Ocean Currents

Abstract

THE differences of barometric pressure to which Mr. Keith Johnston refers (NATURE, Jan. 19, p. 227) have a well-ascertained geographical existence, but his suggestion that they may originate or direct the Ocean Currents is clearly inadmissible. The high pressure over a large patch of the North Atlantic to the south or south-west of the Azores—and similarly in each of the other oceanic basins—is there permanently; and whatever disturbance might be produced by it was produced once for all when the high pressure was first formed. It would then displace a certain quantity of the water over which it rested, would thrust it out, and keep that particular part of the ocean at a slightly lower level than that over which the pressure of the air was not so great. But having done this, the adjacent bodies of water would be in hydrostatic equilibrium, and the high pressure could not continue to thrust water out towards the place of low pressure. My meaning may be at once illustrated by putting one end of an open glass tube into a basin of water, and partially exhausting the air inside it. The adjacent surface is thus exposed to a higher pressure than the surface inside the tube, and a certain portion of the water is immediately thrust from the place of greater to the place of less pressure; the column of water inside the tube is raised until the weight of the excess balances the difference of pressures. At that height it remains, and no further movement takes place, so long as the relative pressures remain the same. A fluctuation of the pressures will give rise to alternate ingoing or outgoing currents, but a continuous stream in one direction can only be produced by a continuous increase or decrease of one or the other pressure.