It is customary among engineers and architects, in making allowance for the strain to be borne t)y any l)art of a structure, to assign to the miaterials use(l a strength sufficient to w;thstan(d a strain somewhiat greater than the structure is ever likely to he subjected to. By experiment it can be found, for exam)ple, what is the "breaking load" of a woo(ldn or ironi b?aim of given (limiensions, and an empirical la'w is establ!shled which will give us approximately the breaking, load of

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... , wlen we know the dlimensions, illaterial, etc.; but in or(ler to cover all possil)le (lifferenceswhich may exist in various btans. a L'oflicie'it of Safe'/y is eithlr intro(luced inito the forimtila itself' or is app)lied to the resuilt obtained froin the forimutila. 'I'his co-dioicient stouldti b(e uIrge enoughi to cover, not only the lairgest possil)le deviation between ex)erilment and theory, but also to imieet all uloforesten emiergenicies, suchi as tinim and age inevitably bring.