Under-CURRENTS, are distinct from the upper or ap-
parent, and in different places set or drive a contrary
way. Dr Smith makes it highly probable, that in
the Downs, in the straits of Gibraltar, &c. there is
an under-current, whereby as much water is carried
out as is brought in by the upper currents. This he
argues from the offing between the North and South
Foreland, where it runs tide and half-tide, i. e. it is
ebb or flood in that part of the Downs three hours
before it is so off at sea: a certain sign, that though
the tide of flood runs aloft, yet the tide of ebb runs
under-foot, i. e. close by the ground; and so at the
tide of ebb it will flow under foot. This he confirms
by an experiment in the Baltic sound, communicated
to him by an able seaman present at the making of it.
Being there then with one of the king's frigates, they
went with their pinnacle into the mid-stream, and
were carried violently by the current. Soon after
that, they sunk a basket with a large cannon bullet to
a certain depth of water, which gave a check to the
boat's motion; and sinking it still lower and lower,
the boat was driven a-head to the windward against
the upper current, the current aloft not being above
four or five fathom deep. He added, that the lower
basket was let down, the stronger the under-current
was found.
From this principle, it is easy to account for that
continual indraught of water out of the Atlantic into
the Mediterranean through the straits of Gibraltar, a
passage about 20 miles broad; yet without any sen-
sible rising of the water along the coasts of Barbary,
&c. or any overflowing of the land, which there lies
very low.—Dr Halley, however, solves the currents
setting in at the straits without overflowing the banks,
by the great evaporation, without supposing any under
current.