The Canton mail. [volume] (Canton, Miss.) 18??-1882, 1875-10-23

quently rises to 1902 occasionally to 1862
Now, as it is not the heat of the air that
thus raises the surtace-temperature, but
direct solar radiation, | can see no Other
reason why the temperature of the Equa-
torial Atlantic does not rise as high as
that 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 Gas happens along the
Guinea coast), the surface-temperaturt
rises.
Another strong indication of this as
cent is afforded by the specific-gravity
observations, very carefully and syste
matically made by the physicist of the
Challenger" 1t was long since oh
served by Humboldt, that whilst the
salinity of oceanic surf;sce-water increase,
as either tropic is approached from it.
Temperate zone, it diminishes again OF
passing from either tropic towards the
equator; and this fact was confirmed by
the observations of LEM, on whom it oh
viously made a great impression. The
progressive increase between the poles
and the tropics is obviously referrible tc
increased evaporation; but what is tn,
cause of the equatorial reduction? Tc
attribute it to the admixture of the fresh
water which desoends 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
and South America is utterly inadequate
to produce such reduction, which is as
distinct in mid-ocean as it is nearer either
t;on i afforded rs LEM pointed out. and
aS in IGNORANCE of his work. | had nv
elf 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
saiinity 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-
lions between St. Thomas's and Bermuda
gave 1.0272 as the specific gravity of
ltropicaD surface-water, and 1.0263 as
that of (polar) bottom water. But Il
mean of seventeen observations between
Cape Verde and Bahia gave 1'0263 as
the specific gravity of (equatoriaD sur
while mean of eight obser-
vations gave 1-0261 as the specific gray
ity of (polar) bottom-water,-ana 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
If South America as far as Abrolhos
Island, lat 200 S., the Challenger" then
,rossed the South Atlantic to the Cape of
food Hope first stretching obliquely to
he island of Tristan d'Aunha in lat. $50,
md then keeping in nearly the same par
lllel to the Cape. Though the depth of
his basin is less than that of the North
Atlantic, the general temperature of its
,Ottom was found to be about two de-
,rees lower, averaging 330 while the iso-
herm of $50 Law at about six hundred
athoms above the bottom thus showing
he existence of stratum of water
# one thousand eight hundred feet in
colder than any that is found
he North Atlantic. The isotherm of 400
gain, which, If the North Atlantic at
Ike distance from the equator, Law at
Iepth. here Law at between three hundred
nd five hundred fathoms; thus showing
hat the whole mass of water filling the
;outb Atlantic basin beneath that depth
las been subjected to the influence of
IL
hove the isotberm of 40o is thicker than
Inder tbe equator II il only about half
5 thick as the corresponding stratum in
he North Atlantic. and contains much
ess heat: the surface-temperature in this
ect;on nowhere rises above 500, and be-
np generally much lower, though taken
n the early summer of the southern hem
snbere
The thermal condition of the South
tIantic then. differ, from that of the
Atlantic in the greater cold
,ess of the vast mass of water occupyinp
he deener part of it's basin; and o. in
he inferior warmth of its upper stratum.
The former is clearly attributable to the
omplete freedom of communication be-
ween the South Atlantic and the ant
rctic area: the lattef is probably due to
eyeraI influences in combination, as
hall now explain.
In the first place, as has long been
nowu to physical peographers, the great
of land in the northern
emisnbere raises its General temperature:
he solar radiation raising the tempera-
n re of tho land whilst II expends itself
rben falIing ON the surface OF the sea, in
eyaporation. Hence the ther-
,al equator lies to the north of the
Equator; and the isotherm
Thich corresponds to the parallel of (say)
% south. lies in the northern hemisphere
carer the parallel of 400.
But, secondly, the peculiar direction of
he American coast-line gives a northerIy
Iant to nv far the Larger proportion of
he great Equatorial drift-current of the
tIantic. Too much attention, as it
GEMS to me, has been fixed on the part
f it which is impelIed into the Gulf of
Iexico. and which issues forth from it
) the Gulf Stream: and too little upon
hat very part which strikes the chain of
he Antilles and the peninsula of Flor
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