TRANSACTIONS OF SECTION G. 1183 
how far an existing practical rule is founded on reason, how far on custom, and how 
far on error.’ ‘There is thus an ample text for many discourses; but, as I am not 
writing a treatise on engineering, but merely delivering a brief address, I will 
confine my attention at present to a particular case of the branch of mechanical 
science referred to in the last clause of Professor Rankine’s definition, and will ask 
you to consider how far the existing practical rules respecting the strength of 
metallic bridges are ‘ founded on reason, how far on custom, and how far on error.’ 
The first question obviously is, What are the rules adopted by engineers and 
Government departments at the present time ?—and it is one not easily answered. I 
have for some time past been receiving communications from leading Continental 
and American engineers, asking me what is my practice as regards the admissible 
intensity of stress on iron and steel bridges, and in replying I have invited similar 
communications from themselves. Asa result, I am able to say that at the present 
time absolute chaos prevails. The old foundations are shaken, and engineers have 
not come to any agreement respecting the rebuilding of the structure. The 
variance in the strength of existing bridges is such as to be apparent to the 
educated eye without any calculation. If the wheels of a miniature brougham 
were fitted to a heavy cart, the incident would excite the derision even of our 
street boys, and yet equal want of reason and method is to be found in hundreds 
of bridges in all countries. It is an open secret that nearly all the large railway 
companies are strengthening their bridges, and necessarily so, for I could cite cases 
where the working stress on the iron has exceeded by 250 per cent. that considered 
admissible by leading American and German bridge builders in similar structures. 
In the case of old bridges the variance in strength is often partly due to errors 
in hypothesis and miscalculation of stresses. In the present day engineers of all 
countries are in accord as to the principles of estimating the magnitude of the 
stresses on the different members of a structure, but not so in proportioning the 
members to resist those stresses. The practical result is that a bridge which 
would be passed by the English Board of Trade would require to be strengthened 
5 per cent. in some parts and 60 per cent. in others before it would be accepted by 
the German Government or by any of the leading railway companies in America. 
This undesirable state of affairs arises from the fact that in our ownand some other 
countries many engineers still persistently ignore the fact that a bar of iron may 
be broken in two ways—namely, by the single application of a heavy stress or by 
the repeated application of a comparatively light stress. An athlete’s muscles haye 
often been likened to a bar of iron, but, if ‘ fatigue’ be in question, the simile is 
very wide of the truth. Intermittent action—the alternative pull and thrust of the 
rower, or of the labourer turning a winch—is what the muscle likes and the bar of 
iron abhors. Troopers dismount to rest their horses, but to relieve a bar of iron tem- 
porarily of load only serves to fatigue it. Half a century ago Braithwaite correctly 
attributed the failure of some girders, carrying a large brewery vat, to the vessel 
being sometimes full and sometimes empty, the repeated deflection, although im- 
perceptibly slow and wholly free from vibration, deteriorating the metal, until, in 
the course of years, the girders broke. These girders were of cast iron; but it was 
equally well known that wrought iron was similarly affected, for in 1842 Nasmyth 
called the attention of this Section to the fact that the ‘alternate strain’ in axles 
rendered them weak and brittle, and suggested annealing as a remedy, he haying 
found that an axle which would snap with one blow when worn would bear 
eighteen blows when new or after being annealed. 
So important a matter as the action of intermittent stresses could not escape 
the attention of the Royal Commissioners appointed in 1849 to consider the appli- 
cation of iron to railway structures, and some significant and sufficiently conclusive 
experiments were made by Captain Douglas Galton and others. Cast-iron bars 
3 inches square and 13 feet 6 inches span between the supports were deflected, both 
by the slow action of a cam and the percussive action of a swinging pendulum 
weight. When the deflection was that due to one-third of the breaking 
weight, about 50,000 successive bendings by the cam broke one of the bars, and 
about 1,000 blows from the pendulum another. When the deflection was increased 
from one-third to one-half, about 500 applications of the cam, and 100 blows, 
