ON THE ENDURANCE OF METALS. 425 



published in one or more of the engineering journals of the time ; and it 

 has recently been republished in the article on ' Elasticity ' by Sir William 

 Thomson in the ' Encyclopeedia Britannica.') 



In the year 1849 a series of experiments undertaken with the view to as- 

 certain the effect of repeated changes of load on iron structures was carried 

 out by Capt. (now Sir Henry) James, R.E., and Lieut, (now Sir Douglas) 

 Galton, R.E., in conjunction with Professor R. Willis, on behalf of the 

 Commissioners appointed to inquire into the application of Iron to Rail- 

 way Structures. In the course of these experiments, cast-iron bars, 3 

 inches square, placed on supports 14 feet apart, were subjected to a suc- 

 cession of blows from a swinging weight ; the general result obtained was, 

 that when the blow was powerful enough to bend the bars through one- 

 half of their ultimate deflection (that is to say, the deflection which cor- 

 responds to their fracture by dead pressure), no bar was able to with- 

 stand 4,000 of such blows in succession ; but all the bars (when sound) 

 resisted the effects of 4,000 blows, each bending them through one-third 

 of their ultimate deflection. Other cast-iron bars of the same dimensions 

 were deflected slowly by a revolving cam four times per minute, whilst 

 others, in addition to deflection by the cam, were subjected to violent 

 tremor. The results of these experiments were that when the deflection, 

 repeated in some cases 100,000 times, was equal to one-third the ultimate 

 deflection, the strength of the bars, as shown by subsequently breaking 

 them under a dead load, was not reduced. When, however, the depres- 

 sions produced were equal to one-half of the ultimate deflection, the bars 

 were actually broken by less than 900 depressions. Experiments were also 

 carried out by slowly drawing a load from end to end of the experimental 

 bars, and by running a truck loaded with various weights over the bars at 

 velocities up to 30 miles per hour, in order to test the effect of the rate of 

 motion on the deflection, with the result that it was found to increase 

 steadily with an increase of speed, until at 30 miles per hour it amounted 

 to more than double the statical deflection. To compare the results of 

 these experiments with the effects produced in actual practice, careful 

 observations were undertaken of the deflections of two railway bridges on 

 the South Eastern Railway during the passage of a locomotive engine at 

 various rates of speed, and with the engine at rest upon the bridge ; in 

 these cases the deflection produced by the engine passing at 50 miles per 

 hour was observed to be one-seventh greater than that due to the same 

 load at rest. 



The investigations of Professors Willis and Stokes, taken in con- 

 junction with these experiments, show that the great relative increase of 

 deflection arising from velocity was due to the comparatively small size of 

 the experimental bars and great deflections employed, and that the increase 

 would be greater for short bridges than for long ones. Thus the increase 

 of the statical deflection may at the highest speeds amount to one-half for 

 bridges of 20 feet span, while for bridges of 50 feet the increase would 

 not be greater than one-seventh, and would rapidly diminish as the spans 

 become greater. 



After the publication of the Report of the Royal Commission on the 

 use of Iron in Railway Structures in 1849, the effect of repeated stresses 

 on iron appears to have received no further attention until 1860-61, when 

 Sir W. Fairbairn carried out his well-known experiments for the Board of 

 Trade on a wrought-iron girder. The girder was 22 feet in length, 16 



