tance of properties of solids. tivated precisely the same branches of science which thematician; he concentrated his efforts more me- § 3. THOMAS YOUNG-Strength of Materials, and Art of Construction (continued).—TELFORD (342.) It is a circumstance not uninstructive as to the (343.) The eighteenth century was in this, as in so many Progress other departments of science, sluggish and mechaniduring the cal, or else abstract and ultra-geometrical. The eighteenth learned labours of Euler and the Bernouillis on century. elastic curves, and the strength of pillars, were for the most part elegant mathematical amusements, and with the exception of the experiments of Musschenbroek in the earlier half of the century, and the skilful but more limited researches of Coulomb at its close, little valuable in the way of precise theory or of accurate data derived from practice had been added to this important branch of mechanical engineering. Robison, indeed, with the peculiar tact and skill which I have already ascribed to him, wrote several papers (contributed to an early edition of the Encyclopædia Britannica, and printed in his collected works) full of acute observation and reasoning, adapted to the imperfect experiments of his time, and connected by sound scientific deductions, which are still well worthy of careful perusal; but it was to the penetration of Dr THOMAS YOUNG,' who partook strongly of Robison's mechanical tastes, whilst he surpassed him in facility of mathematical resource, that we owe a great revision of the doctrine of the strength of materials. In the "Syllabus of Lec tures" (1802), into which he condensed, in a manner tion of Extension. The forces tending to alter the figure or dimensions (345.) of substances usually called solid may be thus clas- Applicasified: (1.) Extending forces, or such as produce force to elongation in a body when applied in a direct man- solids. ner. (2.) Compressive forces. (3.) Force producing detrusion, or the slipping of one portion of the substance over another. (4.) Force producing flexure. (5.) Torsion or twisting force. The resistance of bodies to extension was examined by Hooke and Gravesande, and is held to be directly as the area of section of the body, and to increase directly as the amount of elongation produced, at least within certain limits. The measure of this resistance Young termed (not very happily) Modulus of Elasticity, expressing the Modulus of force required to produce unit of elongation (or to Elasticity. double the length) of a prism of the substance under experiment. This quantity may be measured either by the length of a depending prism of the substance which would produce the requisite strain, or more simply by the strain expressed in pounds or tons, which, supposing the elongations to increase without limit as the extending forces, would double the length of the prism under experiment. Thus, in round numbers, a bar of wrought iron an inch square will be extended Toboo part by a pressure of one ton-hence the modulus of elasticity is about 10,000 tons. The elasticity of wrought iron remains perfect to about half the breaking weight, after which the elongations appear to double for each addition of about or of the breaking weight. Thus, in a recent experiment by Mr Edwin Clark, a bar of 1 I reserve to the chapter on Optics a fuller account of Young and his writings. wrought iron, one inch square and ten feet long, extended of its length for every ton of weight up to 12 tons, from which point the extensions nearly doubled successively for every two tons of load, and the bar was finally torn asunder by 23 tons. (346.) The compression of bodies proceeds (like the exCompress- tension) at first uniformly with the load. Some boing force. dies resist compression more than extension (as cast iron); some the reverse (as wrought iron). Substances give way under compression after different fashions. Hard bodies divide into prisms parallel to the compressing force; slender elastic bodies bend laterally; soft bodies bulge horizontally; bodies of a medium hardness divide into wedges, and the surfaces slide along the plane of spontaneous fissure. Detrusion. Detrusion marks more particularly the mode of giving way by the sliding of surfaces in the interior of solids. Though seldom due to force directly applied, it is an important element in most cases of the rupture of semiductile solids. beams. (347.) The force of flexure is that by which the resistFlexure of ance of the greater number of solids is most easily overcome, but which it is of most importance to resist; as when a beam is fastened by one end into a wall, and loaded at the other, or when it spans a horizontal space. It had not escaped the notice of James Bernouilli, Duhamel, and other writers of the earlier part of the 17th century, that the fibres on the concave side of a loaded beam are in a state of compression and not of extension, and that there is therefore a point, or rather a line, in every beam, in which the fibres are neither extended nor compressed. But the clear modification of the theory prevalent in the time of Leibnitz and Marriotte which this consideration introduced, was probably first developed by Coulomb, Robison, and Young, who in their