Cuar. IV., § 3.] 
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 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 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 ideaof Robert Stephenson was desired to construct a rail- 
pereey way bridge across the Menai Strait,—subject to the 
pees | 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 
ial 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. Asa 
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. 
(361.) The comparatively great strength of tubes was 
pean 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 
MECHANICS.—MR STEPHENSON. 
877 
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 zine plates (long 
previously used for roofs) came to the engineer's 
assistance. A combination of two longitudinally cor- 
rugated or gofered 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 pee: | Ye 
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 com- 
posed, are united by mechanical pressure by means 
of hot rivets; and it may safely be affirmed that 
without this ingenious and perfect method of com- 
bination (which is due to Mr Fairbairn), the struc- 
ture would have been impossible. 
The success of this astonishing piece of engineer- 362.) 
ing has been complete; the stiffness of the tubes, Its com- 
whether under constant pressure or during the rapid Piet — 
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 Pcponeible 
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 mc 
is also due for the manner in which the experiments aiid Hodg- 
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 
