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354 THE ATLANTIC. [ c h a p . V.
the conditions of temperature, on which the distribution
oi animals mainly depends, are nearly similar.
The Density o f Sea-icater.—Tbe specific gravity of
tbe surface-Avater was determined daily by Mr. J. Y.
Bucbanan, tbe cbemist to tbe Expedition, witb great
accuracy; the specific gravity of the hottom-Avater
Avas also determined so far as possible at every observing
station, and every opportunity Avas taken
to procure for pbysical and chemical examination
samples of Avater from intermediate depths. Qn our
return home through the Pacific, Mr. Buchanan,
at my request, prepared a preliminary report on his
method of investigation and on the general results
of his work, which I received at Valparaiso; and
from that report the folloAving summary of specific
gravity conditions in the Atlantic, according to the
first year’s observations, is taken. The apparatus
in use for procuring water from the hottom and
from intermediate depths has heen already described
(vol. i. p. 34 et seq.).
Representiug tbe specific gravity of distilled water
at 4°C. by 100,000, Mr. Bucbanan found tbat of
ocean-Avater at 15°-56 C. to vary between tbe extremes
of 102780 and 102400; so tbat, to be of
any value at all, tbe possible error in tbe results
must not exceed 10. Tbe hydrometer used for tbese
observations is fully descrilied in a paper presented
to tbe Royal Society by Mr. Bucbanan early in
1875, and published in abstract in the ‘Proceedings’
for that year. Its description is briefly as follows :__
The stem, Avhicb carries a millimetre-scale 10 centimetres
long, has an outside diameter of about 3
millimetres, tbe external volume of the divided
CHAP. V.J GENERAL CONCLUSIONS. 35 D
portion being 0'8607 cubic centimetre ; tbe mean
volume of tbe body is 160'15 cubic centimetres,
and tbe weight of the glass instrument is 160’0405
grammes. With this volume and Aveight it floats
in distilled water of 16° 0., at about the lowest
diAusion (100) of the scale. In order to make it
serviceable for heavier waters, a small brass table
is made to rest on tbe top of the stem, of such a
weight that it depresses the instrum.ent in distilled
Avater of 16° C. to about the topmost division (0) of
the scale. By means of a series of six weights,
multiples by 1, 2, 3, 4, 5, and 6 of the weight of
the table, specific gravities between 1-00000 and
1‘03400 can be observed. It is not necessary that
these weights should he accurate multiples of the
weight of the tahle; it is sufficient if they approach
it within a centigramme, and their actual weight
be known with accuracy. The weights of the table
and weights in actual use are :—
Weight of ta b le ...................................... 0-8360 gramme.
„ of weight No. 1................... 0-8560 „
I I ................... 1-6010
” )) I I I ................... 2-4225 grammes.
IV .................. 3-1245
V .................. 4-0710
V I .................. 4-8245
Eor ocean-waters the hydrometer is always used Avith
the table and either No. IV. or No. V. weight.
When the mechanical part of the construction of
tbe instrument Avas finished, Avith the exception of
the closing of the top of the stem (which instead was
AA'idened into a funnel-shape large enough to receiA c
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