CHAP. IV., 3.] 



MECHANICS. ME STEPHENSON. 



79 



the inven- 

 tion. 



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 in- 

 clude three portions ; an u^er 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 incom- 

 parably greater than the upper ; in wrought iron a 

 slight predominance should be given to the upper 

 flange for the converse reason. 



(359.) Hence it will be easily understood how, when Mr 



The idea of Robert Stephenson was desired to construct a rail- 



l!rid tU e b ae" wa y Dri % e 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. 



(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 

 assumed the practical shape, now to be seen at Con- 

 way, and near Bangor. in North Wales. It is un- 

 fortunately notorious that there has existed an un- 

 happy rivalry as to the share of merit due to the 

 several persons who of necessity were jointly con- 

 cerned in the completion even of the design of these 

 astonishing works. Unfortunately for Mr Stephen- 

 son's tranquillity, the tremendous responsibility of 

 this novel, gigantic, and costly experiment, was 

 thrown upon him during the very height of the com- 

 mercial and engineering excitement (not unjustly 

 called mania') which prevailed in 1845 and 1846, on 

 the subject of railway projects. Instead of the un- 

 interrupted 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, confi- 

 dential, 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 comparatively great strength of tubes was 

 a fact known from the time of Galileo. Their de- 

 fect was a liability to crumple or pucker. Round, 

 oval, and rectangular tubes were tried, and the last 

 (Mr Stephenson' s original conception) were, as might 

 be supposed, found to be stronger than the other 

 two. When supported at the ends, and loaded in 



(361.) 

 The cellu- 

 lar top. 



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 exces- 

 sive stiffness of corrugated iron and zinc plates (long 

 previously used for roofs) came to the engineer's 

 assistance. A combination of two longitudinally cor- 

 rugated or goffered wrought-iron plates running 

 along the top of the model, and forming long and 

 nearly cylindrical cells, was found to give the re- 

 quired 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 s n re s *J f 

 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 ingeni- 

 ous and perfect method of combination (which is due 

 to Mr Fairbairn, who had previously obtained a patent 

 for it), the structure would have been impossible. 



The success of this astonishing piece of engineer- (362.) 

 ing lias been complete ; the stiffness of the tubes, Its com ' 

 whether under constant pressure or during the rapid e g S e s 

 transit of trains, is almost incredibly great. 



To Mr Robert Stephenson is clearly due the credit (363.) 

 of undertaking, on his sole responsibility, a project Mr ste ' 

 of equal boldness and novelty, and of contriving, not ^ eg tj^itle 

 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 J. 16 . 88 . 1 ' 3 . 



. % . i_ Tr? , Fairbairn 



is also due for the manner in which the experiments and Ho 

 were managed, and the principles established by these kinson. 

 educed. Mr Fairbairn, a practical engineer of Man- 

 chester, 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 Hodg- 

 kinson, well known for his able enquiries into the 

 strength of pillars and girders of different forms, 

 conducted the mathematical enquiries, and deter- 

 mined the relative strengths of the models. His con- 

 fidence in the result was less encouraging than that 

 of his coadjutor, which serves to show the greatness 



