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unexceptionally. I therefore, as already mentioned in the
t’oregoing, consider 0° theoretically and practically to be the
natural starting-point of all volumetric experiments connected
with arctic hydrography. Consequently I intend to refer the
observed numbers of specific gravity to the common standard
73-73 C, notwithstanding the areometer teas adjusted conformably to
’ _L 15°
another, viz. C.
The practical advantages will best be seen from the
following 1 example.
Suppose the areometer to have attained its equilibrium
in the water. Then the weight of the instrument is
counterbalanced by the weight of the displaced volume of water —
therefore it sinks in the fluid to a certain mark and not
further. In order to obtain satisfactory results the glass body
of the areometer must have the temperature of the fluid.
Only in the case of the dilatation of the fluid being equal to
that of the glass it would be immaterial to our purpose, at
what temperature the measurement was performed, and the
readings of the areometer could be used without any
corrections whatever. The coefficient of dilatation of sea-water, as
may be seen from the tables and plates of the foregoing
paper, is highly dependant upon its amount of salt and
likewise upon its temperature. At + 15° C and + 17°.s C, the
points, to which ordinarily areometric measurements are
referred, the difference of the expansion of sea-water and of
glass is very great and necessitates careful corrections, but
at temperatures not exceeding a few degrees on either side of
0° C we may totally dispense with every correction depending
on the temperature and still obtain numbers representing the
specific gravity of the water with correctness in the fourth
decimal.
The areometer used on the Vega and afterwards forwarded
for my inspection was graduated so as to allow observations
of O.0001 of the specific gravity. For the present problems
of hydrography this approximation is quite sufficient, because
it enables us to calculate the saltness of the water unto O.01 %.
1 The following application of the principle of Archimedes could
easily be disguised in a more mathematical garb. The advantage, however,
of more generalized formula; would not in the eyes of the author compensate
for the loss of a plain manner of demonstration, intelligible to every student
of hydrography.
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