locked together so as to make the adjoining ends deflect equally and simultaneously. The other end of each draw is locked to the masonry of the outer rest-pier, which acts as an anchorage. It is not necessary to make the shore arms of the same length as the channel arms; but if there be a difference, there must be compensating weight so as to balance each span over the centre of the pivot-pier, and there must be a vertical, close surface provided at the end of each short arm so as to equalize as well as possible the moments of the wind-pressure on the two arms. This class of bridge is probably not very rigid, but it can he made quite satisfactory and effective.

The pull-back draw is also a very unusual type, and will always be so, for the reason that the first cost is great and its operation is expensive. This type may be divided into two classes: first, structures with one span over the entire opening; and, second, structures with two spans over the entire opening, meeting at mid-channel, as in the case of the double, rotating, cantilever draw. The first class requires a truss-bridge nearly, if not quite, twice as long as the width of channel between pier centres, the bottom chords thereof running on two groups of rollers that travel just half as fast as the bridge when the span is moved longitudinally. Although the shore arm may be made shorter than the channel arm, still its weight must be such that its moment will be somewhat greater than the tipping moment of the weight of the channel arm just as it leaves the farther pier. A disappearing platform will be required so as to leave space on the approach for the shore arm to move back, or else the whole bridge will have to be rotated slightly about a horizontal axis so that it can roll up onto the approach. Either method is very clumsy, and the operation of the bridge consequently must be slow.

The double pull-back draw is similar to the single pull-back draw just described, except that the far end of each span has to be anchored down to a mass of masonry when the bridge is closed and ready for traffic, and the ends meeting at mid-channel must be locked together as in the case of the double, rotating, cantilever draw.