An illustration of a modern blast furnace plant is given in Fig. 250. A is the furnace, in which the iron ore and fuel are arranged in alternate layers. The hot air blast comes in through pipes _t_ at the bottom, called tuyeres. As gas escapes through the opening _b_ at the top, it is first cleared of dust in the settler and washer B, and then passes through the pipe C to the regenerators D D D, where it is made to heat the incoming air. The gas mixed with some air burns in the regenerators, and, after heating a mass of brick within the regenerators red hot, escapes by the underground passageway to the chimney on the right. When the bricks are sufficiently hot in one of the regenerators, gas is turned off therefrom, and into another regenerator, and fresh air from pipe H is passed through the bricks of the heated regenerator, and being heated passes out pipe F at the top and thence to the pipe G and tuyeres _t_, to promote the chemical reactions in the blast furnace. In the earlier blast furnaces a vast amount of heat was allowed to escape and was wasted. The utilization of this heat engaged the attention of Aubertot in France, 1810-14; Teague in England (British patent No. 6,211, of 1832); Budd (British patent No. 10,475, of 1845), and others. To enable the escaping hot gases to be employed for heating the hot blast regenerators a charging device is now used, as seen at a in Fig. 250, in which the admission of ore and fuel is regulated by a large conical valve, and the gases are compelled to pass out at _b_ and be utilized. Among the world’s largest blast furnaces may be mentioned the Austrian Alpine Montan Gesellschaft, which concern owns thirty-two furnaces. This is said to be the largest number owned by any one concern in the world, but most of them are of small size and run on charcoal iron. The furnaces of the United States are, however, of the largest yield, and the leading ones of these are: No. Annual capacity Furnaces. in tons. Carnegie Steel Co. 17 2,200,000 Federal Steel Co. 19 1,900,000 Tennessee Coal and Iron Co. 20 1,307,000 National Steel Co. 12 1,205,000 The present annual output of pig iron in the United States is about ten million tons, of which these four companies make about one-half. [Illustration: FIG. 251.--PUDDLING FURNACE.]