and the corresponding bridges of simple-truss or cantilever type depend greatly upon the governing conditions for the substructure, and that, as these almost invariably differ essentially at proposed crossings, it is impracticable to determine in general the economics of substructure for such com-

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parisons, each case having to be treated upon its own merits. While this is certainly true, it is practicable, nevertheless, to outline some general principles that will enable a bridge designer to determine readily the suitability of any proposed crossing for an arch layout, as affected by substructure conditions; and the author offers the following: