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Bøjen løfter sig. og bringes ned igjen paa Merke ved at
man helder rent Vand fra Buretten i Fordunstningsskaalen.
Det ydre Kar dd hænger i Slingrebøjler. Paa
Bunden af Fordunstningsskaalen ligger et Thermometer til
Maaling af det fordunstende Vands Temperatur. Da det
ydre Vands Bæreevne forandrer sig med Temperaturen,
idet Bøjen flyder lettere i ’koldt end i varmt Vand,
trænger den af Buretten aflæste Vandmængde en Rettelse, for
at kunne give den virkelig fordunstede Vandmængde, i de
Tilfælder, at det ydre Vand havde en anden Temperatur
ved Forsøgets Afslutning end ved dets Begyndelse. Et
Thermometer t, der hænger ned i det ydre Vand, tjener
til at aflæse dettes Temperatur ved hvert Forsøgs
Begyndelse og Ende.
Fordunstningsmaaleren stod paa Agterdækket, paa en
Kappe (w) lige i Nærheden af Regnmaaleren (z) — (Fig. 2
Side 2). Denne Plads var vistnok ikke den heldigste, men
en anden var vanskelig at finde, og den havde den Fordel,
at Apparatet her kunde staa uforstyrret af andre
Gjøre-maal. Det havde intet ’fag, saaat det optog alt det Regn,
som faldt , derpaa. Naar den faldne Regnmængde var
større end den fordunstede Vandmængde, vilde Bøjen, idet
Ringen r toges bort, staa med Merket under Vand. Jeg
tog da med en Pipette en Del Vand ud af Skaalen a,
indtil Instrumentet kom paa Merke. Denne Vandmængde
udgjør, regnet efter dens Vægt, Forskjellen mellem
Regnmængden og Fordunstningsmængden. Da den første af
disse findes ved Hjelp af Regnmaaleren, kan den sidste
beregnes. Da den med Pipetten udtagne Vandmængde er
salt Vand, trænger den paa Pipetten aflæste (Mængde en
Correction, for at udtrykke Forskjellen mellem den Mængde
fersid Vand, der er faldt som Regn og den, der er fordunstet.
Naar Fordunstningen var større end Regnmængden, var
Fremgangsmaaden den sædvanlige, og
Fordunstningsmængden beregnedes som Summen af det af Buretten heldte
Vancl og den faldne Regnmængde.
Paa vor Rejse i 1870 var Vejret ialmindelighed
meget uroligt, og Skibets Rulling og navnlig Sætning
bevirkede meget ofte, at Vandet i Fordunstningsskaalen blev
slængt ud af denne. Derfor fik jeg kun faa
Fordunstnings-observationer den Sommer. Indhivningsmaskinen havde,
naar den var i Gang, en lignende Virkning paa
Fordunstningsmaaleren som Søgangen. I Solskin opvarmedes det
ydre Kar dd saameget, at hele Apparatets Temperatur steg
mange Grader over Luftens ’og Havvandets. Fra
Skorstenen faldt der Sodfiller i Fordunstningskarret.
For at undgaa disse Ulemper, construerede jeg et nyt
Apparat, (Fig. 13) der blev brugt i 1877 og i 1878, efter
samme Princip, men med mindre Dimensioner. Fordunst-
Den norske Nordhnvsexpedition. H. Mohn: Meteorologi.
similar to a part of the sea-surface itself. At the close of
the experiment, the leaden ring was removed, the buoy
rose, and was brought down again to the mark on the
stem by pouring fresh water from the burette into the
evapoi’ating dish.
The outer vessel d d was suspended in gimbals. At
the bottom of the evaporating dish lay a thermometer, for
measuring the temperature of the evaporating water. As
the specific gravity of the outer water varies with the
temperature, and the buoy will float higher in cold water than
in warm, the quantity of fresh water read off on the
burette requires a correction in order to determine the true
quantity of water lost by evaporation, provided the
outer water have a different temperature at the close than
at the beginning of the experiment. A thermometer t.
partially immersed in the outer water, serves for reading
off the said temperature at the beginning and end ol’ each
experiment.
The atmometer was mounted on the afterdeck, on
the cap of a hatchway (n), in immediate proximity to the
rain-gauge (z) — fig. 2, p. 2. This was certainly not the
best of places; but a better could hardly have been found
where the apparatus was out of harm’s way and caused
no inconvenience. The atmometer had no cover, and
accordingly received all the rain that fell on its surface.
When the quantity of fallen rain was greater than that of
the water evaporated, the buoy would, on the ring r being
removed, remain depressed below the mark on the stem,
the latter being under water. I then took with a pipette
a quantity of water from the dish a, till the instrument
reached the mark. This quantity of water gives by its
weight the difference between the quantity of fallen rain
and of the water evaporated. The former being found from
observation of the rain-gauge, the latter can be computed.
The water taken out with the pipette being salt, requires
a correction, in order to find the difference between the
fresh water, fallen as rain, and the fresh water evaporated.
When the amount of evaporation had execeeded the
quantity of fallen rain, the instrument was adjusted in
the usual way, the evaporation being computed as the sum
of the water poured from the burette and the quantity of
rain fallen.
During the cruise in 1876 the weather was as a rule
very boisterous, and the rolling, and more particularly the
pitching, of the vessel, caused the water to be repeatedly
thrown out of the evaporating dish; hence, only a few
experiments could be made that summer. The vibration
caused by the donkey-engine produced an effect on the
atmometer similar to that occasioned by a rough sea. When
the sun was shining, the outer vessel dd became so heated
that the temperature of the whole apparatus rose many
degrees above that of the air and the water of the sea.
Flakes of soot, too, fell from the funnel on the water
evaporating.
With a view to obviate these drawbacks, I constructed
a new apparatus (Fig. 13), which was used in 1877 and
1878. on the same principle but of smaller dimensions.
18
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