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102                      METALLURGY   OF   CAST   IRON.
one or two inches higher as they approach the last bed, so as to conform closely to the incline of the main or "iron runner," as it is generally called, which has a fall of about eighteen inches in one hundred feet. A greater fall than this would generally cause the iron to flow with too great a rush, and should it get away from the furnace any faster than usual, the chances are it could not be controlled, and instead of its being distributed as desired throughout all'the pig beds, the lower two or three beds would be overflowed, and a 4t boil " easily started by reason of a large area of floor space being all covered with a plate of fluid metal, permitting no escape of gas and steam from the sand cores between the pigs. The founder often receives pigs united together, and often much thicker in depth than usual. These are called " jump cores,'* and are formed by reason of the body of sand in the mold separating the pigs, being raised or pressed to one side by the action of too quick a flow, poor sand, or a little "boil." It has been no uncommon occurrence for metal to come so fast down the iron runner that it could not be controlled, and by reason of covering over a large area, cause a whole tap to go under the drop, or, worse still, require dynamite to break it up sufficiently small to be charged into the furnace, along with the ore, or sold for scrap metal to be re-melted in air furnaces or big cupolas.
The making of the iron runner is generally the work of the " keeper." Figs. 24, 25, 26 and 27 show different views of such runners, and Fig. 34, page 104, a perspective view of the whole.
After a furnace has been tapped, the metal often comes slowly, to prevent it from chilling until itso the roof the moment it struckat 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     "