FRESH AIR 27 



which enters the room is thoroughly fresh. Figure 16 shows 

 how a schoolroom is supplied with warm fresh air. The actual 

 labor involved in furnace heating and in hot-water heating is 

 practically the same, since coal must be fed to the fire, and ashes 

 must be removed ; but the hot-water system has the advantage 

 of economy and cleanliness. 



Fresh air. Currents of fresh air are essential to normal, 

 healthy living; and 2000 cubic feet of fresh moving air per 

 hour is desirable for each individual. If a gentle breeze is 

 blowing, the barely perceptible lowering of a window will give 

 the needed air currents, even if there are no additional drafts 

 of fresh air through other openings. Most houses are so loosely 

 constructed that small currents of fresh air constantly enter 

 through cracks and crevices in loose doors, floors, and windows. 

 But except in case of high winds, this supply is never sufficient 

 in itself. It should be increased by currents of air admitted 

 through slightly lowered windows, or brought in by means of 

 a special ventilating system. 



In the preceding section, we learned that many houses 

 heated by hot water are supplied with fresh-air pipes which 

 admit fresh air into the 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 more coal to heat the larger quantity of water, 

 but the additional expense is more than compensated by the 

 gain in health. 



Wind and currents. The gentlest summer breezes and 

 the fiercest blasts of winter are produced by the unequal heat- 

 ing 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 



