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this belt, if traced along any given meridian, would be coincident with the sea-
level in the first of these positions, whilst it would attain its maximum elevation
above the sea-level in the last ; or, taking the mean of such oscillation, that an
arc of a circle might be described somewhat nearer to the level of the sea than
the upper margin of the belt, which would denote the mean annual altitude of
the snow-line.
Inasmuch as the temperatures of the ocean and atmosphere are in a great
measure mutually dependent on each other, and both derive their heat from
the same sources, we might natm-ally expect to find a line of uniform temperature
in the sea which would bear some evident relation to that in the atmosphere ;
and such is the case, although the space above and below these respective
boundary-lines is subject to widely different laws. Thus, in the ocean we meet
with an isothermal curve, which attains its maximum depth below the surface at
the equator, and is coincident with the sea-level (that is to say, rises to the
surface) in lat. 56° 62', dipping down again as it advances from that point to
the pole. Although this isothermal curve is liable to slight oscillation from
similar causes that bring about the oscillation of the aerial belt, the variation is so
small as to be hardly perceptible ; and accordingly, a series of two larger and
two smaller arcs, described along any given meridian, will represent the line of
uniform temperature of the sea with sufficient accuracy for present purposes. But
whilst the temperature of the atmosphere beyond the line of perpetual congelation
goes on gradually decreasing*, that of the water below the isothermal line remains
constant to the bottom. AVere it not for the operation of the law on which the
latter phenomenon depends, the entire ocean would long since have become soU-
dified and both sea and land rendered unfit for the habitation of living organisms.
Unlike other bodies which expand and become lighter with every rise in temperature,
water attains its maximum density, not under the lowest degree of cold, but
at 39°'5 Fahr. ; and consequently, as soon as the superficial layer of sea is cooled
down towards this degree, it descends and allows a fresh portion to ascend and
be in turn cooled. This process is continued until the whole upper stratum is
reduced in temperature to 39°'5, when, instead of contracting further, it
begins to expand and get lighter than the water beneath, floats on it, becomes
• Aeconfing to M. Fourier, the temperature of the ethereal regions has been estimated a t —50° P.,
whilst M. Pouiflet estimates it at —220° Fahr. (Somerville’s ‘ Physical Geography,’ vol. ii. p. 83.)
further cooled down, and at 28J°* is converted into ice. In this state, having
arrived at its maximum lightness, its position remains unchanged ; and being a
bad conductor of heat, the circulation between the superficial and lower strata
ceases and thereby prevents the congelation of the entire body of water, which
would take place did the density increase indefinitely with the decrease of
temperature so as to permit of
congelation commencing from the
bottom instead of the surface of
the sea.
'Thus, under the operation of an
apparently exceptional law, the
equilibrium of the oceanic circulation
is maintained ; for whilst at
the equator the mean temperature
of the surface-layer of water, which
is 82°, gradually decreases until at
a depth of 1200 fathoms it becomes
stationary at 39°*5 Fahr.,
and retains that temperature to the
bottom, within the polar regions,
and extending to lat. 56° 25' in
either hemisphere, the temperature
increases from the surface downwards
to the isothermal line, beyond
which it remains uniform as
in the former case. Hence in lat.
56° 25', the temperature is uniform
the whole way from the surface to
the bottom, and, as has been found
by observation, about lat. 70° the
isothermal line occurs at 750 fathoms
below the surface. The
annexed diagram will give a general
* The freezing-point of salt water.
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