The eliminator is composed of a series of irregular sheet-metal surfaces so arranged that the air is required to abruptly change its direction several times in its passage of a short distance. The impact of the air against the surfaces and the centrifugal force exerted by the sudden changes of direction throw out the excess moisture and any remaining suspended matter the air may contain. The saturated air from the eliminator passes through a heater where the temperature is raised to that of the rooms. In the rise of temperature the air which is saturated is rendered capable of absorbing more moisture, and hence has been dehumidified. The rise of temperature and the corresponding decrease in relative humidity is intended to be such as to leave in the finished air the desired percentage of moisture. =Air-cooling Plants.=--The use of air-washing and humidifying plants so far mentioned has been confined to elimination of dust and the addition of moisture to air, under winter conditions. The same type of apparatus, used in summer, becomes a cooling plant, and by observance of the necessary requirements may be used to produce agreeable atmospheric conditions during hot weather. When used for such purpose the air is washed, by passing it through falling water which frees it from dust and reduces its temperature. It is then further cooled by passing over cold surfaces that take the place of the heaters used in cold weather. The cooling surfaces are pipe coils kept cold by the contained water which comes from the water supply or from a refrigerating plant. The temperature and humidity are thus changed to suit the requirements of the conditioned air. During the hot weather of summer the most disagreeable atmospheric condition is that caused by humidity near saturation, at a time of relatively high temperature. Under such conditions the cooling plant not only cools the air, but causes a precipitation of the moisture on the cold surfaces which are kept below the dew-point. The air is cooled and dehumidified to a point such that the conditioned air produces an agreeable atmosphere. The regulation of the degree to which the air is cooled is accomplished by the same general methods as are used in heating. =Humidity Control.=--The method of regulating atmospheric humidity in a humidifying plant will be determined by the conditions under which it is intended to work. There are a variety of means employed that may be used to bring about the same effects, each of which is particularly suited to certain requirements. The present object is to describe the essential features of airconditioning plants, by use of illustrations representing each of the three methods mentioned above. That of the ventilation of a school building under winter conditions will be taken as an example.