aury, the distinguished
American hydrographer and meteorologist, advocated a theory of
gravitation as the chief cause of the currents, claiming that difference
in density, due to difference in temperature and saltness, would
sufficiently account for the oceanic circulation. This theory gained
great popularity through the wide circulation of Maury's Physical
Geography of the Sea, which is said to have passed through more editions
than any other scientific book of the period; but it was ably and
vigorously combated by Dr. James Croll, the Scottish geologist, in his
Climate and Time, and latterly the old theory that ocean currents are
due to the trade-winds has again come into favor. Indeed, very recently
a model has been constructed, with the aid of which it is said to have
been demonstrated that prevailing winds in the direction of the actual
trade-winds would produce such a current as the Gulf Stream.

Meantime, however, it is by no means sure that gravitation does not
enter into the case to the extent of producing an insensible general
oceanic circulation, independent of the Gulf Stream and similar marked
currents, and similar in its larger outlines to the polar-equatorial
circulation of the air. The idea of such oceanic circulation was first
suggested in detail by Professor Lenz, of St. Petersburg, in 1845, but
it was not generally recognized until Dr. Carpenter independently hit
upon the idea more than twenty years later. The plausibility of the
conception is obvious; yet the alleged fact of such circulation has been
hotly disputed, and the question is still sub judice.

But whether or not such general circulation of ocean water takes place,
it is beyond dispute that the recognized currents carry an enormous
quantity of heat from the tropics towards the poles. Dr. Croll, who has
perhaps given more attention to the physics of the subject than almost
any other person, computes that the Gulf Stream conveys to the North
Atlantic one-fourth as much heat as that body receives directly from the
sun, and he argues that were it not for the transportation of heat by
this and similar Pacific currents, only a narrow tropical region of the
globe would be warm enough for habitation by the existing faunas. Dr.
Croll argues that a slight change in the relative values of northern
and southern trade-winds (such as he believes has taken place at various
periods in the past) would suffice to so alter the equatorial current
which now fe