Chap. XXV.

ON STONE BRIDGES.

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manner; and all abrupt changes from one form to another should be guarded against, asthey produce an unsightly and disagreeable effect.

Staircase of the same Construction, on a circular Plan, may be executed either with

Fig. 2096.

Fig. 2698.

or without a string at the outer end of the steps: the forms of the steps and theirconstruction are sufficiently indicated by the figures which accompany.

CHAP. XXV.

ON STONE BRIDGES.

Having already described several bridges, we shall now proceed to give some account ofthe theory and practice by which the engineer has been enabled to erect those useful, and,in some instances, magnificent structures, which rank among the greatest efforts of hisscience and skill.

On the Situation of Bridges. — This must depend on local circumstances, as the leadingstreets in a city, or the direction of the public roads; but wherever possible, it shouldbe selected at the point where the width of the river is contracted below, and continuesin a straight course for a considerable distance below, and its position should be at rightangles with the river. Skew bridges, on account of the difficulties attending their con-struction, should be avoided as much as possible.

5 b 3