inland sea is upon an average
higher, by 3-1/2 degrees of Fahrenheit, than the eastern part of the
Atlantic Ocean in the same latitude, which must greatly promote its
evaporation. The Black Sea being situated in a higher latitude, and
being the receptacle of rivers flowing from the north, is much colder,
and its expenditure far less; accordingly it does not draw any supply
from the Mediterranean, but, on the contrary, contributes to it by a
current flowing outwards, for the most part of the year, through the
Dardanelles. The discharge, however, at the Bosphorus is so small, when
compared to the volume of water carried in by rivers, as to imply a
great amount of evaporation in the Black Sea.

_Whether salt be precipitated in the Mediterranean._--It is, however,
objected, that evaporation carries away only fresh water, and that the
current from the Atlantic is continually bringing in salt water: why,
then, do not the component parts of the waters of the Mediterranean
vary? or how can they remain so nearly the same as those of the ocean?
Some have imagined that the excess of salt might be carried away by an
under-current running in a contrary direction to the superior; and this
hypothesis appeared to receive confirmation from a late discovery, that
the water taken up about fifty miles within the Straits, from a depth of
670 fathoms, contained a quantity of salt _four times greater_ than the
water of the surface. Dr. Wollaston,[457] who analyzed this water
obtained by Captain Smyth, truly inferred that an under-current of such
denser water flowing outward, if of equal breadth and depth with the
current near the surface, would carry out as much salt below as is
brought in above, although it moved with less than one-fourth part of
the velocity, and would thus prevent a perpetual increase of saltness in
the Mediterranean beyond that existing in the Atlantic. It was also
remarked by others, that the result would be the same, if the swiftness
being equal, the inferior current had only one-fourth of the volume of
the superior. At the same time there appeared reason to conclude that
this great specific gravity was only acquired by water at immense
depths; for two specimens of the water, taken within the Mediterranean,
at the distance of some hundred miles from the Straits, and at depths of
400 and even 450 fathoms, were found by Dr. Wollaston not to exceed in
density that of many ordinary samples of sea-water. Such being the case,