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442                   METALLURGY OF CAST IRON.
One case which has come to the writer's knowledge showed a gain of 29 per cent, by reason of tumbling test bars. For results with chilled bars, see page 434. Mr. Outerbridge was led to demonstrate that shocks could increase the strength of cast iron by first observing that chilled car wheels rarely cracked in ordinary service, after having been used for a considerable length of time. He says if they did not crack when comparatively new, they usually lasted until worn out or condemned for other causes. Mr. Outerbridge found that, up to the point of the shock relieving the internal strains by permitting the individual metallic particles to re-arrange themselves and assume a new condition of molecular equilibrium, any further shock did not increase the strength. He does not say this would injure it, and, in speaking of a few practical deductions for universal application to be drawn from his tests and observation, he says: " Castings such as hammer frames, housings for rolls, cast iron mortars or guns, which are to be subjected to severe blows or strains in actual use, should never be tested to anything approaching the severity of intended service/* Mr. Outerbridge's discovery is a valuable one, and can find practical application in many ways, especially in showing the light-work founder that li tumbling " castings is beneficial; but that it is best, when practical, where there are any fears of castings being broken, to start slowly and gradually increase the speed to the limit generally practiced when " tumbling. "*
* The paper giving all the tests, etc., was originally presented at the meeting of the American Institute of Mining Engineers, in Pittsburg, Pa., February, 1896, and, can be found in its proceedings of that year.nd. Three estra weights, I, are used when necessary. Tlniy are each of the same weight as II. Uc-nee, with one weight, scale division m eorre:,ponds to m y^rammes on the diamond, with two weights i> corresponds to '/o grammes,with thr<*e weights to -\n ^nimmes, and \-ith tour weights to ,|o grammes, the other scale division:, in Volume XXIII. of the              .*