It is quite another question, however, whether this theoretic
circulation, true cause as it may be, is competent to give rise to such
movements of sea-water, in mass, as those currents, which have commonly
been regarded as northern extensions of the Gulf-stream. I shall not
venture to touch upon this complicated problem; but I may take occasion
to remark that the cause of a much simpler phenomenon--the stream of
Atlantic water which sets through the Straits of Gibraltar, eastward, at
the rate of two or three miles an hour or more, does not seem to be so
clearly made out as is desirable.

The facts appear to be that the water of the Mediterranean is very
slightly denser than that of the Atlantic (1.0278 to 1.0265), and that
the deep water of the Mediterranean is slightly denser than that of the
surface; while the deep water of the Atlantic is, if anything, lighter
than that of the surface. Moreover, while a rapid superficial current is
setting in (always, save in exceptionally violent easterly winds) through
the Straits of Gibraltar, from the Atlantic to the Mediterranean, a deep
undercurrent (together with variable side currents) is setting out
through the Straits, from the Mediterranean to the Atlantic.

Dr. Carpenter adopts, without hesitation, the view that the cause of this
indraught of Atlantic water is to be sought in the much more rapid
evaporation which takes place from the surface of the Mediterranean than
from that of the Atlantic; and thus, by lowering the level of the former,
gives rise to an indraught from the latter.

But is there any sound foundation for the three assumptions involved
here? Firstly, that the evaporation from the Mediterranean, as a whole,
is much greater than that from the Atlantic under corresponding
parallels; secondly, that the rainfall over the Mediterranean makes up