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 heats the hand grasping the poker, although only the opposite end of the poker has actually been in the fire.  Heat from the fire passed into the poker, traveled along it, and warmed it.  When heat flows in this way from a warm part of a body to a colder part, the process is called conduction.  A flatiron is heated by conduction, the heat from the warm stove passing into the cold flatiron and gradually heating it.

In convection, air and water circulate freely, carrying heat with them; in conduction, heat flows from a warm region toward a cold region, but there is no apparent motion of any kind.

Heat travels more readily through some substances than through others.  All metals conduct heat well; irons placed on the fire become heated throughout and cannot be grasped with the bare hand; iron utensils are frequently made with wooden handles, because wood is a poor conductor and does not allow heat from the iron to pass through it to the hand.  For the same reason a burning match may be held without discomfort until the flame almost reaches the hand.

Stoves and radiators are made of metal, because metals conduct heat readily, and as fast as heat is generated within the stove by the burning of fuel, or introduced into the radiator by the hot water, the heat is conducted through the metal and escapes into the room.