ient in itself and should not be depended upon
alone. At night, or at any other time when gas lights are required,
the need for ventilation increases, because every gas light in a room
uses up the same amount of air as four people.

[Illustration: FIG. 17.--The air which goes to the schoolrooms is
warmed by passage over the radiators.]

In the preceding Section, we learned that many houses heated by hot
water are supplied with fresh-air pipes which admit fresh air into
separate rooms or into suites of rooms. In some cases the amount which
enters is so great that the air in a room is changed three or four
times an hour. The constant inflow of cold air and exit of warm air
necessitates larger radiators and more hot water and hence more coal
to heat the larger quantity of water, but the additional expense is
more than compensated by the gain in health.

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 o