heaped up around the sides of the pail. Hence, in the middle
latitudes there is a greater weight of air than at the poles, and
this tends to press the lower air to higher latitudes. Centrifugal
force, however, balances this pressure, so long as the lower air
moves with the velocity of the upper strata; but as the friction of
the earth retards its motion and diminishes its centrifugal force,
it gradually yields to the pressure of the air above it, and moves
toward the poles. Near the polar circles it is again retarded by its
increasing centrifugal force, and it returns through the middle
regions of the atmosphere.

Thus there are two systems of atmospheric circulation in each
hemisphere. The principal one extends from the equator to high
middle latitudes and partly overlies the other, which extends from
the tropical calms to the polar circles. These two circulations move
in opposite directions; like two wheels, when one communicates its
motion to the other by the contact of their circumferences.

In the middle latitudes the lower current of the principal
circulation lies upon the upper current of the secondary circulation,
and both move together toward the equator. This principal lower
current first touches the earth's surface beyond the tropical calms,
and having lost its relative eastward motion and now tending westward,
it appears as the trade-wind, very regular and constant; while the
upper secondary current returns, without reaching the tropics, as an
undercurrent, and in our latitude appears as the prevailing
northeastward wind,--a very feeble motion, usually lost in the
weather winds and other disturbances, and only appearing distinctly
in the general average.