) Sclvweizerisehe Meteorolo<rische Beobachtiuigen 1875.
4.mm90. or 03 per cent during the day (8 a.m. to 8
p.m.), and 2.m"’83, or 37 per cent during the night. The
temperature of the air was about 25° C. Velocity of the
wind inconsiderable, 3 to 4 metres per second.
The rate of evaporation is dependent on the
temperature of the evaporating water, the amount of salt, the
humidity of the air, the pressure of the air, and the velocity
of the wind. In the atmometer, the temperature can hardly
be kept so low as it is at the sea-surface, and moreover,
the temperature of the air is as a rule higher around the
apparatus than above the sea. The proportion of salt in
the water is, practically speaking, the same for both, the
sea-water in the atmometer having been changed every
day. The velocity of the wind, however, is different, partly
since the ship in which the apparatus is mounted moves
through the water, and partly because the velocity of the
wind is less on deck than on the sea-surface. Owing to
this circumstance, the evaporation as measured on board
will generally be less than the evaporation going on at the
surface of the sea. And moreover, the surface of the ocean,
by reason of the waves, and more especially of the spray,
presents, as compared with the calm, horizontal surface of
the water in the atmometer, a much greater evaporating
surface. The evaporation of sea-water, as measured with
our apparatus, cannot, therefore, represent the full amount
of evaporation at the sea-surface, but has, at most, only a
relative value.
Our computations, made according to Weilenmann’s
formulae,1 of the evaporation experiments described above,
did not give a trustworthy result. For 1877, the year in
which we had the calmest sea, the formula agrees best
with my measurements; there is, however, a mean
difference (for 12 hours) between the computed and the observed
values for the depth of the water evaporated of + 0.mra6.
As stated above, this difference was estimated, from the
accuracy with which the buoy could be adjusted to the
mark, at + 0."""1. and the results found with the two
different apparatus agree within + 0.mm37. For 1878,
the coefficient for the term containing the velocity of
the wind came out with a negative sign. For the
observations from the West Indies, the constants were widely
different from those found in 1877, and the mean difference
between the observed and the computed depth of the water
evaporated in 12 hours was + l.mm5. Hence, the
conditions under which the apparatus gave its results, would
not appear to agree with the assumptions of the formula.
In particular, I will observe, that on board of the
"Vørin-gen," the velocity of the wind introduced into the formula
was measured from the roof of the roundhouse, whereas
the atmometer was far less exposed to the full force of
the wind. Nor do the velocities of the wind in both places
at all times bear the same relation to each other. In
1878, the weather was often very boisterous, and the ap-
’) Schweizerisehe Meteorologische Beobaehtungen, 1875.
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