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VIII. Experiments to determine the effect of Impact , Vibratory Action, and long- 
continued Changes of Load on Wr ought-iron Girders. 
By W. Fajrbairn, LL.B., F.JR.S. 
Received January 20, — Read February 4, 1864. 
A question of great importance to science and the security, of life and property has 
been left in abeyance for a number of years, — namely, to determine by direct experi- 
ment to what extent vibratory action, accompanied by alternate severe strains, affects 
the cohesive force of bodies. It is immaterial whether the body be crystalline, 
homogeneous, or elongated into fibre, such as cast or wrought iron ; the question to be 
solved is, how long will a body of this description sustain a series of strains produced 
by impact (or the repeated application of a given force) before it breaks 1 In the case 
of bridges and girders, this is a subject on which no reliable information has yet been 
given which may be considered as a safe measure of strength for the guidance of the 
architect and engineer. It is true that regulations have been established by the Lords 
Commissioners for Trade ; but they appear to have had their origin on limited data, 
and in cases where the material and workmanship are good they may be relied upon 
as sufficient for the public safety. What, however, is wanted is experimental data to 
enable the engineer to comply satisfactorily with the conditions of the Board of Trade, 
and cordially to unite with the Government in affording ample security to constructions 
in cases where the lives of the public are at stake. 
To remove all doubts on this question, I have been enabled, through the liberality 
and at the request of the Board of Trade, to undertake a series of experiments to deter- 
mine, or to endeavour to ascertain, whether a continuous change of load, and the 
strains produced by those changes, have any effect (and to what extent) upon the ulti- 
mate strength of the structure, — or, in other words, to ascertain the rate of endurance 
the material is able to sustain under these trials. 
To comply with this request, a wrought-iron beam was constructed, representing the 
girders of a bridge of questionable strength, to be employed to determine, experi- 
mentally, the strength and durability of such a structure. This beam was made of the 
ordinary construction, of moderately good, but not the best quality of iron, and sub- 
jected to vibration and a perpetual change of load until the cohesive powers of the 
material were destroyed. 
Of the resisting-powers of material under the severe treatment of a continuous change 
of strain, such as that which the axles of carriages and locomotive engines undergo 
when rolling over iron-jointed rails and rough roads, we are very imperfectly informed. 
mdccclxiv. 2 u 