respective publications insisted upon it with great judgment; and it is difficult to overrate its importance in mechanical engineering, although the first great canon of Galileo remains still true, that the ultimate strength of a solid rectangular beam varies as the breadth and as the square of the depth. The writings of Young and Robison did not immediately attract the attention of practical men, and Coulomb, who was by far the ablest French experimenter on subjects of mixed mechanics, seems to have done less on the theory of strains producing flexure than in the case of torsion, which he studied with so much success, and applied to such excellent purpose. Nevertheless in his memoir on the Resistance of Masonry, in the 7th vol. of the "Memoires Presentés" (1776), he had already laid down very clearly the effect of compression on a beam.1 It is, however, to Young that we owe the application of these principles in unfolding their legitimate consequences. In a series of remarkable propositions contained in the writings I have quoted (344), he assigns numerical relations between the flexure of a beam under almost every supposable circumstance, and the resistance of the material to direct strains. These results have been extensively used by all subsequent writers. They are not equally verified in all classes of substances. This, however, is not wonderful; flexure is not due to direct compressive and extending strains alone; deformation may take place in a solid without appreciable change of density, thus giving rise to some of the nicest questions in molecular physics. The laws of Torsion, as laid down by Coulomb, _ (348.) have been mentioned in the last section (340). Torsion. The mathematical investigations of Young on (349.) mechanical problems were conducted with bold di- Young's rectness and in defiance of the generalizing methods writings. mechanical and symmetrical notation of foreign writers on such subjects. But his pre-eminent sagacity in laying hold on the salient points of the questions he discussed, and in conducting his argument to a practical conclusion, was unequalled, and deserves imitation.2 A great revival in the study of the properties of (350.) elastic matter, as regards strength, took place about General the year 1820, probably in consequence of the in- use of wrought troduction of wrought iron into the construction of iron. suspension bridges, which has been attended with important results. THOMAS TELFORD, though neither the contriver of (351.) suspension bridges, nor the introducer of them into Telford; Britain,3 deserves notice from the superior boldness suspension bridges. and solidity of the noblest work of the kind which has yet been executed-the Menai Bridge. Telford (and the same may be said of his contemporary Rennie) was more distinguished as a man of judgment, integrity, and experience, than as eminently original or philosophical. In this respect both yield to Smeaton, who, with Watt, was the founder (each in his own department) of modern engineering. But the beautiful and truly workmanlike structure of the Menai Bridge inaugurated the era of the extensive introduction of that admirable material, wROUGHT IRON, into great permanent structures exposed to heavy strains. Cast iron had been used much earlier, as in the bridge erected at Colebrookdale in 1777 by Mr Derby, and in the very beautiful arch at Sunderland, which dates from 1796. The span of the Menai Bridge is 580 feet, the whole quantity of iron used was 2186 tons, the transverse section of the suspending chains or bars was 260 square inches, supporting a strain of 1094 tons. This 1 Reprinted in the Théorie des Machines Simples, Paris, 1821. 2 A copious selection from Young's mechanical writings may be found in his Miscell. Works edited by Dr Peacock, vol. ii. 3 Captain Samuel Brown erected the first considerable chain bridge in this country across the Tweed in 1819. son. was a work quite unexampled at the time of its erection (1826), and showed a sagacious confidence in the employment of a material then comparatively little trusted. (352.) Telford never made extensive experiments on the Data of re- resistance of solids. Some special ones were indeed sistance. made under his direction on wrought iron in particular, but in general he seems to have relied upon the old ones of Musschenbroek and Buffon. The Tredgold, Barlow, and two persons who first in recent times vigorously Hodgkin- applied themselves to the practical determination of the data of resistance so long deficient, were Tredgold, a private engineer, and Professor Barlow of Woolwich. The data they obtained have since been generally used, not only in this, but in other countries. Tredgold's works (on Carpentry, Strength of Timber, &c.) show a very great aptitude in applying the results of science to practice, and an acquaintance with both which is rarely attained. Mr Eaton Hodgkinson has made many valuable additions to Tredgold's work, and has contributed an excellent paper on the strength of pillars to the Philosophical Transactions (1840.) (353.) Geometry of the To Mr Hodgkinson we are also indebted for a useful investigation (in the Manchester Transactions) into the figure assumed by the chains of suscatenary. pension bridges. The elegant properties of the simple or geometrical catenary were fully investigated a century and a half since by the Bernouillis and by David Gregory, but the application of suspended structures of immense weight to purposes of utility suggested new problems. Amongst these, perhaps the most interesting was the catenary of uniform strength, in which the section of the suspending chains is made everywhere proportional to the strain which they have to resist at that particular point. Its equation was investigated by Mr Davies Gilbert in 1826. An elegant and valuable contribution to the geometry of catenarian curves was made by the late Professor Wallace of Edinburgh,' with particular application to curves of equilibration for bridges of masonry after the ingenious manner of Robison mentioned in Art. (334). (354.) In connection with suspension bridges, and also M. Navier. with researches on the yielding of elastic materials, we must record the name of M. Navier, a very eminent French engineer and writer on practical and theoretical mechanics. His work on suspension bridges (1823) is one of the earliest and best. He is also well known for his physico-mathematical researches on the yielding of elastic solids to pressure 1 Edinburgh Transactions, vol. xiv. under given circumstances, in the course of which he came into collision with Poisson, who gave a somewhat different theory. The subject is one of extreme difficulty, owing to our ignorance of the molecular constitution of bodies; and it is believed that all these investigations were so far erroneous that they were based upon the assumption of a single constant to represent the resistance of bodies to change of form and dimension. These (form and dimension) are two very different things, and require distinct treatment." British mathematicians have lately paid much attention to these enquiries, with the prospect of a solid improvement in engineering theories.3 son. The art of bridging over great spaces has been (355.) pushed, by the requirements of the railway system, to Mr Robert an astonishing extent, and under circumstances of Stephenpeculiar difficulty. I shall connect these improvements with the name of Mr ROBERT STEPHENSON, the inventor of the Tubular Bridge, a work which, in its very simplicity, is a triumph of art, and being nothing more than a hollow beam of somewhat peculiar construction, supported at the ends, it is an admirable instance of a structure of which the stability may be easily reduced to calculation. Wooden The wooden bridges of Switzerland were for a long (356.) time unequalled as skilful works of carpentry. During bridges in the last century the Rhine at Schaffhausen was crossed Switzerby two spans of 171 and 193 feet. At Trenton, in land and America, the river Delaware is crossed by a wooden America. bridge, of which one arch is 200 feet in span. It is on the bow principle, an elastic wooden arch, convex upwards, being skilfully braced and united to a level roadway passing through the spring of the arches. The American lattice bridge, very simply and skilfully contrived, has great firmness, owing to the depth of the framing, and exercises no horizontal thrust on the piers. The widest spanned wooden bridge in the world, 340 feet, across the Schuylkill, at Philadelphia, designed by Wernwag, combines the bow and Îattice principle. In these we might see foreshadowed in some faint (357.) degree the principle of the TUBULAR BRIDGE, the The tubular greatest discovery in construction of our day. But bridge. in reality the idea of it arose from a different consideration. During the first ten or fifteen years of railway ex- (358.) perience, engineers had gradually acquired a correct Railway girderperception of the manner in which cast and wrought bridges. iron may most effectually and economically be formed 2 About twenty years ago, the present writer showed that India rubber, which possesses to such a remarkable extent the quality which may be termed cubical flexibility, is yet scarcely at all compressible-in fact, just as much as water, and no more. Though not otherwise published, he has been in the habit of demonstrating this in his annual course of lectures. 3 Professor Stokes in Cambridge Transactions, vol. viii.; Mr Clerk Maxwell in Edinburgh Transactions, vol. xx. Mr M. Rankine in Cambridge and Dublin Math. Journal for 1851 and 1852. Experimental data are still deficient; but M. Wertheim has lately published some valuable ones (which are still in progress) in the Annales de Chimie. (359.) the tubular into girders or beams supported at the ends, and adapted for sustaining enormous loads. Such beams were constructed, often of single castings, so as to include three portions; an upper flange, a lower flange, and a web, or thinner vertical plate connecting the two. The relative section of the upper and lower flange was made to vary with the material. In cast iron, which yields far more easily to tensile than to compressive strains, the lower flange should be almost incomparably greater than the upper; in wrought iron a slight predominance should be given to the upper flange for the converse reason. Hence it will be easily understood how, when Mr The idea of Robert Stephenson