The main central key-stone is constructed of large fire-bricks made on purpose; against that key-stone the two arches press, having their abutments at the sides against the walls. The highest point of the roof is only 8 inches above the melted metal. The sole of the hearth is composed of a layer of sand 8 inches thick, resting upon a bed of iron or of brickwork. The edge of the fire-bridge is only 3 inches above the fluid iron.
In from 2 to 4 hours from 1 to 3 tons of metal may be founded in such a furnace, according to its size; but it ought always to be heated to whiteness before the iron is introduced. 100 pounds of cast iron require from 1 to 11⁄2 cubic foot of coal to melt them. The waste varies from 5 to 9 per cent.
I shall conclude the subject of iron with a few miscellaneous observations and statistical tables. Previously to the discovery by Mr. Cort, in 1785, of the methods of puddling and rolling or shingling iron, this country imported 70,000 tons of this metal from Russia and Sweden; an enormous quantity for the time, if we consider that the cotton and other automatic manufactures, which now consume so vast a quantity of iron, were then in their infancy; and that two years ago, the whole of our importation from these countries did not exceed 40,000 tons. From the following table of the prices of bar iron in successive years, we may infer the successive rates of improvement and economy, with slight vicissitudes.
| Years. | Per Ton. | ||||
|---|---|---|---|---|---|
| £ | s. | £ | s. | ||
| 1824 | 9 | 0 | to | 10 | 0 |
| 1825 | 10 | 0 | — | 14 | 0 |
| 1826 | 8 | 10 | — | 10 | 0 |
| 1827 | 8 | 0 | — | 9 | 0 |
| 1828 | 7 | 10 | — | 8 | 0 |
| 1829 | 5 | 10 | — | 7 | 0 |
| 1830 | 5 | 5 | — | 6 | 0 |
| 1831 | 5 | 5 | — | 5 | 10 |
| 1832 | 5 | 0 | — | 5 | 10 |
| 1833 | 5 | 10 | — | 6 | 0 |
| 1834 | 6 | 0 | — | 6 | 10 |
| 1835 | 5 | 10 | — | 7 | 0 |
I have been informed upon good authority that the total production of iron in Great Britain, in the year 1836, was almost exactly ONE MILLION OF TONS!
The export of iron that year, in bars, rods, pigs, castings, wire, anchors, hoops, nails, and old iron, amounted to 189,390 tons; in unwrought steel to 3,014, and in cutlery, to 21,072; in whole to 213,478: leaving apparently for internal consumption 776,522 tons, from which however one tenth probably should be deducted for waste, in the conversion of the bar iron. Hence 700,000 tons may be taken as the approximate quantity of iron made use of in the United Kingdom, in the year 1836.
The years 1835 and 1836 being those of the railway mania over the world, produced a considerable temporary rise in the price of bar iron; but as this increased demand caused the construction of a great many more smelting and refining furnaces, it has tended eventually to lower the prices; an effect also to be ascribed to the more general use of the hot blast.
The relative cost of making cast iron at Merthyr Tydvil in South Wales, and at Glasgow, was as follows, eight or nine years ago.
| At Merthyr. | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| s. | Tons. | Cwts. | Qrs. | £ | s. | d. | ||||
| Raw mine | at | 10 | per ton, | 3 | 7 | 0 | 1 | 13 | 6 | |
| Coal | at | 6 | 2 | 16 | 0 | 0 | 16 | 6 | ||
| Limestone | 1 | 5 | 2 | 0 | 1 | 4 | ||||
| Other charges | 0 | 9 | 1 | |||||||
| Total Cost | 3 | 0 | 5 | |||||||
| At Glasgow. | ||||||||||
| s. | d. | Tons. | Cwts. | £ | s. | d. | ||||
| Raw mine at | 4 | 6 | 3 | 10 | 0 | 16 | 3 | |||
| Splint Coal at | 2 | 5 | 5 | 15 | 0 | 14 | 0 | |||
| Limestone at | 0 | 3 | 0 | 14 | 0 | 3 | 6 | |||
| Coals for the engine | 1 | 10 | 0 | 3 | 0 | |||||
| Other charges | 1 | 1 | 0 | |||||||
| Total cost | 2 | 17 | 9 | |||||||