Under-CURRENTS, are distinct from the upper or apparent, 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 oscillation 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 afloat, 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 found, 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 pinnace 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 afloat not being above four or five fathom deep. He added, that the lower the basket was let down, the stronger the under-current was found.
From this principle, it is easy to account for that continual indraft of water out of the Atlantic into the Mediterranean through the straits of Gibraltar, a passage about 20 miles broad; yet without any sensible 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.