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vægt ved 17" 5 C. Disse Observationer bleve dog selv
paa de to sidste Togter ikke anstillede i.samme
Udstrækning som Egenvægtsbestemmelserne. der ogsaa oprindelig
vare bestemte til i förste Række at tjene som Maal for
den samlede Saltgehalt.
Expeditionen var for Egenvægtsbestemmelser forsynet
med flere Sæt Glasaræometre fra Dr. Kiichler i Ilmenau,
17° 5
indrettede til at vise Søvandets Eirenvægt ved , _ saa-’
iro
ledes at et Sæt viste Egenvægter fra 1 til 1.007. et andet
fra 1.006 til 1.013, et tredie fra 1.012 til 1.019. et fjerde
fra 1.018 til 1.025 og et femte fra 1.024 til 1.031.
Aræo-metrene vare inddelte i Delstreger af Værdi 0.0002. medens
Afstanden mellem disse Delstreger paa Scalaen beløb sig
til meget nær 1.5mm, saaledes at man maatte kunne aflæse
uden stor Feil det 5te Decimal. Under Aflæsningen af
Vandprøvernes speciliske Vægt anbragtes clisse i en i
dobbelt Slingrebøile ophængt Glascylinder, hvis indre Diameter
belob sig til omtrent det tredobbelte af Aræometrets
Corpus. hvorefter dette omhyggeligt renset og aftørret
neddykkedes i Vædsken og tillodes at svømme frit i nogen
Tid. indtil det. havde antaget Vandets Temperatur.
Aflæsningen foretoges nu langs den undre Rand af Vædskens
Niveau, idet samtidig Vandets Temperatur iagttoges paa et
controlleret Thermometer, inddelt i Delstreger af Værdi
0.0 2.
Paa Grund af det af Expeditionen benyttede
Dampskibs fortrinlige Egenskaber som Søskib voldte disse
Observationer i nogenlunde roligt Veir ingensomhelst
Vanskeligheder, selv naar Kursen sattes ret mod Vinden, hvorimod
Skibets Duvning i meget haardt Veir altid ytrede sig i
mærkbare om end smaa Bevægelser hos Aræometret. Hvor
Vandprøverne optoges i saa uroligt Veir, at Bestemmelsen
paa Grund deraf kunde medføre forøget Usikkerhed, bleve
de altid hensatte nogle Dage. indtil de kunde undersøges
under mere gunstige Vilkaar.
Disse saaledes aflæste Egenvægter maa imidlertid i 2
Henseender forbedres, idet man paa den ene Side maa
anvende passende Correctioner for at faa de ved meget
forskjellige Temperaturer aflæste Egenvægter reducerede til
den fælles Normaltemperatur 17." 5. og paa den anden Side
maa befrie dem for. Aræometrenes constante Feil.
Hvad for det Første Correctionerne for Temperaturen
angaar, da give de af flere Videnskabsmænd udførte
Bestemmelser af Søvandets Volumforandring med Temperaturen
Midlerne til at beregne clisse, idet baade Hjibbarcl2, L. F.
1 Naar her som ofte senere brages Betegningsmaaden Egenvægt ved
t°
^ö, da menes dermed Egenvægt ved t° i Forhola til destilleret Vand
af T° som Enhed. Alle Temperaturangivelser i denne Afhandling ere
udtrykte i. Grader Celsius.
2 Maury’s Sailing Directions 1858, — 1 — 237.
Expedition. K those amounts in cubic centimetres
corresponding to lcc of the solution of silver, and 8 the specific
gravity of the standard sample at 17° 5 C. These
observations, however, were not instituted even on the two last
voyages to the same extent as those based on
determinations of specific gravity, the method by which, as
originally agreed upon, the total amount of salt was to be chiefly
computed.
For performing specific gravity determinations, the
Expedition had been supplied by Dr. Kiichler of Ilmenau
with clivers, sets of glass areometers, adapted to show the
17" 5
specific gravity of sea-water at ^^ .one set indicating
specific gravities from 1 to 1.007. another from 1.006 to
1.013. a third from 1.012 to 1.019, a fourth from 1.018
to 1.025, and a fifth from 1.024 to 1.031. The areometers
were graduated in degrees of 0.0002, the interspaces on
the scale measuring however very nearly* 1.5"""; and hence
you could read off with comparative accuracy to the fifth
decimal. When about to read the specific gravity, the
samples of water were poured into a glass cylinder
suspended in gimbals, the inner diameter of the cylinder being
triple that of the areometer, which, carefully wiped and
dried, was immersed in the fluid t and suffered to float
freely for some time till of the same temperature as the
water. The specific gravity was now read in the ordinary
way. the temperature of the water, as shown by a tested
thermometer graduated in fifths of a degree Centigrade,
being simultaneously observed.
The steamer selected for the Expedition being an
excellent sea-boat, these observations were attended with
no difficulty whatever in moderately fair weather, even
when steaming dead against the wind; pitching, however,
was found to have a distinctly disturbing effect on the
areometer, and therefore all samples of water drawn when
it was in any way violent! so as to give reason for
apprehending greater uncertainty in the determinations if
performed at once, were stored for a few clays, till the weather
had improved.
These readings of specific gravity have, however, a
twofold need of correction, arising on the one hand from the
very different temperatures at which the specific gravities
were read, involving the necessity of their reduction by
proper corrections to the normal temperature 17° 5, aud
ou the other, the constant error of the areometer, which
has also to be eliminated.
As regards the corrections for temperature, these
may be computed by the determinations performed by
clivers men of science of the extent to which the volinne of
sea-water varies with the temperature, Hubbard,2 L. F.
1 The expression, specific gravity at signifies specific’gravity at
t°, with distilled water of T° as the unit of comparison. All statements
of temperature in this Memoir are given in degrees Celsius.
2 Maury’s Sailing Directions, 1858, 1, p. 237.
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