Dr. Arnott was one of the first to draw attention to the value of a chimney as a means of drawing off the foul air from the interior of an apartment. He invented a ventilator consisting of a well-balanced metallic valve, intended by its instantaneous action to close against down draught and so prevent the escape of smoke into a room during the use of fires. If the fire is not alight, what is known as the register of the stove should be closed, or a tight-fitting board placed in front of the fireplace, with the adoption of all chimney-ventilators fixed near the ceiling. [Illustration: 27. Harding’s Ventilator.] Harding’s ventilators are better known in the north of England than the south. They are recommended by Pridgin Teale, surgeon to the General Infirmary at Leeds, as a means of securing freshness of atmosphere without draught, and free from all mixture of dust, soot, or fog. The outside air is conducted through a grate and aperture in the wall about 7 ft. 6 in. above the floor level, where it is made to pass through a series of small tubes fixed at an angle of about 30° with the wall. The currents of air are said to be compressed while passing through the tubes, but to expand and diffuse in all directions as soon as they are liberated into the apartment. In all filtering arrangements it must be remembered that if air is to pass through a screen or filter without retarding the current entering the room through a tube, the area of the screen must be greater than the area of section of the tube. This can be effected by placing the screen diagonally within the tube which admits the air. In some buildings the filter is dispensed with, and the apparatus is used simply to diffuse the air as it enters the room. An outlet for the vitiated air is provided by the chimney flue, either through the fireplace or by a mica valve placed in the flue near the ceiling. In rooms where flues do not exist an air extractor is provided, consisting of two perforated cones and a central tube. The external air impinging upon the perforated cones is deflected, creating an induced current up the vertical tube, drawing the foul air from the interior of the room, and expelling it through the perforations. In fixing the extractor, a wooden base or frame is placed on the ridge and covered with lead to make it watertight; the extractor is then placed over this and fixed in the ordinary manner. A small inner cone is provided simply to prevent rain from getting into the tube. Harding’s extractors are so designed that they may be easily fixed inside an ornamental turret without in any way affecting their action. They can be obtained in London from Strode & Co., at prices varying from 15_s._ to 6_l._ and upwards. Their action is illustrated in Fig. 27: _a_, wall; _b_, grating outside; _c_, filter.