channel, we find the cost to be $770,000. For the bascule, we enter with a "clear height" of "50 feet or less," and find the cost to be $820,000.

Example No 4

What would be the comparative costs in Example No. 3, if the bascule were not permitted to encroach on one corner of the waterway clearance?

The cost for the vertical lift is $770,000 as before. Entering Fig. 30e for the bascule with a "clear height" of 140 feet, by interpolating we find the cost to be about $845,000.

Example No. 5

What are the comparative costs of a double-track-railway, Class 60, bascule and the corresponding vertical lift for a 120-foot clear-opening without flanking-spans, the "clear height" being 50 feet with span raised, and 20 feet with span down?

From the lower group of curves in Fig. 30e, we find the cost of the vertical lift to be $170,000, and that of the bascule $200,000.

Example No. 6

What would be the comparing costs for Example No. 5, if flanking truss spans were used?

Entering the upper group of curves of Fig. 30e, we find $460,000 for the vertical lift, and $497,000 for the bascule.

Example No. 7

In a double-track-railway bridge, with flanking truss spans, having a clearance above high water, with span down, of 20 feet, what will be the vertical clearances for a vertical-lift span of equal cost with a bascule, when the clear horizontal opening is 100 ft., 110 ft., 120 ft., or 130 ft.?

From Fig. 30f we find the following: