drops upon the hearth, together with the silica, lime and clay; these form a slag which floats on the melted iron, and is drawn off from time to time, as occasion may require. The molten iron is allowed at intervals to flow off through a hole in the side of the hearth. After having heated to 200° or more the air requisite for the combustion of the charcoal or coke, it is forced into the blast-furnace by a blowing-engine or other suitable apparatus, and a temperature of probably 2000°, or 2600° Fahr., is obtained.

In proportion as the melted iron and slag are removed from beneath, fresh charges of ore, lime and fuel are introduced at the top, and in this manner the smelting is often continued for five or six years, according as the furnace holds out. The following table shows the materials used and the resulting products:

Aron ore-Carburetted iron (cast-iron).

Flux-Carbonic oxide and carbonic acid (gases of combustion).

Fuel-Silicates of lime and alumina (slag).

The siliceous slags from the blast-furnace usually have a green or blue colour, which is due to the oxides of iron and manganese dissolved in it whilst in a state of lusion; it is frequently formed into square blocks, and used for building stones.

The metal obtained by the above process is termed crude cast-iron; it is by no means pure, but is chemically combined with carbon, and also contains small proportions of other foreign bodies, such as silica, alumina, manganese, &c. A hundredweight of iron will take up at the hottest white heat from about four

to five pounds of carbon, likewise some silicon from the silicic acid, and aluminum from the clay. Traces of sulphur, phosphorus, and arsenic are also sometimes present.

As the molten crude iron flows from the hearth of the furnace, it is directed into a trough or channel formed in a bed of sand, from which other channels branch off on either side; the iron cast in the main channel is termed the sow, and that in the smaller ones the pigs, whence the term pig-iron.

Of the two kinds of iron commonly known in commerce, grey iron has a granular texture, and admits of being filed and bored with facility; hence it is suitable for castings.

White iron is of a silvery whiteness, and too hard to be worked with steel instruments, and is most suitable for the manufacture of malleable iron and steel. Crude white iron, by remelting and very slow cooling, is changed to grey, and, on the other hand, grey iron is changed to white by heating and then suddenly cooling it. Thus, by pouring molten metal into a cold mould it acquires a very hard surface, and presents what is termed a chilled casting.

Castings may be locally chilled by forming the mould with means for cooling that part of the surface which corresponds with the portion of the casting required to be hard.

Malleable iron is obtained from crude iron by depriving it of its carbon, which is done by various processes of oxidation, such as the following:

1. The carbon is oxidised by the action of atmospheric air on the molten iron, which is kept stirred to expose new surfaces to its action whilst in a refinery

or a puddling furnace.

dling.)

(The old method of pud

2. Air is forced through the iron while in a state of fusion in a vessel termed a converter, through the bottom of which the air is blown. (Bessemer's method.)

3. Super-heated steam is forced through the molten metal, thus oxidising the carbon, and also removing sulphur and phosphorus as sulphuretted and phosphoretted hydrogen. (Galy-Cazalat's method.)

4. The melted metal is acted on with certain salts, such as nitrate of soda, &c., by which the carbon is oxidised out. (Heaton's process.)

In all these processes the carbon escapes as carbonic oxide or carbonic acid. The process invented by GalyCazalat has not yet been applied on a large scale in England, but in France it has been found to yield results eminently satisfactory, and it certainly is a very elegant process, both the constituent gases of the water which are evolved by its decomposition being rendered subservient to some useful purpose.

The quality of malleable iron is improved by being well hammered and cut up, and again worked up in the forge, as by these means its quality is rendered more uniform and its texture more homogeneous; thus scrap iron, that is old iron re-worked, is much esteemed for certain manufactures, such as for gun-barrels, &c., requiring great strength and soundness.

Steel is iron containing a certain quantity of carbon, but not so much as is found in cast-iron; it may be prepared in either of the two following ways:

1. By keeping bars of wrought-iron at a temperature close upon the melting point in contact with powdered

charcoal, access of air being prevented for a length of time, dependent on the size of the bars. This process is called cementation, and evidently the bars will be more carbonised on the exteriors than the centres; hence, to obtain the steel uniform, the cemented bars must be cut up and re-wrought into bars or plates, as may be required.

2. By carrying the refining of crude iron to such a point that there is sufficient carbon left in it to form steel, and then arresting the process. This method gives better results at a much reduced cost of production.

Malleable iron, for general commercial purposes, is manufactured in the following forms :

Bars.-Round, square, flat, elliptical.

Do.-Angle, tee, and flanged, having sections L T and H, also half-H iron or channel iron bars.

Special forms frequently used, as railway-bars, sashbars, deck-beams, rolled-girders, &c.

The rolls in which the bars and plates are formed are adjustable, so that any required thickness may be obtained.

In ordering iron of a manufacturer for any considerable work, it is usual, after the working drawings have been finally settled, to go carefully through them, and note the sizes of all the plates and bars required, then from these data the order list for the rolling-mills can be made out.

Medium-sized bars will run up to 25 feet in length, and similar angle-iron bars up to 30 feet; but when

bars or plates exceed certain gross weights per plate or per bar, the price per ton is increased, otherwise the longer the bars the better, as reducing the number of joints in a structure.

Some years back a method of plate-welding was introduced, to supersede rivetting, by Mr. Bertram, but it has not come largely into use, although it was found that joints thus made were equally strong with the rest of the plate when experimentally tried. Probably the practical difficulties in manipulation have militated against its adoption.

CHAPTER II.

STRAINS ON STRUCTURES.

THE strains which are brought to bear upon the different elements of structures are five in number, namely, tension, compression, transverse or bending strain, shearing, and torsion or twisting strain. The two first are direct, and the third may be resolved into them. Shearing force tends to cut or shear off some portion of material, such for instance as the head of a rivet. Twisting strain does not often occur in the elements of structures, being more common in machinery,-it may, however, be resolved into shearing force.

A strain of a direct character may act upon an element lying in the same direction-such is the stress produced by a load on a column or a weight hanging at the extremity of a chain or vertical suspending rod; but, on the other hand, the strain may not act in the direction of the sustaining framework, being borne by two or more inclined bars. In such a case the intensity of