276. In this system, the air is permitted to circulate without any artificial force. In the better class of dwellings, special flues are provided, by which the foul air may pass out; but in the majority of cases it escapes only through incidental outlets, such as openings around the window casings, loosely fitting window sashes, cracks in the plastering and walls, through transoms, under doors, etc.

The problem of ventilation by this system is always combined with the question of heating. The air is moved solely by heat, which is usually applied before it enters the room. The quantity of fresh air that may enter the room depends upon its temperature, and upon the rate at which heat is lost by cooling, etc. The main object is usually to maintain a certain temperature in the apartment, the fresh air being regarded principally as a carrier of heat. If the cooling process goes on slowly, the quantity of hot air admitted must be reduced, regardless of the needs of ventilation. Thus, this system operates so as to provide the greatest supply of fresh air in very cold, windy weather, and the least in moderate, still, and humid weather, just the reverse of what should be the case. The natural ventilation system is useful in small buildings only; it is a failure in large or crowded buildings, such as churches, schools, etc.