was desired to construct a railbridge de- way bridge across the Menai Strait, subject to the rived from onerous condition imposed by the Admiralty, that it them. should (even at the abutments) be without lateral struts or diagonal pieces below the roadway, he should have entertained the idea of a gigantic girder with a top and bottom flange of proportionate extent, with a deep web uniting them, or rather of two such girders placed side by side, thus forming square tubes, of which the lower flanges should constitute the bottom, the upper flanges the top, and the two webs the sides. tion. (360.) It is not for me in this place to explain how, step Progress of by step, the idea of a tubular bridge of wrought iron the inven- assumed the practical shape, now to be seen at Conway, and near Bangor, in North Wales. It is unfortunately notorious that there has existed an unhappy rivalry as to the share of merit due to the several persons who of necessity were jointly concerned in the completion even of the design of these astonishing works. Unfortunately for Mr Stephenson's tranquillity, the tremendous responsibility of this novel, gigantic, and costly experiment, was thrown upon him during the very height of the commercial and engineering excitement (not unjustly called mania) which prevailed in 1845 and 1846, on the subject of railway projects. Instead of the uninterrupted leisure which he required to superintend his preliminary experiments, to consider his plans, and perform his calculations, Mr Stephenson, as well as every other engineer of eminence was at that time engaged all day and a great part of the night in the unparalleled worry of Parliamentary contests. As a matter of course, much was trusted to able, confidential, and highly paid assistants. The experiments on models of different forms, which alone cost many thousand pounds, could not all be conducted in the presence of the chief engineer. Yet he alone was responsible for the failure or success of the plan. the middle, model tubes of this form invariably gave way at the top. How to strengthen the top against compressive strains was the question. The excessive stiffness of corrugated iron and zinc plates (long previously used for roofs) came to the engineer's assistance. A combination of two longitudinally corrugated or goffered wrought-iron plates running along the top of the model, and forming long and nearly cylindrical cells, was found to give the required stiffness with the least increase of weight. Ultimately a square arrangement of cells was adopted, principally to give facility for painting and repair. The tubes were hindered from racking by means of numerous wrought-iron frames employed to stiffen them, whose section resembled the letter T, and which were called T irons. Suitable diaphragms were also inserted at short distances along the tubes. The widest spaces to be spanned at the Menai Strait were Dimensions 460 feet, there being two intervals of this width, and and two of 230 feet. The tubes are 30 feet high and 14 strength of the bridge. broad, containing a transverse section of about 1500 square inches of wrought iron. The weight of one principal tube is about 1450 tons, and its strength, measured by the breaking load at the centre, above 2300 tons. This last number is calculated from the experiments on the breaking weight of models, one of which was on no less than a sixth of the true scale. The boiler plates, of which the tubes are composed, are united by mechanical pressure by means of hot rivets; and it may safely be affirmed that without this ingenious and perfect method of combination (which is due to Mr Fairbairn), the structure would have been impossible. The success of this astonishing piece of engineer- (362.) ing has been complete; the stiffness of the tubes, Its comwhether under constant pressure or during the rapid plete suctransit of trains, is almost incredibly great. cess. Messrs To Mr Robert Stephenson is clearly due the credit (363.) of undertaking, on his sole responsibility, a project Mr Steof equal boldness and novelty, and of contriving, not phenson the responsible perhaps in every detail, but in its totality, the means inventor; by which so signal a triumph of art and of science was carried into effect, an honour to his own age, and a lesson to posterity. To Mr Fairbairn and Mr Hodg-assisted by kinson, his assistants, selected by himself, much praise Fairbairn is also due for the manner in which the experiments and Hodgwere managed, and the principles established by these kinson. educed. Mr Fairbairn, a practical engineer of Manchester, well known for his experience and sagacity, gave to Mr Stephenson, as a matter of honour, the full benefit of both; and his confidence in the result helped no doubt to sustain the manly courage of his principal amidst a storm of opposition. Mr Hodgkinson, well known for his able enquiries into the strength of pillars and girders of different forms, conducted the mathematical enquiries, and determined the relative strengths of the models. His confidence in the result was less encouraging than that of his coadjutor, which serves to show the greatness (364.) Commission on railway bridges. (365.) (366.) bart Bru nel. of the