other
general principles, seems simple enough once it is pointed out.

With that broad sweep of imagination which characterized him, Humboldt
speaks of the atmosphere as the "aerial ocean, in the lower strata
and on the shoals of which we live," and he studies the atmospheric
phenomena always in relation to those of that other ocean of water. In
each of these oceans there are vast permanent currents, flowing
always in determinate directions, which enormously modify the climatic
conditions of every zone. The ocean of air is a vast maelstrom, boiling
up always under the influence of the sun's heat at the equator, and
flowing as an upper current towards either pole, while an undercurrent
from the poles, which becomes the trade-winds, flows towards the equator
to supply its place.

But the superheated equatorial air, becoming chilled, descends to the
surface in temperate latitudes, and continues its poleward journey as
the anti-trade-winds. The trade-winds are deflected towards the west,
because in approaching the equator they constantly pass over surfaces of
the earth having a greater and greater velocity of rotation, and so, as
it were, tend to lag behind--an explanation which Hadley pointed out in
1735, but which was not accepted until Dalton independently worked it
out and promulgated it in 1793. For the opposite reason, the anti-trades
are deflected towards the east; hence it is that the western, borders
of continents in temperate zones are bathed in moist sea-breezes, while
their eastern borders lack this cold-dispelling influence.

In the ocean of water the main currents run as more sharply
circumscribed streams--veritable rivers in the sea. Of these the best
known and most sharply circumscribed is the familiar Gulf Stream,
which has its origin in an equatorial current, impelled westward by
trade-winds, which is deflected northward in the main at Cape St. Roque,
entering the Caribbean Sea and Gulf of Mexico, to emerge finally through
the Strait of Florida, and journey off across the Atlantic to warm the
shores of Europe.

Such, at least, is the Gulf Stream as Humboldt understood it. Since his
time, however, ocean currents in general, and this one in particular,
have been the subject of no end of controversy, it being hotly disputed
whether either causes or effects of the Gulf Stream are just what
Humboldt, in common with others of his time, conceived them to be. About
the middle of the century Lieutenant M. F. M