cal belt, to prevent the occurrence of a sharp angle, and
for this end the edges are slightly rounded off, and a
space is formed called the belly. The lowest division
of the furnace is in shape a quadrangle, and is com-
posed of large slabs of refractory sandstone, which are
cemented together with fire-clay. This division is a little
smaller at the bottom than at the point where it meets
the boshes, and its angles are gradually rounded off;
but this difference in size at its two extremities is, in
many instances, so trifling that the hearth (as this part
is named) may almost be termed a quadrangular prism.
The bottom of the furnace is composed of large fire-
stones, which are supported on a mass of masonry, and
numerous channels are left open, through which any
moisture from the brickwork may escape; while, in
order to keep the whole building thoroughly dry, two
arched galleries traverse the foundations, and intersect
each other at right angles beneath the axis of the in-
ternal cavity of the furnace. Only three of the sides
of the hearth are continued to the stone, which consti-
tutes the bottom of the arrangement. The fourth is
merely brought to a certain distance within that of the
base, and is there supported by strong cast-iron bearers
r girders, firmly fixed into the masonry of the walls.
'n these bearers rests a heavy block of sandstone, called

B

18

METALS USED IN CONSTRUCTION.

the tymp. About five or six inches beneath the also a little in advance of it, is placed a stone ca dam-stone, which is in the shape of a prism. securely fixed by means of a strong piece of ir peculiar shape, covering its outer side, and cal dam-plate. The part of the furnace beneath th is called the crucible, in which the fused meta lected, till a proper quantity has been accumula tapping. The blast from the blowing machine ducted into the furnace by means of the tuyers are fitted into holes perforated a little above t of the tymp, in the face of the hearth (oppos dam) and its two sides. To save waste of workmen passing from one tuyer to another, it i to have four vaulted galleries constructed, whi nect the various arches by which the tuyers an of the crucible are approached. In practice it to build two or more of these blast furnaces side, and it is found advantageous, when the si permits, to place them at the foot of a declivity, a way as will render it easy to connect their s with the adjoining high ground by means of a By this arrangement the furnaces are easily s with the necessary ore and fuel; but if, from the of the country, this method cannot be adopte

i

either raised by means of an inclined plane and waggons worked by the engine of the blowing machine, or by a moveable platform raised by compressed air or hydraulic pressure. Generally, the tops of the furnaces are enlarged by a platform, as shown in fig. 1, and which, to prevent accidents, is railed round; and when the moveable stage is used, the ore, etc., is raised in iron waggons, which can be readily transferred to a continuation of railway on the platform of the furnace, conducting them directly to the throat where the waggons are tilted and their contents shot out. That the moisture may readily escape, and the brickwork be prevented from splitting through the pressure of confined watery vapour, the whole of the masonry constituting the exterior casing of the apparatus is traversed by numerous small channels, which greatly facilitate the drying of the mass. Stout iron bands, as shown in fig. 1, are likewise made, to bind the work strongly together. These bands are on the outside of the work, and are made to bind tightly by wedged keys or screws and These horizontal bands are also held together by long vertical bars, to which they are attached by loop-eyes or strong screw-bolts, and by this means great strength and solidity are communicated to the building. The most common height of these furnaces

nuts.

is from 45 ft. to 50 ft., not including the chimney, which is from 8 ft. to 10 ft. in length, and frequently formed of only one course of bricks, strongly bound together by stout iron rings and girders. In some cases these chimneys are so firmly ironed that their surface is half covered with metal. Usually two doors are left on the opposite sides of the base for the introduction of the ore and fuel. A large annular plate of cast-iron protects and holds together the throat or trunnel-hole, and on it the foundation of the chimney reposes. The inclination of the boshes will depend on the nature of the cast-iron which it is desired to produce, as its quality appears to be greatly influenced by the slope which is given to this part of the furnace. They are commonly less sloped when very dark iron for the purposes of casting is manufactured than when grey iron for subsequent conversion into bars is required. The limit of this difference generally is about 5 deg., the usual angle formed by the sides of the boshes with the perpendicular being from 55° to 60°. For the erection of a pair of blast furnaces of 40 ft. in height, 32,000 common bricks for the outside masonry, and 80,000 fire-bricks for the refractory lining on the inside, are required. The blast must be regulated according to the amount of the metal required to be run down,

and the size of the furnaces. As a rough average, the power required to produce the requisite supply is 25horse power each furnace, but the following will form the best guide:-1-horse power will produce 2 tons of metal weekly; and as a criterion as to the amount of air required, it may be taken as a fair average that 60 cubic feet per minute will produce 1 ton of metal per week.

"The blast is admitted into the hearth at a pressure which varies within considerable limits, according to the season of the year or the nature of the fuel employed. The air being more rarefied in summer than in winter, will contain in a given bulk a less amount of oxygen, and therefore a larger quantity is blown into the furnace to produce a similar result. But little pressure is used with very light and easily combustible fuels, such as the various kinds of charcoal; but when a very dense fuel, such as coke, is employed, the air requires to be more strongly compressed. The small charcoal-fed furnaces of the Continent are frequently blown with not more than half-a-pound pressure on the square inch; but this is more or less modified according to the nature of the charges with which it is supplied. The pressure for coke varies from 24 lb. to 34 lb. per square inch; but these numbers represent the extreme limits, the average