These are plates of any shape in plan, arched from the edges towards the centre; the arch has a very slight rise, and forms a dome or groined surface, according as the plate is round or square.

Such plates will bear a very great weight, and are applicable to fireproof floors, bridges, and several other purposes.

Flitch Plates are made in widths up to 18 inches for use in flitch girders (see p. 278, Part II.) They are generally of common iron, as they require no bending or smithing, nothing but a few holes punched.

Sheet Iron is so called when the material is of a thickness equal to or less than No. 4 B.W.G. - i.e. "239 inch; above that thickness the material is called plate iron.

It is generally of superior quality and higher price (as there are so many more sheets to the ton, and consequently extra labour in rolling, etc., than in plate iron) and its thickness is specified in terms of the Birmingham wire gauge (B.W.G.)

The following table shows the classification of sheet iron as to thickness -

Name of Class.

B.W. Gauge.

Thickness.

From

To

From

To

Singles

4

20

•239

•035

Doubles

20

25

•035

•020

Lattens

25

27

•020

•016

Sheet iron is not much required for engineering purposes, but in many places it is used for roofing churches, houses, sheds, etc.

Corrugated Sheet Iron is made by passing sheets between grooved rollers, which force and bend them into a series of parallel waves or corrugations. These enormously increase the stiffness and strength of the sheets, and adapt them for several purposes for which the flat sheets would be too weak.

The sheets must be of good quality to stand the process, or they will crack.

The sheets are in sizes, generally about 6 feet by 3 feet 2 inches, or 8 feet by 3 feet 2 inches, before corrugation; with corrugations 5 inches apart, which reduce the width from 3 feet 2 inches to 2 feet 6 inches.

The thicknesses and weights are as follows : -

Birmingham

Wire Gauge.

B.W.G.

Thickness in inches.

Weight per square in lbs.

Width of

Flutes.

Uses.

No. 16 . .

•065

380

5 in.

Where great strength is required.

„ 17 . . .

•056

320

,,

For first-class work generally.

„ 18 ...

•049

280

,,,,

„ 19 ...

•042

252

,,

„ 20 ...

•035

224

3 in.

„ 21 ...

•032

205

,,

„ 22 :..

•020

185

,,

„ 23 ...

•025

165

,,

Sent to Colonies.

„ 24 . . .

•022

150

,,

„ 26 .

•020

112

,,

"The flutes or corrugations are made of various widths, those most usual in England being 3, 4, and 5 inch. Sheets with 5 inch flutes are commonly preferred by engineers. The depth D is generally 1/4 of the width A, and the proportions can only be modified in the manufacture by making special new dies. Sheets with flutes wider than 5 inch are occasionally used when great strength is required, but in such cases the thinner gauges of iron should not be employed."1

The ordinary form of corrugation is shown in Fig. 144, the sheets, when used for roof-covering, being laid with the corrugations parallel to the slope. A special form, shown in Fig. 145, is sometimes made for sheets intended to be laid with the corrugations parallel to the ridge.

Corrugated iron is generally galvanised (see next paragraph).

Mallet s Buckled Plates 300172

Fig. 145.

Mallet s Buckled Plates 300173

Fig. 144.

1 Matheson's Works in Iron.

Galvanised Iron is iron covered with a coating of zinc by the process described at p. 335.

The quality of galvanised sheets depends upon the kind and thickness of the Iron, the purity of the zinc, and the care with which the process has been conducted.

Continuous Galvanised Iron Roofing Sheets are made in lengths of 100 and 200 feet, and of the undermentioned widths, gauges, and weights. They are very suitable for light roofs and save much expense and trouble as there are so few joints required.

31 gauge 24

inches wide, weighs

8 ounces per square foot.

28 „ 24, or 30

,, ,,

11 ,, ,,

26 „ 24, 30 or 36

,, ,,

13 ,, ,,

24 „ 24, 30 or 36

,, ,,

18 ,, ,,

Hoop Iron is made of the widths and gauges specified in the list of extras, p. 292.

It is not much use in building, except as an additional bond in brickwork, for which purpose it is generally about 11/2 inch wide and of No. 16 BWG, tarred and sanded, and laid as described in Part II. p. 228.

Mitis Wrought Iron Castings are produced by Mr. Nordenfeldt's patent process. They are made from scrap iron containing a very small quantity of carbon. This is melted in crucibles, and has to be raised to a very great heat produced by the use of •naphtha as fuel. The molten metal is poured into the moulds from ladles in which the contents are kept hot by means of a surface blast. The best castings are made from the highest class of irons, such as Swedish or Best Yorkshire. Raw material containing as much as 1/4 per cent of phosphorus makes the castings too brittle. These castings may be readily run into any form required, and the inventor claims for them that their strength is 20 per cent greater than that of the best forgings.

A specimen mentioned by Mr. Warren1 in his paper on cast steel gave the following results:

Tensile test, 28 tons per square inch. Elongation in 2 inches, 12.8 per cent.