In the year 1812, Mr. William Evetts, of Sheffield, took out a patent for improved reverberatory furnaces for melting metals, in which he introduced what he termed an air conductor, for the purpose of conveying a stream of pure air upon the surfaces of the metallic substances under reduction. This air conductor consisted of a vertical passage or tube, made in the bridge or wall of brickwork at the back of the furnace, the lower end of which opened into the ash pit, where it was widened, and the size of the aperture regulated by a valve, which valve was operated upon by a long rod passing through the front enclosure of the ash pit; the upper extremity of the air tube or passage did not pass vertically through the bridge, but had a horizontal turn given to it, by which the jet was thrown upon the substances under operation, or against the current of heated vapours, before they passed over the bridge; and this minor stream of fresh air was found to impart sufficient oxygen to the carbonaceous matter of the smoke, and burn it.