392 IRON MANUFACTURE England the birth of the iron manufacture dates back to the days of the early Britons, and relics of the iron smelting of the Romans are abundant. In the 16th century the iron manu- facture had increased to such an extent that, in order to check the rapid destruction of the forests, restrictions were laid on the cutting of wood for charcoal, by laws enacted in 1558, 1581, and 1584. As a consequence the iron manufacture declined, until in the early part of the 18th century there were but 59 iron works in the country. The supply of iron at this time was mainly from Russia, Sweden, and Spain. Numerous attempts were made to use mineral or pit coal for smelting iron, and pat- ents for alleged discoveries were freely granted ; but no manufacture based on mineral coal was established till 1619, when Dud Dudley ob- tained his patents. He successfully carried on the 'manufacture for many years, making iron cheaply and of good quality ; but, being much persecuted by envious rivals, he finally suc- cumbed to his misfortunes, and dying left no record of his invention behind. It was not till 1735 that the next successful attempt was made to use mineral coal. At that time the difficul- ties of the case were again conquered by Abra- ham Darby, who, before using the coal in the furnace, submitted it to the same process as wood undergoes in its conversion into char- coal; in other words, he converted the coal into coke. From this time the progress of the English iron industry was rapid. In the cen- tury following Darby's discovery, bellows gave way to blowing cylinders, and water power to steam, which greatly increased the efficiency and yield of the furnace ; while the application of the hot blast by Neilson in Scotland, and the utilization of the waste gases by Aubertotin France, added vastly to its economy of working. The blast furnace consists of a vertical shaft of circular section lined with fire brick. The lowest part is ordinarily in the form of a cylin- der, and is known as the hearth. In the ma- sonry of the hearth are built the tuyeres (two to eight in number), which are hollow trun- cated cones of metal supplied with a constant current of cold water. Into these tuyeres pro- ject the nozzles of the pipes that supply the blast. The part of the hearth below the tuyeres is called the crucible ; in it the iron and slag accumulate until tapped off. The hearth is prolonged toward the front of the furnace (fore hearth), and is closed by the dam and covered in by the tymp arch. At the bottom of the dam is a channel communicating with the bot- tom of the crucible through which the iron is tapped off, and on the upper edge of the dam is a notch (cinder notch) over which the cinder flows. The tymp arch is covered by the tymp, a long hollow casting through which water circulates. The sloping walls connecting the hearth with the widest part (belly) of the fur- nace are called the boshes, the angle which they form with the horizontal line being called the angle of the boshes. In many furnaces the hearth expands into and is continuous with the boshes. From the widest part of the fur- nace the walls usually slope inward toward the mouth, which may be either permanently open, or provided with a mechanical arrangement by which it is kept closed except during charging. There are openings in the walls of the furnace, close to the top of the stack where it is closed, and some distance down where it is open, to conduct off the escaping gas. Until compara- tively recently furnaces were built entirely of masonry, the outer walls consisting of massive stone work. At the present day this heavy construction has been almost entirely super- seded by slender stacks encased in brick work and surrounded by sheet iron. In building a blast furnace, the main body of the shaft is supported on pillars, usually of cast iron, and is entirely independent of the boshes and hearth, which are put in subsequently, and can be re- moved and repaired without interfering with the upper portion of the furnace. In the Biitt- genbach system of construction the main shaft consists of one layer only of fire brick 18 in. thick, without outer casing of any kind. The charging floor on the top of the furnace is sup- ported by hollow cast-iron columns, which serve also to conduct down the gas to the stoves. .This construction is mainly recom- mended by its cheapness. The dimensions of blast furnaces vary greatly. The height ranges from 30 to 100 ft., the greatest diameter from 6 to 30 ft., and the capacity from 500 to 40,000 cubic feet. The relative dimensions of hearth, boshes, and throat likewise vary greatly. The cause of this great variation in dimensions is partly due to differences in ores and fuels, and partly to the fact that there are no fixed prin- ciples of blast-furnace construction that have found general acceptance. The more refractory the ore and the more dense the fuel, the larger, as a rule, is the furnace ; but as to the proper outline of the interior there is great difference of opinion. Most metallurgists are so far agreed as to have abandoned the flat boshes, narrow mouths, and abrupt changes of outline of the older furnaces ; but further than this there is no uniformity in modern blast-furnace construc- tion. The following are some of the considera- tions which' should determine the dimensions and outlines of blast furnaces. The tempera- ture attained in the hearth, upon which the nature and quality of the iron depend, is the result of a number of factors, such as pressure or penetration of blast, character and amount of fuel, and diameter of hearth. The last is readily fixed when the others are known. The regular working of the furnace depends in a great measure on the regular descent of the charges. Now, as the charge diminishes in bulk in descending, owing to the reduction of the iron and the combustion of the fuel, the capacity of the furnace should diminish corre- spondingly ; that is, the walls should taper downward. The amount of this tapering, or in other words the angle of the boshes, should
