It need hardly be stated that, whatever might be the result of Mr. Stephenson's abstract calculations on these points, his practical decision was one that necessarily involved the most painful responsibility; which indeed, if possible, was increased by the reflection that the Directors of the Chester and Holyhead Railway placed such implicit confidence in his judgment and caution, that they were prepared to adopt almost whatever expedient he might, on mature consideration, recommend. In war, the mangled corpse of the projector of an enterprise is usually considered a sufficient atonement for his want of success; indeed, the leader of the forlorn-hope, who dies in the breach, is not only honourably recollected by his survivors, but by a glorious resurrection occasionally lives in the History of his country but when a man of science fails in an important undertaking involving the capital of his employers and the lives of the public, in losing his reputation he loses that which never can be revived! Unawed, however, by these reflections, Mr. Stephenson after mature calculations—in which his practical experience of ironship building must have greatly assisted him-confidently announced, first to his employers and afterwards to a Committee of the House of Commons, by whom he was rigidly examined, that he had devised the means of accomplishing that which was required; and further, that he was ready to execute his design. The great difficulty had been in the conception and gestation of his project; and thus his severest mental labour was over before the work was commenced, and while the stream, as it hurried through the Menai Straits, as yet saw not on its banks a single workman. The outline or principle of his invention was, that the required transit of passengers and goods across the Conway and Menai Straits should be effected through low, long, hollow, straight tubes one for up-trains, the other for down ones-composed of wrought-iron boiler-plates,' firmly riveted together. He conceived that, in order to turn aside the force of the wind, these tubes ought, like common water-pipes, to be made oval or elliptical, and that they should be constructed at their final elevation on temporary platforms, upheld by chains which-notwithstanding the evident objection, in theory as well as in practice, to an M admixture of moveable and immoveable parts-might of course subsequently be allowed to give to the bridge an auxiliary support, although Mr. Stephenson's experience enabled him to declare to the Committee of the House of Commons very positively that no such extra assistance would be required. He proposed that the extremities of the tubes should rest on stout abutments of masonry, terminating the large embankment by which from either side of the country each was to be approached; the intermediate portions of the aërial passage reposing at the requisite elevation upon three massive and lofty towers. Of these one was to be constructed at high-water mark on each side of the Straits. The third, no less than 221 feet in height, was to be erected as nearly as possible in the middle of the stream, on a tiny rock, which, covered with 10 feet of water at high tide, although at low water it protruded above the surface, had long been considered as a grievance by boatmen and travellers incompetent to foresee the important service it was destined to perform. The four lengths of each of the twin tubes, when supported as described, were to be as follows — Feet. From Carnarvon embankment, terminating in its abutment, to the tower at high-water mark . 274 From the latter tower to Britannia tower, situated upon Britannia rock in the middle of the stream 472 From Britannia tower to that at high-water mark on the 472 From the Anglesey tower to the abutment terminating the embankment which approaches it 274 Total length of each tube 1492 2984 Total length of both tubes Notwithstanding the bare proposal of this magnificent conception was unanswerable evidence of the confidence which the projector himself entertained of its principles, yet, in justice to his profession, to his employers, to the public, as well as to himself, Mr. Stephenson deemed it proper to recommend that, during the construction of the towers and other necessary preparations, a series of searching experiments should be made by the most competent persons that could be selected, in order to ascertain the precise shape and thickness of the immense wrought-iron aërial galleries that were to be constructed, as also the exact amount of weight they would practically bear. In short, the object of the proposed experiments was to ensure that neither more nor less materials should be used than were absolutely requisite, it being evident that every pound of unnecessary weight that could be abstracted would, pro tanto, add to the strength and security of the structure. Although it was foreseen, and very candidly foretold, that these experiments would be exceedingly expensive, the Directors of the Company readily acceded to the requisition, and accordingly, without loss of time, the proposed investigation was, at Mr. Stephenson's recommendation, solely confided to Mr. William Fairbairn, a ship-builder and boiler-maker, who was justly supposed to possess more practical experience of the power and strength of iron than any other person that could have been selected. Mr. Fairbairn, however, after having conducted several very important investigations, deemed it necessary to apply to Mr. Stephenson for permission to call in the aid and assistance of Mr. Hodgkinson,' a powerful mathematician, now professor in the University of London, and whom Mr. Stephenson, in his report to the Directors, dated Feb 9, 1846, declared to be 'distinguished as the first scientific authority on the strength of iron beams. To these two competent authorities Mr. Stephenson subsequently added one of his own confidential assistants, Mr. Edwin Clark, a practical engineer of the highest mathematical attainments, who regularly recorded and reported to Mr. Stephenson the result of every experiment,—to whom the construction and lifting of the Britannia galleries were eventually solely intrusted,--and by whom an elaborate description of that work has just been published.