The curves of Figs. 30e and 30f are based upon the assumption that, for the vertical lift span, the motors, operating machinery, and machinery house are located on the moving span, thus increasing the weight of the said span by 10 per cent or more. While this is the ideal location for this machinery, especially when the operator is on the span, it is considerably

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cheaper to place the said machinery in the towers. If this be done, the clear heights at which the cost of the vertical lift will just equal that of the bascule will be increased 10 feet or more; but the author has plotted the curves on the other basis, because he considers the additional cost justified.