36 METALLURGY OF CAST IRON.
runner. When a furnace of the size shown is full of stock (coke, ore, and lime) the weight bearing down on the hearth (when a furnace is working properly) is about 100 tons of coke, 160 tons of ore, and 35 tons of lime, a total of about 300 tons. Such a weight must be very effective in crushing the stock in the reduced body of the bosh, so as to greatly retard the penetration of blast, and is one reason for the high pressure found necessary in furnace practice. This also shows the necessity for good foundations.
Decreasing the diameter of the stack from its larger portion joining the bosh up to the top, as shown in Fig. 10, is mainly to assist in pfeventing the stock from '' scaffolding,'' which means '' hanging up.' * (See page 55.) There is no end to the different angles, etc., given to furnaces, each style having its advocates. We now have Hawden and Howson of Middlesbrough, England, who are using a plan of turning present forms upside down. We might also mention that strictly straight furnaces have been tried,.but these, it is said, have proved a failure, as a study of these pages would lead us to believe. There are over five hundred blast furnaces in the United States today and many of them differ more or less in their "lines," etc. The shape or " lines " now generally adopted in this country for coke furnaces are more in accordance with those shown in Fig. 10, in which the hearth is about half the diameter of the largest part of the bosh, and the throat or top of the stack about two-thirds of the bosh's largest diameter, in a height of about eighty feet.
The construction and principle of furnace tuyeres is shown at B, Fig. 6. For the size of furnace shown,lowed in ordersent in the ore. Both these manganese metals are jf|V