feet. For clear heights much exceeding these, the bascule leaf will have to be longer than the vertical-lift span, unless an encroachment on the clearance at one corner be permitted, which is not usually the case. For a clear height of 150 feet, this excess length will generally be at least 10 or

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12 feet, and often still more. This limitation does not apply to the Brown balance-beam type, which can be rotated 90°.

In making the comparisons, it was necessary to consider two cases—first, when there are long flanking-spans, and, second, when there are not. Typical layouts  are  shown  in  Figs.  30c  and 30d.  It  will  be  noted  that  in