[Illustration: FIG. 16.--Fresh air from outside circulates over the radiators and then rises into the rooms to be heated.]
11. Fresh Air. Fresh air is essential to normal healthy living, and 2000 cubic feet of air per hour is desirable for each individual. If a gentle breeze is blowing, a barely perceptible opening of a window will give the needed amount, even if there are no additional drafts of fresh air into the room through cracks. Most houses are so loosely constructed that fresh air enters imperceptibly in many ways, and whether we will or no, we receive some fresh air. The supply is, however, never sufficient 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.