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OPERATING   BLAST   FURNACES.                        51
generally conceded that the small bell, as in Fig. n, sends the coarse material to the outside circle, while the larger bell, Fig. 12, sends it to the inner circle, and the coarse material may descend faster than the fine stock. Fnrnacemen are now largely using small bells.
The action of stock In passing down through a furnace should attain, if possible, an occasional shifting movement, so as to retard the formation of any solid mass of the stock. This is best achieved in a taper stack, as the stock in passing downward should assume an action somewhat similar to that illustrated in the various levels, A, B, C, D, E, and F, seen in Fig. 10, page 49. When stock is dropped by a bell, such as in the size of the furnace shown, it is generally, if all is working well, distributed in a form somewhat like that in the mounds M M, seen at the level A, which is called the l ' stock line,'' and is generally ten feet below the level of the bell. The stock in settling down to fill the increasing diameter of a tapering stack must have a spreading* out or leveling action taking place, or in other words, the outside would descend faster than the inside stock. It seems reasonable that the tendency of the stock in settling would be to have the angles constantly leveling themselves somew'bat after the idea illustrated at the various strata B, C, D, and E, Fig. 10, until it has reached the bosh at F, when reaction would take place and the stock in descending would be retarded by the walls of decreasing diameter and cause the center portion to travel faster than the side, until at the last stratum, I, the center stock would, have traveled ahead of the side stock as shown at R. Before this point is reached, however, the reactiony they are often composed of about equal parts of silica and alumina. Bricks should contain silica or alumina in proportion to the amount of heat or friction they are                                                 if j