1184 REPORT—1885. 
sufficed to rupture two of the specimens. Slow-moving weights on bars and on a 
small wrought-iron box girder gave analogcus results ; and the deduction drawn by 
the experimenters at the time was that ‘iron bars scarcely bear the reiterated 
application of one-third the breaking weight without injury, hence the prudence 
of always making beams capable of bearing six times the greatest weight that 
could be laid upon them.’ 
Although these experiments were entirely confirmatory of all previous expe- 
rience, they would appear to have little influenced the practice of engineers, since 
Fairbairn, more than ten years later, in a communication to this Section, said that 
opinions were still much divided upon the question whether the continuous change 
of load which many wrought-iron structures undergo has any permanent effect upon 
their ultimate powers of resistance. To assist in settling the question he com- 
municated to the Association the results of some experiments carried out by himself 
and Professor Unwin on a little riveted ‘girder 20 feet span and 16 inches deep. 
Once more the same important but disregarded facts were enforced on the attention 
of engineers. About 5,000 applications of a load equal to four-tenths of the 
calculated breaking load fractured the beam with the small ultimate deflection of 
three-eighths of an inch, and subsequently, when repaired, the beam broke with 
one-third of the load and a deflection of but a quarter of an inch, which sufficiently 
indicated how small a margin the factor of safety of four, then currently adopted, 
allowed for defective manufacture, inferior material, and errors in calculation. 
Still nothing was done, and the general practice of engineers and the Board of 
Trade regulations continued unaltered. 
Soon after the introduction of wrought-iron bridges on railways, the testimony 
of practical working was added to that of experiments. In 1848 several girder 
bridges of unduly light proportions were erected in America, and one of 66 feet 
span broke down under the action of the rolling load in the same manner as Fair- 
bairn’s little experimental girder. Again, in early American timber bridges the 
vertical tie-rods were often subject to stresses oscillating between one ton and ten 
tons per square inch and upwards. Many of these broke, as did also the suspension 
bolts in platforms subjected to similar stresses. In my own experience, dozens of 
broken flange-plates and angle-bars, and hundreds of sheared rivets, have been the 
silent witnesses of the destructive action of a live load. Like evidence was afforded 
by early-constructed iron ships deficient in girder strength. Under the alternating 
stresses due to the action of the waves weaknesses not at first apparent would, in 
the course of time, be developed, and additional strength, in the way of stringers 
and otherwise, become imperative. 
If none of the preceding evidence had been forthcoming, the results of the 
historical series of experiments carried out by Wohler for the Prussian Ministry of 
Commerce would alone be conclusive. For the first time a truly scientific method 
of investigation was followed, and an attempt was made to determine the laws 
governing the already proved destructive action of intermittent stresses. In 
previous experiments the bar or girder was alternately fully loaded and wholly 
relieved of load. Wohler was not satisfied with this, but tested also the result of 
a partial relief of load. The striking fact was soon evidenced, on testing specimens 
under varying tensions, that the amount of the variation was as necessary to be 
considered as that of the maximum stress. Thus, an iron bar having a tensile 
strength of 24 tons per square inch broke with about 100,000 applications of a 
stress varying from nz to 21 tons, but resisted 4,000,000 applications of the 21 tons 
when the minimum stress was varied from nl to 11} tons. The alternations of 
stress in the case of some test pieces numbered no less than 132,000,000; and too 
much credit cannot be bestowed by engineers upon Wohler for the ingenuity and 
patience which characterised his researches. As a result, it is proved beyond all 
further question that any bar or beam of cast iron, wrought iron, or steel may be 
fractured by the continued repetition of comparatively small stresses, and that, as the 
differences of stress increase, themaximum stress capable of being sustained diminishes. 
Various formule based upon the preceding experiments have been proposed for 
the determination of the proper sectional area of the members of metallic structures. 
These formule differ in some essential respects, and doubtless many experiments 