* * With the sanction and under the immediate supervision of Robert Stephenson, Civil Engineer. A Description of the Britannia and Conway Tubular Bridges; including an Historical Account of the Design and Erection, and Details of the Preliminary Experiments, with the Theories deduced from them. Also, General Inquiries on Beams, and on the Application of Riveted Wrought-Iron Plates to Purposes of Construction; with Practical Rules and Deductions, illustrated by Experiments. By Edwin Clark, Assistant Engineer. With Diagrams and a folio volume of Plates and Drawings, illustrative of the Progress of the Works. London: Published for the Author, by John Weale, 59, High Holborn, 1849.' The practicability of Mr. Stephenson's hollow-beam project having thus, at his own suggestion, been subjected to a just and rigid investigation, we shall have the pleasure of briefly detailing a few of the most interesting and unexpected results; previous, however, to doing so, we will endeavour to offer to those of our readers who may not be conversant with the subject a short practical explanation of the simple principle upon which a beam, whether of wood or iron, is enabled to support the weight inflicted upon it. If human beings can but attain what they desire, they seldom alloy the gratification they receive by reflecting-even for a moment on the sufferings which their fellow-creatures may have undergone in procuring for them the luxury in question. Dives sometimes extols his coals, his wine, his food, his raiment, his house, his carriages, and his horses, and yet how seldom does he either allude to or ruminate on the hardships and misery which, for his enjoyment, have been endured in coal-pits, leadmines, sugar-plantations, cotton-fields, manufactories, smeltinghouses, in horticultural and agricultural labour, by the sons and daughters of Lazarus !— and if this heartless apathy characterises human beings with reference to each other, it may naturally enough be expected that, provided inanimate objects answer our purpose, we think not of them at all. For instance, if a beam without bending or cracking bears-as it usually does-the weight which the builder has imposed upon it, who cares how it suffers or where it suffers? For want, therefore, of a few moments' reflection on this subject, most people, in looking up at a common ceiling-girder, consider that the corresponding upper and lower parts thereof must at all events, pari passu, suffer equally; whereas these upper and lower strata suffer from causes as diametrically opposite to each other as the climates of the pole and of the equator of the earth; that is to say, the top of the beam throughout its whole length suffers from severe compression, the bottom from severe extension, and thus, while the particles of the one are violently jammed together, the particles of the other are on the point of separation; in short, the difference between the two is precisely that which exists between the opposite punishments of vertically crushing a man to death under a heavy weight, and of horizontally tearing him to pieces by horses! Now this theory, confused as it may appear in words, can at once be simply and most beautifully illustrated by a common small straight stick freshly cut from a living shrub. In its natural form, the bark or rind around the stick is equally smooth or quiescent throughout; whereas, if the little bough firmly held in each hand be bent downwards, so as to form a bow, or, in other words, to represent a beam under heavy pressure, two opposite results will instantly appear; namely, the rind in the centre of the upper half of the stick will, like a smile. puckering an old man's face, be crumpled up; while on the opposite side immediately beneath, it will, like the unwrinkled cheeks of Boreas, be severely distended-thus denoting or rather demonstrating what we have stated, namely, that beneath the rind the wood of the upper part of the stick is severely compressed, while that underneath it is as violently stretched ;indeed if the little experiment be continued by bending the bow till it breaks, the splinters of the upper fracture will be seen to interlace or cross each other, while those beneath will be divorced by a chasm. But it is evident on reflection that these opposite results of compression and extension must, as they approach each other, respectively diminish in degree, until in the middle of the beam, termed by mathematicians its neutral axis,' the two antagonist forces, like the anger of the Kilkenny cats, or, rather, like stillwater between tide and backstream, become neutralised, and, the lamina of the beam consequently offering no resistance either to the one power or to the other, they are literally useless. As therefore it appears that the main strength of a beam consists in its power to resist compression and extension, and that the middle is comparatively useless, it follows that in order to obtain the greatest possible amount of strength, the given quantity of material to be used should be accumulated at the top and bottom where the strain is the greatest-or in plain terms the middle of the beam, whether of wood or iron, should be bored out. All iron girders, all beams in houses, in fact all things in domestic or naval architecture that bear weight, are subject to the same law. The reader has now before him the simple philosophical principle upon which Mr. Stephenson, when he found that he |