The drift of these observations and what has preceded, is to prove that something like 12,000 feet of capacity and air at 563° C. (1,045° F.) are very near the useful limits as to size of furnace and temperature of blast; at all events about one cwt. of coke is all the saving by an immensely larger furnace fed with air at 1,600° F.

In the discussions which have taken place at the meetings of the Iron and Steel Institute and elsewhere, the question has often been asked, whether capacity, gained by enlarging the diameter of the furnace instead of adding to its height, would be attended with an equal economy of fuel.

If one may venture to give an opinion on a matter which has not been made the subject of direct experiment, I would say, provided the descending current of materials were so circumstanced that it was exposed to the ascending gases as long and as completely in the furnace where capacity was attained by increased diameter, as in that where the same object was secured by increased height, then a low furnace would do its work as well as a high one of the same cubic capacity.

It is my belief, however, that the conditions referred to cannot be secured in a low furnace of large diameter; and that in consequence no such economy as has attended the loftier furnaces would be obtained. This opinion rests on the following grounds.

We will take the case of a furnace smelting Cleveland stone with a well having a diameter of eight feet. As is well understood, it is necessary to keep the size of a furnace at the tuyeres within moderate dimensions, in order to permit the blast to penetrate to the centre of the materials exposed to its action. Capacity is needed, however, in the upper zones of the structure, in order to permit the solids, while descending to the hearth, to have several hours exposure to the heated gases. To facilitate this, the boshes have an inwards inclination, at a certain angle or slope. I have constructed a diagram, Plate V., in which a furnace of 18,000 cubic feet is comprised in a building of 60 feet from the hearth to the charging plates, the approved angle of the bosh for working Cleveland stone being preserved.

I entertain little doubt, that the practised eye of any furnace manager would condemn, without a trial, such a section as that given in the diagram; and this probably for the same reasons as those which appear to myself to be valid.

Plate to shew imperfect action in a furnace of large capacity but deficient in height.

The inconvenience of having the materials for the smelter in a fine state of division is recognized by all experience, and the obvious cause is their greater impermeability to the heated gases. The more the coarse dust-like matter is divided and distributed by larger blocks the better. Now the quantity of this dust being the same in the low as in the high furnace, but having in the same period of time to pass through 80 feet in the loftier or through 60 feet in the lower furnace, the annulus it forms from the distributing cone will be proportionately thinner in the furnace of 80 feet. Probably however a greater evil would attend the increase of diameter in the furnace itself, from say 20 to 32 feet, in the facility it would afford to the largest pieces of ore, etc., for separating themselves more completely from the smaller fragments. Such separation not only offers very open spaces, through which the gases will rush rapidly upwards from the tuyeres, but the increased horizontal section (two-and-a-half times greater at the top of the bosh in the lower furnace than in the higher) lends itself in an increased degree to the premature escape of the gaseous matters in question. This mode of action possesses the obvious inconvenience of preventing that action between solids and gases in the furnace, which has already received sufficient notice: and therefore no more need be said to prove that a waste of fuel must necessarily accompany the substitution, for increased height, of mere capacity obtained by enlarging the diameter of the furnace.

Allusion was made (Sec. III., p. 41) to the cost at which the ores of iron were freed from the earths in the form of slag, during the process of smelting. To illustrate this a table has been construeted on the following principles.

The heat generated by the formation of carbonic acid has been shown to be but a little in excess of that required for tearing away the oxygen from the iron. We will therefore omit this source of expenditure of heat out of the estimate, and regard all the fuel as burnt to the state of carbonic oxide: the amount we will assume to be 19:42 units per 20 units of iron. In like manner carbon impregnation, being a process of a heat producing character, is also eliminated from the account. We have then :

Exclusive of reduction of the oxide of iron, left out for the reason already assigned, we have the following figures, setting forth the coke consumed in each section of the work performed when smelting 20 units of Cleveland iron.

It will thus be noticed that, in smelting Cleveland stone, fully 52 units of coke are expended, for every 20 units of pig produced, in merely melting the slag. Properly speaking, to this must be added the fuel required to effect the change in the limestone, placed under the heads Nos. 2 and 3, and together 3:130 units: making in all 8.924 units of coke required for offices in connection with the formation and fusing of the slag.

From what has preceded, it follows as a matter of course that a mineral coal, from which there is little or nothing to be expelled, ought, when subjected to a high temperature, to fulfil the conditions required in the blast furnace. That variety of fossil coal known as anthracite is admirably adapted for the purpose in question; for being so free from volatile ingredients, its interior is little changed by the intense heat to which it has been exposed during its passage through the furnace. Owing probably to its tendency to splinter at high temperatures, a very