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440                      METALLURGY   OF   CAST   IRON.
practical sense exhibited in looking to high transverse and deflection combined with a low contraction, than to impact blows on the side of a test bar. A practical way to apply an impact test is to the castings themselves. The car wheel men teach a lesson in this respect. Here we find that some select from a large stock one wheel out of every hundred, and if by dropping a i4o-pound weight on the hubs of the sample wheels from a height of 12 feet the sample wheels stand five blows each, all the other wheels are then accepted, providing they have stood the thermal test described on page 443, and which shows, in connection with the above impact tests, the absurdity of thinking to be guided by impact blows on the side of test bars.
The power of castings to withstand shocks or blows is often far more affected by their proportion or design than by the quality of iron composing them. There is altogether too much indifference exhibited by de-signers of machinery in proportioning castings so as to have the least possible internal contraction strain in them. Some designers seem to ignore wholly the fact that a light body will contract more than a heavy one. Many castings have been made, the iron in which would test all right as far as test bars were concerned, but subjecting them to shocks or blows, would imply that the iron was not of the right character. This again illustrates the impracticability of some impact tests on bars and shows that a weak, high-contraction iron can' often be of much more value in a well-proportioned casting than the reverse kind of iron in an ill-propor-tioned one.
A. E. Outerbridge's shock tests form an interesting study in this connection.    In these tests, Mr. Outer- t.2 grammes on the diamond. 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              .*