314 
DR. FAIRBAIRN ON THE EFFECT OF IMPACT, VIBRATORY ACTION, 
duced by passing weights over bars with different velocities, and subjecting others to 
reiterated strain corresponding to loads equal to some fractional part of the breaking- 
weight. The latter experiments were made with cams, caused to revolve by steam 
machinery, which depressed the bars and allowed them to resume their natural 
position for a large number of times. Two cams were used ; one communicated a highly 
vibratory motion to the bar during the deflection, and the other greatly depressed the 
bar subjected to it, and released it suddenly when the ultimate deflection due to the 
load had been obtained, the rate of deflections being from four to seven per minute. 
Three bars, subjected by the first-mentioned cam to a deflection equal to what would 
have been produced by one-third of the statical breaking-weight obtained from similar 
bars, received 10,000 successive depressions, and when afterwards broken by statical 
pressure, bore as much as similar bars subjected to dead weight only. Of two bars sub- 
jected to a deflection equal to what would have been caused by half the statical breaking- 
weight, one broke with 28,602 depressions, the other withstood 30,000, and did not 
appear weakened to resist statical pressure. 
Of the bars subjected to the second cam, three bore 10,000 depressions, each giving 
it a deflection equal to what would be produced by one-third of the statical breaking- 
weight, without having their strength to resist statical pressure apparently at all 
impaired; one broke with 51,538 such depressions, and one bore 100,000 without any 
apparent diminution of strength ; whilst three bars, subjected by the same cam to a 
deflection equal to what would be produced by half the statical breaking-weight, broke 
with 490, 617 and 900 depressions respectively. It must therefore be concluded that 
iron bars will scarcely bear the reiterated application of one-third their breaking-weight 
without injury. 
A bar of wrought iron 2 inches square in section and 9 feet long between the sup- 
ports, was subjected to 100,000 depressions, by means of the first-mentioned or rough 
cam, each depression producing a strain corresponding to about f ths of the strain that 
permanently injured a similar bar. These depressions only produced a permanent set 
of -015 inch. 
Three wrought-iron bars were subjected to 10,000 depressions each from the step- 
cam, depressing them through ^ inch, f inch, and f inch respectively, without pro- 
ducing any perceptible permanent set. A bar depressed through 1 inch obtained 
a set of '06 inch, and one depressed 300 times through 2 inches acquired a set of 
TO 8 inch. The largest deflection which did not produce any permanent set appears, 
by an experiment on a similar bar, to be that due to rather more than half the statical 
weight which permanently injured it. 
A small box girder of boiler-plate riveted, 6 in. by 6 in. in section and 9 ft. long, 
was also subjected to depressions by means of the rough cam, principally with the view 
of ascertaining whether any effect would be produced on the rivets by the repeated 
strain; but a strain corresponding to 3752 lbs. repeated 43,370 times did not produce 
any appreciable effect. 
