making general economic investigations for arch bridges, because no two examples thereof are alike. In case of a succession of long spans in which there is a choice of rise, a tentative layout should be made using the anticipated economic ratio of rise to span-length when both substructure and superstructure costs are considered, then the costs of piers and spans should

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be computed, the uneconomic effect of departing from the established economic ratio of rise to span-length, given in Figs. 26f to 26i, inclusive, being employed to calculate the weights of metal from those shown in certain other diagrams.

Next, this work should be repeated for a slightly greater ratio of rise to span-length, and then for a slightly smaller ratio thereof. These three sets of computations  would  probably  determine  the  best  ratio  to  adopt;