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dudes that, as a general rule, both iu tbe Atlantic
and tbe Pacific Oceans, between tbe parallels of 40°
N. and 40° S., tbe specific gravity reduced to 15°-56 C.,
is greatest at or near tbe surface, and decreases more
or less regularly until a minimum is reached, generally
400 fathoms from the surface, AA'hence there
is a slow rise, the hottom-water being slightly
heavier.
Prom Mr. Buchanan’s report, and from the specific
gravity Tables of tbe year 1873, we come then to tbe
broad conclusion that tbe density of the upper-layers
of the North Atlantic is considerably higher tban that
of any other part of tbe ocean, and the specific gravity
tables for tbe spring months of 1876 give tbe same
result. I need scarcely say tbat tbis is exactly wbat
must bave been anticipated, if my view be correct,
tbat tbe movement of deep-water in tbe Atlantic is
mainly due to excess of evaporation over precipitation
in its northern portion.
An element of great uncertainty is undoubtedly
introduced into the determination of the specific
gravity of surface by tbe weather. Tbese determinations
were made as usual daily for tbe section between
Stations 323 and 335 from Monte Video to Tristan
d’Acunba in the beginning of March, 1876, and the
mean of tbese, the temperature reduced to ]5°-56 C.,
was 1-02620. Of the eighteen days occupied in running
tbe section, nine were dry and fine, and on nine
rain fell either continuously or in showers. Tbe
mean for the nine dry days was 1-02639, and for tbe
nine wet days 1-02591. Tbe maximum surface specific
gravity for the section (1-02680) was at Station 323,
at tbe point wbere probably the Brazil current has
most effect on tbe surface, and tbe minimum (1-02494)
Avas at Station 326 after a heavy fall of rain. Tbe
mean specific gravity of tbe surface-water at tbe
temperature at which it was procured was 1-02502.
The specific gravity of the hottom-water Avas determined
at ten stations on the section. Reduced to
a temperature of 15°-56 0., the mean was 1-02601 ;
the maximum, 1-02650, was at Station 323 at a
depth of 1,900 fathoms ; and the minimum, 1-02580,
was at Station 326 at 2,775 fathoms. The mean
specific gravity of the hottom-water at the depth at
which it was procured was 1-02811, shoAving a difference
between the two means of 0-00210, due to
difference of temperature alone.
It seems from these observations th a t the differences
of surface specific gravity due to differences of
salinity along the section are very small, and that,
AA'ith the exception possibly of Station 323, Avhich is
abnormal in many respects, they depend mainly on
the rainfall.
The difference between the mean surface specific
gravity, the temperature reduced to 15°-56 C., and
the mean hottom specific gravity under tlie same
conditions is also very slight ; the actual specific
gravity at every point is practically determined hy
the temperature ; and cons«quently the hands of
equal density are, like the bands of equal temperature,
virtually continuous with those of the Southern
Sea.
The Amount o f Carbonic Acid contained in Seawater.—
I give in Appendix C. to this chapter a table
of carbonic acid determinations from Mr. Rucbanan’s
‘ Laboratory Work ; ’ and tbe substance of tbe feAV