12. Winds and Currents. The gentlest summer breezes and the fiercest
blasts of winter are produced by the unequal heating of air. We have
seen that the air nearest to a stove or hot object becomes hotter than
the adjacent air, that it tends to expand and is replaced and pushed
upward and outward by colder, heavier air falling downward. We have
learned also that the moving liquid or gas carries with it heat which
it gradually gives out to surrounding bodies.

When a liquid or a gas moves away from a hot object, carrying heat
with it, the process is called _convection_.

Convection is responsible for winds and ocean currents, for land and
sea breezes, and other daily phenomena.

The Gulf Stream illustrates the transference of heat by convection. A
large body of water is strongly heated at the equator, and then moves
away, carrying heat with it to distant regions, such as England and
Norway.

Owing to the shape of the earth and its position with respect to the
sun, different portions of the earth are unequally heated. In those
portions where the earth is greatly heated, the air likewise will be
heated; there will be a tendency for the air to rise, and for the cold
air from surrounding regions to rush in to fill its place. In this way
winds are produced. There are many circumstances which modify winds
and currents, and it is not always easy to explain their direction
and velocity, but one very definite cause is the unequal heating of
the surface of the earth.

13. Conduction. A poker used in stirring a fire becomes hot and