layers. For example, Professor Rankine gives the following

Such considerable differences are, however, the exceptions rather than the rule, and do not appear in the timbers most used.

With the flanged forms obtainable in wrought-iron beams, similar variations are not likely to occur, and there is no sensible error in assuming that the ultimate resistances of the flanges correspond to the tensile and compressive strengths obtained by direct pull or thrust.

A few examples of the great variety of forms in which iron beams are made will be found in Fig. 116. It is unnecessary

to repeat what has already been said as to the increased strength to resist bending obtained by using these flanged forms, instead of the solid rectangular sections which are

*

unavoidable with wood. But it may be proper to mention that this essential difference between wood and iron affects the relative efficiencies not merely of deck beams, but also of ribs, longitudinal frames or strengtheners, pillars, and many other parts of the structure of a ship.

The simple angle-iron is sometimes used as a beam; its form may be seen from the sections f, g, h, in Fig. 116, and differs from the T-iron in having a top flange on one side only of the vertical web. Neither the angle nor the T form is well adapted for resisting bending strains, because of the absence of a bottom flange. The angle-bulb (b) is a great improvement in this respect, and is used for light decks or platforms. Z-iron (c) is used for frames behind armour in ironclads, and for longitudinal stiffeners, but not often for beams. H-iron (d) is expensive, and is not used so much as the made-beam (g) of similar cross-section. Not unfrequently, instead of having double angle-irons on the upper edge of the made-beams, to a deck covered with iron or steel plating, only single angle-irons are worked, a portion of the deck plating above the beam then forming the upper flange. Sections e and ƒ may be regarded as interchangeable: the latter was formerly much in use, but since the iron manufacturers have made such advances as to be able to produce the section e with ease, and at moderate cost, the shipbuilder naturally prefers to obtain the finished form. The box-beam h is only used where exceptional strength is required, to support some concentrated load, or to furnish a very strong tie. Of the other sections sometimes used it is needless to speak; they all, or nearly all, exhibit the general characteristic of top and bottom flanges or bulbs connected by a thin vertical web. Even for the largest ships, beams of these sections are now procurable in one length, which is another great advantage as compared with the two-piece or three-piece wood beams required in large ships.

*Sec page 357 as to beams; also page 353 as to ribs.

A practical rule, not pretending to exactness, for comparing the strengths of beams may have some interest. For the flanged iron beams such as are generally used in ships, the ultimate breaking strengths of any cross-section may be expressed approximately by the formula

Breaking strength = 20 tons x sectional area

depth
3

The areas of the flanges being expressed in square inches, and the depth in inches, the breaking strength will represent a moment in inch-tons. For example, take a beam of section d, Fig. 116, suppose it 12 inches deep, and its top and bottom flanges to be each 6 inches wide, the web and flanges being inch thick. Then, approximately,

Breaking strength = 20 tons x (12+6+6) 1 × 3 × 12
= 960 inch-tons.

For a solid wood beam of rectangular cross-section the approximate rule for teak or oak would be,

Breaking strength=3 tons x sectional area ×

depth
6

The weight of the iron beam taken as our example would be about 40 lbs. per foot of length, the sectional area of a teak beam of equal weight would be about 120 square inches: suppose it to be 12 inches deep by 10 inches broad. Then

Breaking strength (approximate) = 3 tons x 120 x 12 = 720 inch-tons.

As regards ultimate strength, the iron beam is therefore one-third stronger than the wood beam of equal weight. But here the necessity for taking account of working strengths as well as breaking strengths must be remembered. The comparatively large factors of safety required with timber increases the advantage of iron, even when each beam is in a single piece. The scarphs of the wood beam further detract from its strength in wake of them. And,

moreover, it must not be overlooked that, while the strength of the iron (20 tons per square inch) may be safely looked for, the strength of the wood may vary over a very extensive range.

Putting the working strengths instead of the breaking strengths, the case stands approximately as follows:

Working strength of iron beam

=

weight of iron.

Weight of timber (per cubic foot) = (say)

Sectional area of timber beam = 10 times sectional area

of iron beam of equal weight.

=

=

= 23,

Hence, finally, for equal weights and equal depths, Working strength of iron beam 4 x 1 x 1 Working strength of wood beam× 10 × 1 which represents a very considerable gain in favour of iron. Besides being procurable in single pieces of a flanged form, iron plates and bars can be combined readily to produce that form; on the other hand, wood must be used in rectangular or, at least, solid timbers, and cannot readily have many pieces combined into a flanged form. Examples of this difference have already been given. Refer, for instance, to the contrast between the solid timber ribs spaced closely in the wood ship (see Fig. 102, page 322) and the flanged transverse frames with the adjoining segments of plating in the iron ship (Fig. 103, page 324). As another contrast, compare the strong longitudinal frames or girders, to which the adjacent parts of the inner and outer skins form flanges in the ironclad ship (Fig. 104, page 331), with the solid binding strakes or keelsons of a wood ship. Many other illustrations of the facility with which iron can be thrown into the form best adapted for resisting bending strains will present themselves to the

student interested in the detailed structural arrangements: but we cannot now enlarge upon this important feature. Nor need we do more than recall attention to the fact that when the ship, as a whole, is treated as a girder resisting longitudinal bending moments, the component parts of the flanges in that girder are mainly exposed to tensile and compressive strains, in resisting which iron gains upon wood in the manner explained above; the web of the girder is simultaneously subjected to racking or distorting strains, against which the superior edge connections in an iron ship make the skin greatly more efficient than the skin of a wood ship.

From this brief sketch it will be understood why iron ships are lighter in proportion to their strength than wood ships of the same form and dimensions; as also why it is possible with iron to construct ships of sizes, proportions, and speeds unattainable with wood. It is, of course, possible by ill-considered structural arrangements to throw away much of the advantage that may be gained by using iron hulls. Bad combinations, improper distribution of the material, imperfect fastenings, and other faults may lead to the production of weak, yet heavy, iron ships. It has been shown in the preceding pages that even now, in the magnificent iron ocean steamships which lie so far beyond the possibilities of wood construction, all has not been done that might be accomplished towards associating lightness with strength. This statement, however, is only tantamount to an assertion that the capabilities of iron as a shipbuilding material have not yet been fully developed in the mercantile marine; and in support of this view we can refer to the remarkable results attained in the armoured ships of the Royal Navy, or the still more notable case of the Great Eastern.

Next, as to the comparative durability of iron and wood ships. For some years after the introduction of iron ships this was a matter of dispute, but lengthened experience