responsibility of the engineer-in-chief. As neither Mr Fairbairn nor Mr Hodgkinson could have incurred any just blame had the vast structure when on the eve of completion doubled up under its own weight, and blocked up, perhaps for ever, the navigation of half the Menai Strait, so neither can they possibly claim more than a subordinate share in the success of the undertaking.1 I shall here only refer to the work of Mr Edwin Clark, the resident engineer of the Britannia Bridge, for farther details of its principles and construction, and to the report of a royal commission (published in 1849) on the application of iron to railway structures, for many curious researches connected with the subject of this section. In particular, we find a theoretical and practical solution of the very delicate problem of the influence of the speed of passing loads on the deflection of bridges, to which Professors Willis and Stokes, and Colonel James, R.E., are contributors. Mr Robert Stephenson is the son of Mr George Stephenson, who will be mentioned in a succeeding Mr Ste phenson. section. He was born in 1803; educated (in part) at Other the University of Edinburgh, under Leslie, Hope, and works of Jameson; he long occupied the chief position in the locomotive factory established by his father at Newcastle, having in the first place constructed under his direction the celebrated "Rocket" engine which gained the prize at the opening of the Liverpool and Manchester railway. To his own exertions, both before and after that period, the locomotive owes much of its present perfection. He surveyed and principally carried through the London and Birmingham railway, the second great line in the kingdom; and he has been engaged in a large proportion of the most remarkable engineering works connected with railways, both in this country and abroad. He has personally superintended the construction of railways amidst the blowing sands of Egypt, and in Norway with its heavy winter snows and deeply frozen soil. His high personal character, both for skill and integrity, has everywhere procured him the respect and confidence of his profession and of the public. § 4. BRUNEL.-Self-acting Machinery.—The Thames Tunnel.—Mr BABBAGE'S Calculating Engines. Sir MARC ISAMBART BRUNEL, born at Hacqueville Marc Isam- in Normandy on the 25th April 1769, was one of the most inventive mechanicians and engineers of his day. As his genius gave a strong impression to contemporary art, we associate his name with the progress of civil engineering in the earlier part of the present century, particularly in connection with mechanism. Like most of his eminent coevals in the same profession, he had not the benefit of a scientific education; but he more than most of them supplied its defects by a singular capacity for correct induction and by great mechanical ingenuity. Though a native of France, it was in Great Britain that his talents were to find their full scope, and it became his thoroughly adopted country. career as a civil engineer, his boyish tastes having already indicated this as his natural calling. He executed some considerable works, and planned many more; it is stated that he there devised the essential parts of his block machinery. About 1799 he decided on settling in England. cess. It is probable that his talents and ingenuity alone (368.) recommended him to a government employment at a Employed by the Engtime when the mere fact of his being a Frenchman lish gomust have acted as a powerful obstacle to his suc- vernment. Those who recollect the vivacity and bright intelligence of even his later years, will understand that in his more active days it must have been difficult to refuse Brunel at least a hearing. And it is to the credit of Lord Spencer, then one of the Lords of the Admiralty, and of General Sir Samuel Bentham, inspector of naval works, that Brunel was engaged in 1802 to superintend the erection of his celebrated block machinery. The invention of self-acting machinery to super- (369.) sede the work of artisans was of course not new. Self-acting The saw-mill and the spinning-jenny were already in machinery. 1 It has been alleged that Mr Stephenson's original proposal to allow the suspension chains (which were primarily intended to be used in putting together the tubes in their final positions) to remain in aid of the rigidity of the structure, manifested a want of confidence in his own great idea. But a dispassionate consideration of his evidence before the committee of the House of Commons would alone clearly show (independent of Mr Stephenson's declarations on the subject) that he was forced into the admission that the chains might give an ulterior guarantee against miscarriage of the whole plan, simply to save the bill from being thrown out by the not unnatural incredulity of those to whom a proposal so new, so gigantic, and affecting the lives of so many persons, as well as so great pecuniary and other interests, was for the first time and suddenly proposed. Besides this, even his own coadjutors did not all entirely support him. Mr Hodgkinson, whose character for scientific knowledge carried great weight with the committee, recommended in his report the ultimate additional security of chains. |