TABLE XII.-EXPERIMENTS ON WROUGHT-IRON BEAMS (continued).

This experiment shows the superior quality of wrought-iron beams in giving timely notice before fracture; it further exhibits weakness on the top sides of the beams, a circumstance requiring great attention in their construction, which in some recent experiments, instituted for attaining the section of greatest strength, has been strikingly developed.

*

14. In the preceding experiments, we have endeavoured to compare the strength, as well as the weight of the beams or frames which form the ribs of ships. As regards the strengths with equal weights, it is in favour of oak; but the circumstance of the fastenings by rivets in the sheathing being so much superior to those of timber, the iron ship builder is enabled to dispense with one-half the number of frames, and consequently a great reduction of weight is effected and more strength obtained in the vessel as a whole, than could possibly be accomplished in the timber-built ship, however ingenious the construction or the arrangement and distribution of the material. The very act of caulking the joints of a wooden vessel has a tendency to loosen the fastenings, whereas, in the iron ship, there are no actual joints, for the whole being bound together en masse, the same, or nearly the same, strength is obtained as if the whole ship were composed of solid plates and ribs.

Fig. 39.

15. The best sectional form of beams for the decks of ships is probably that exhibited in Table XII., which, along with the box beam of the annexed form, fig. 39, for supporting the shafts and paddle-boxes of steamers, is that generally used in the construction of vessels of this description. Other forms have been adopted, particularly those suggested by Mr. Kennedy of Liverpool, alluded to in the appendix of 'Useful Information.'

16. Having carefully investigated the different properties of wrought iron in its varied forms of construction, and conceiving that the results obtained from the experiments may be useful in a variety of circumstances connected with the useful arts, I have endeavoured to collect them in the abstract, in order that the practical builder and engineer may the more readily ascertain

* See the Author's work on the construction of the Britannia and Conway Tubular Bridges.

the comparative value of the different forms of beams, the properties of the material, and their adaptation to any particular construction in which he may be engaged. Should further information be required, we must then refer to the experiments, in which will be found the facts more in detail, and which are probably better calculated to satisfy the inquiring mind and to effect that conviction essential to success.

17. I have not attempted any inquiry into the laws of oxidation, the adhesion of barnacles and marine vegetation, and the means necessary to prevent such evils. This is a subject which does not come within the province of the present inquiry, and more properly belongs to that of the chemist. I would however briefly notice, that in the whole of my experience I have had little to complain of from the effects of oxidation, as that destructive process, as regards iron, appears to be greatly mitigated, if not almost suspended, by constant use, and under the influence of vibration the chemical action appears to be rendered nugatory, if it does not entirely cease, and that under circumstances exceedingly difficult to explain. This is an investigation not unworthy the attention of our best chemists, to whom the causes may be known, but which are at present, as far as I know, unaccounted for. For example, I may mention that an iron ship, if kept constantly in use, or nearly so, will last for a number of years exposed to all the changes of weather and temperature without any sensible appearance of decay. The same may be said of iron rails, over which are passing daily such enormous weights, and at such velocities as almost to neutralise the action of the atmosphere. All these are striking examples of the durability of wrought iron, which may be considered an important element of its security, and a recommendation for its extended application. There is another circumstance in connection with this subject to which it may be necessary in this place to advert, and that is the effect which a long continuance in salt water has upon the hull of an iron ship. It is well known that a long immersion of cast iron in the sea will convert it into plumbago, and that a similar process with malleable iron, from its contact with the saline particles of the ocean, produces oxidation; and in case the immersions were long continued, the effects of this destructive process might

endanger the safety of the ship. As yet we have not had sufficient evidence of its effects to enable us to come to any definite conclusion, but it is not improbable that an occasional visit to harbours of fresh water may mitigate, if it does not entirely neutralise, the injurious effects which the material is likely to sustain. With these observations, which I offer with diffidence, I now beg to direct attention to the abstract deduced from the experiments.*

Abstract of Results obtained from the experiments.

18. In the first part of this inquiry we have endeavoured to show that 50,000 lbs. per square inch is the mean breaking weight of iron plates, whether torn asunder in the direction of the fibre or across it (see page 72); that the tensile strength of different kinds of timber drawn in the direction of the fibre varies in a given ratio to that of iron; and we have also assumed 70,000 lbs. as the resistance of wrought iron per square inch to a crushing force to be correct: the timber in this comparison being represented by unity, we have the following ratio of strength :

These, for practical purposes, may be taken as a fair measure of the strength of the different woods as compared with that of iron plates.

With reference to the relative resistance of plates and timber to the vertical pressure of a blunt instrument forced through them, for the purpose of ascertaining their comparative powers of resistance when placed under circumstances precisely similar, and subjected to the same force, we observe that the results will be found interesting, inasmuch as the iron plates appear to follow a different law in their resistance to pressure to that of

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*For the mathematical formulæ relative to the beams in the foregoing experiments, vide Philosophical Transactions, 1850, and Useful Information for Engineers,' 4th edition, p. 304, et sequitur.