TAPPlNG-OUT AND STOPPING-UP FURNACES, ETC. 91
the hearth or crucible is not mtich over four inches below the level of the notch, but continual running and £< fast driving- " of a furnace soon cut out the bottom lining, so that it is no uncommon result for metal to burn the bottom down two to three feet below the level of a notch, as indicated by the dotted line S in Fig. 18. Furnacemen claim it is not until a bottom is cut down for a foot or two that the best output and quality of product can be obtained, and also that a deep bed is very desirable to help maintain a uniform product. Often has a furnace cut the bottom out to such a depth as to force an opening for metal to pass downward through the ground or outward through the sides, about as is indicated by the lines N, M, and H, Fig. 18. The havoc such an escaping body of metal can make, if bursting out, as it often does, into a reservoir of water, which is always more or less deep around the hearth of a furnace at N, can be but partly conceived. The mass of liquid metal in the bed of a furnace often weighs 50 to 100 tons. This often solidifies and lies in a furnace until it is torn down, or the hearth portion removed to permit its being broken by dynamite. It has happened that, through a furnace ' i getting off'' or working badly, the bed of metal has solidified above the level of the notch, so that to tap the metal out of the furnace it would have to be drawn off at the flushing or slag-hole at A, Fig. 18. Some furnaces have run for a week or two in this manner before they were able to get the solidified mass melted down, so as to again draw metal from the notch-hole. A furnace in this condition must be tapped much oftener than when it can be tapped at the regular notch. It is often surprising how rapidly, as about 75 per cent of the heat generated from the solid fuel is utilized. This is attained where one ton of coke will produce one ton of iron; and Sir........................ 2,720 "