Had the builders of those tiny furnaces which dotted the Central Eastern states in the first half of the last century been shown these formidable specifications before having practical knowledge of the subject they would undoubtedly have thrown up their hands and ceased all efforts to meet such apparently impossible conditions. But, as a matter of fact, they had not the disadvantage of this knowledge, for disadvantage in their case it would have been, since the practice to which they were accustomed involved difficulties constituting only a fraction of those outlined above. This was due to the lower temperatures and the lower rate of driving, with its reduction of abrasion both by gas and by stock, and perhaps most important of all they did not dream of such pressures as we now consider right and proper.

In happy ignorance, therefore, of the conditions which their successors would have to meet, they built stacks of stone, truncated pyramids, one side of the base almost as long as the stack was high. In the earliest days all these were built, as I have already described, against and into a hill. Only one tuyere was used. No distinction was made between supporting structure and lining. The stack was simply a truncated pyramid 20 to 40 ft. high, with a hole some 6 or 8 ft. in diameter through it, hardly different from a square chimney with a round flue, except of more squatty proportions. In one side was an archway into the stonework which gave access to the single tuyere, the forehearth which I have previously described, and the tapping hole.

This simple pile of masonry resisted the pressure of the blast, the heat of the combustion, the abrasive action of the stock and the chemical action of its heterogeneous contents.

The gradual increase in output from those days to our own brought changes in construction, and these in turn permitted greater outputs which again required other changes of construction, and so, with each successful step helping toward the next, by slow, painful and costly progress we have passed from those primitive conditions to those prevailing to-day.

Two of the earliest changes were the substitution of firebrick for the lining of the stack and the provision of an archway in each side of the structure with a tuyere in each arch instead of only one. More probably in many cases three of the arches were used for tuyeres; in the fourth the tuyere was left out in order to make room for the fore-hearth, tapping hole, etc. Gradually, as the proper shape of the furnace was evolved, the hearth structure became separated at the base from the piers of masonry which supported the whole stack, and finally it became evident that it was desirable to increase the distance between these so as to provide greater access to all portions of the hearth. In course of time also the stone stack with its binders of hewn timbers gave way to a brick stack banded with flat bands like a tank, the structure passing from square to round in its exterior shape, the lining being of firebrick and fully differentiated from the stack proper.

About 1860 this structure was superseded at one of the Lake Cham-plain furnaces by a shell riveted up of plate iron standing on cast iron columns, and lined with firebrick, the prototype of the structure used to-day.

The photograph of a modern furnace is shown by Fig. 162 (page 238). The stack is at the right. A section drawing of a different but similar furnace, with its hoist is shown in Fig. 163 (page 239).

It will be seen that there is a foundation of concrete 13 ft. below the ground level and 39 ft. in diameter, upon which the whole structure rests. An annular foundation of brickwork upon this supports the bases of the columns, ten in number, each 25 ft. high and 2 1/2 in. thick.

Upon these columns rests a heavy ring of structural material better shown in detail in Fig. 164 (page 240), and to this "mantle" is riveted the shell of 3/4-in. steel plate, 36 ft. in diameter at the bottom and 23 ft. 11 in. in diameter at the top. This shell is riveted up with the same class of workmanship as that used in the best steam boilers. Within this shell is the brickwork about 4 ft. thick at the level of the mantle and maintaining nearly this thickness all the way to the top. Below the top of the mantle the brickwork is thinner down to the tuyeres and then below the tuyeres thicker again, finally uniting with the furnace bottom, which consists of some 8 or 9 ft. of solid firebrick work on the concrete foundation.

Fig. 162. Modern blast furnace stack with stoves and dustcatchers. Built by William B. Pollock Company.

This structure is, in broad outlines, typical of the standard construction of to-day, but the variations of detail are almost as numerous as the individual furnaces. It is impossible to describe all these variations individually, but by dividing the furnace into zones we may deal in some detail with the conditions to be met and the approved forms of construction in each. The zones in which the furnace should be divided for this purpose are the hearth and bottom, the tuyere zone, the bosh, the shaft, and the top.