The herald and mail. [volume] (Columbia, Tenn.) 1873-188?, 1875-10-22

lnuS raises tne suTIdce-IeIlp erdture, but
direct solar radiation, can see no Other
reason why the temperature of the Eoua.
tOrial Atlantic does not rise as high as
shat of the Red Sea, than that the for.
mer is kept down by the continual ascent
of cooler water. Where such an ascent
is prevented by the comparative shallow.
ness of the bottom (as happens along the
Guinea cOast), the face-temperature
rises.
Another strong indication of this as
cent is afforded by the specifc-gravitv
observations, very carefully and syste-
matically made by the physicist of the
Challenger" 1t was long since OBJ
served by Humboldt, that whilst the
salinity of oceanic surface-water increases
as either tropic is approached from its
Temperate zone, it diminishes again on
passing from either tropic towards the
equator; and this fact was confirmed by
the observations of LEM, on whom it OBJ
viOusly made great impression. The
progressive increase between the poles
and the tropics is obviously referribie to
increased evaporation; but what is the
cause of the equatorial reduction? To
attribute it to the admixture of the fresh
water which descends in equatorial
rains seems absurd because all this wa
ter, and more, has been pumped up by
evaporation from the equatorial area
itself. And the quantity of fresh water
discharged by the great rivers of Africa
AID South America is utterly inadequate
to produce such a reduction, which is as
distinct in mid-Ocean as it is nearer either
of these continents. The true eXplana-
ton is afforded, as LEM pointed out, and
as, in ignorance of his work, | had my
self suggested, by the ascent of the polar
overflow which brings all the way to
the equator the low salinity it has de-
rived in the polar areas from the melt
ing of ice and snow. This reduction of
salinity not being sufficient to neutralize
the increase of specific gravity produced
by reduction of temperature, the polar
water continues to underlie the more sa-
line water above it; but rising to the
surface as the latter is draughted off, it
continues to show its polar source by its
low specific gravity, even when its tenn
perature has been raised to the equatO-
rial standard. A mean of eight Observa-
tions between st. Thomas's and Bermuda
(tropical) surface-water, and 1'0263 as
that of (pOlar) bottom water. But a
mean of seventeen observations between
Cape Verde and Bahia gave 1'O263 as
the specific gravity of (equatoriaD sur
tace-water, while a mean of eight Obser-
vatiOns gave 1'0261 as the specific grav-
ity of (polar) bottom-water,-an approx
imatiOn so remarkable, that it can
scarcely be considered as. capable of any
other explanation, than that the equatO-
rial surface-water is really polar water
which has risen up from beneath.
Proceeding sOuthwards along the coast
of South America as far as AbrOlhos
Island, lat 200 s., the " Challenger" then
crossed the South Atlantic to the Cape of
Good Hope; first stretching Obliquely to
the island of Tristan d'Aunha in lat $50,
and then keeping in nearly the same par
allel to the Cape. Though the depth of
this basin is less than that of the North
Atlantic, the general temperature of its
bottom was found to be about two de-
grees lower, averaging $50, while the iso-
therm of $50 lay at about six hundred
fathoms above the bottom thus showing
the existence of a stratum of water
of one thousand eight hundred feet in
thickness, colder than any that is found
(save under eXceptiOnalc ircumstances) in
the North Atlantic. The isOtherm of 40'
again, which, in the North Atlantic at a
like distance from the equator, lay at
eight hundred or nine hundred fathOms'
depth, here lay at between three hundred
and five hundred fathOms; thus showing
that the whole mass of water filling the
South Atlantic basin beneath that depth
has been subjected to the influence of
cold. Though the stratum
above the isOtherm of 40o Is thicker than
under the equator, it is Onl about half
As thick as the corresponding stratum in
the North Atlantic, and contains much
less heat: the surface-temperaturei IN this
section nowhere rises above 60, and be-
ing generally much lower, though taken
in the early summer of the southern hem
sphere.
The thermal condition of the South
Atlantic, then, differs from that of the
North Atlantic-(D in the greater cold
ness of the vast mass of water occupying
the deeper part of its BASIN and (2) in
the inferior warmth of its upper stratum.
The former is clearly attributable to the
freedom of communication be
tween the South Atlantic and the ant
arctic area; the latter is probably due to
shall now explain.
In the first place, as has long been
known to physical geographers, the great
of land in the northern
raisesits general temperature;
the solar radiation raising the tempera
ture of the land, whilst it eXpends itself,
when falling on the surface of the sea, in
evaporation. Hence the tier
NAL equator lies to the north of the
Equator; and the isotherm
which corresponds to the parallel of (say)
300 south, lies in the northern hemisphere
nearer the parallel of 4O0.
But, secondly, the peculiar direction of
the American coast-line gives a nOrtherly
slant to by far the larger proportion of
the great Equatorial drift-current of the
Atlantic. Too much attention, as it
seems to me, has been fixed on the part
Of it which is impelled into the Gulf of
Mexico, and which issues forth from it
As the Gulf Stream; and too little upon
that very part which strikes the chain of
the Antilles and the peninsula of Flor
ida and which must be thus turned back
without entering the gulf. Although
this drift-current is quite superficial, its
depth being estimated by Captain Mares
1t no more than fifty fathoms, yet it seems
to me quite conceivable that the vast
body of water it conveys should make its
way downwards, when its onward motion
is checked, so as to raise the temperature
Of the sub-surface layer, and that this is
the explanation of the marked thickness
IF the stratum of between go and 65o
Fahr., which has been already noticed as
4 peculiar feature of the sections taken in
the western portion of the North Allan
tic (p. 12).
In the third place, the progressive mar
rowing of the North-Atlantic basin from |
The equator to the arctic circle, and the |
widening of the basin of the |
south Atlantic from the equator south
wards, may be expected toeXert precisely |
opposite influences upon the thickness of |
hat upper stratum, which, e. hypothesi, |
S being drawn, in each hemisphere, from |
he Equator towards its pole-tending, in
he first case, to increase its thickness by |
ateral compression, and, in the second, |
O diminish its thickness by lateral ex-
pansion. In virtue of this excess of
thickness, the north-mOving stratum in
he former will possess a far greater power
If resisting the influence of atmospheric
old, than the sOuth-mOving stratum in
I ...,, L.. E,,l =, Z