mere temporary and partial deflections from this grand course. The heated air at the equator rises continually and flows in an upper current towards the pole, getting gradually cooled on its way north. That from the pole flows in an under current towards the equator, getting gradually heated on its way south. We speak only of the Northern Hemisphere, for the sake of simplifying explanation,—the action of the great wind-current in the Southern Hemisphere is precisely similar.
But our broad simple statement about the upper current from the equator, and the under current from the pole, requires a slight modification, which we thought it best not to mingle with the statement itself. The heated air from the equator does indeed commence to flow in an upper current, and the cooled air from the pole in an under current; but, as the upper currents of air are speedily cooled by exposure to space, and the under currents are heated by contact with the earth's surface, they constantly change places—the lower current becoming the upper, and vice versâ. But they do not change direction. The Equatorial Current ascends, rushes north to a point about lat. 30°, where, being sufficiently cooled, it swoops down, and continues its northward rush along the earth. At another point the Polar Current quits the earth, and soaring up, in consequence of its recently acquired heat, becomes the upper current. This change in the two currents takes place twice in their course.
