Bridge Company, has very kindly investigated this question for the author; and in a letter dated January 29th, 1920, he reports as follows concerning a double-track, steam-railway bridge, 1,200 feet long, for which the metal erected is assumed to cost 7.00 per lb.:

After careful study I feel that the three equal spans and the two spans of 350 feet with one of 500 feet should take the same pound price. For the two spans of 300 feet

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and one of 600 feet I would reduce the price $1.00 per ton or to 6.95¢ per pound; and for the two 250-foot spans and one 700-foot span, I would reduce the price $2.00 per ton, or to 6.90¢ per pound.

As the extreme variation in pound price is only one-and-a-half per cent, evidently, as before stated, it is not worth while to make a special diagram recording total cost-ratios for uneconomic, three-span, simple-truss layouts; because those for the weight-ratios thereof will suffice.