H. Using nickel steel instead of carbon steel in the superstructure increases materially the economic span-length.

I. The assumed variations in unit prices with changing market conditions make very little difference in the economic span-lengths. There would have been no difference at all had the prices of all the materials used been assumed to vary in the same proportion; but the superstructure steel,

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erected, ordinarily changes in value somewhat more rapidly than does the substructure of the bridge.

J. There are not many irregularities to be found in comparing the diagrams or the tabulated results of the calculations; and what few exist are small. They are generally due to the adoption of a minimum weight limit for sinking to great depths instead of figuring upon employing temporary loading.

Certain of the cost curves in the diagrams, in combination with other diagrams giving weights of steel distributed between trusses, laterals, and