Illustration from 1911 Encyclopædia Britannica, article Bridges.

Original text: (1) Lifting Bridges.—The bridge with its platform is suspended from girders above by chains and counterweights at the four corners (fig. 33 a).

(2) Rolling Bridges.—The girders are longer than the span and the part overhanging the abutment is counter-weighted so that the centre of gravity is over the abutment when the bridge is rolled forward (fig. 33 b).

(4) Swing or Turning Bridges.— ... Three arrangements are found: (a) the axis of rotation is on a pier at the centre of the river and the bridge is equal armed (fig. 33 c), so that two navigation passages are opened simultaneously. (b) The axis of rotation is on one abutment, and the bridge is then usually unequal armed (fig. 33 d), the shorter arm being over the land. (c) In some small bridges the shorter arm is vertical and the bridge turns on a kind of vertical crane post at the abutment (fig. 33 e).

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