After the conditions already mentioned are satisfied, taste has more to do with the form of the arches than anything else. The forms in use are the old semicircular, the elliptical—usually got at by putting together several circular arches of different radii— and the segmental arch. The semicircular arch was almost exclusively used in the more ancient bridges. This arch is the most solid and most easily constructed. as all the voussoirs may be worked from the same mold. It requires, however, high banking, as its height is equal half its breadth; and where the water-level greatly changes, it is par ticularly unsuitable, from the great height necessary to be given to the piers, to carry the intrados out of water-reach. The elliptical arch and the segmental of GO° are, besides, far more pleasing in appearance.
In possible extent of span, the masonic bridge is far exceeded by suspension and girder bridges. At Chester there is a stone arch with a span of 200 ft.; in the Britannia tubular bridge the span is 460 ft.; in the suspension bridge over the Menai strait, 600 ft.; and in the suspension bridge at Freiburg, Switzerland, 870 feet. The railway bridge across the Tay, near Dundee, of iron girders is remarkable for its great length, 3450 yards. See TUBULAR BRIDGE and SUSPENSION BRIDGES.
The principal objection to the wooden B. is its liability to decay, besides which it is liable to warping, through the swelling and contracting of its beams. The latter objec tion applies also to iron bridges, but in their case, the contractions and expansions may be compensated for, as in the compensation balance of a watch, or the compensation pendulum.
Public bridges are maintainable at the expense of the counties in which they are sit uated; but in many cities and boroughs, the inhabitants have acquired by prescription a liability for this expense, and by the 13 and 14 Viet. c. 64, the management and control of
such bridges is given to the council of the city or borough. If part of a public bridge he within one county or other place on which the liabilty rests, and the other part of the bridge be within another, each party or .bed,y shall repair that part of the bridge which is within its own boundaries. Besides the bridge itself, the county liable is bound by the 22 Henry VIII. c. 5, to repair 300 ft. of the road either way from the bridge. And such is still the state of the law as to all bridges built prior to the passing of the highway act, 5 and 6 Will. IV. c. 50. But by that act it is provided that, in the case of all bridges thereafter to be built, the repair of the road itself passing over or adjoining to a bridge, shall be done by the parish, or other parties bound to the general repair of the highway of which it forms a portion—the county being still subject, however, to its former obli gation as regards "the walls. banks, or fences of the raised causeways, and raised approaches to any bridge, or the land arches thereof." See Stephen's Cam., vol. iii. p. 234. The neglect to make such repairs is treated in la* books as a kind of negative offense; but there are positive offenses against bridges, which in the statutes are called nuisances, as to which, see the 43 Geo. III. c. 59. An act to amend the law in regard to the maintenance and management of roads and bridges in Scotland was passed in 1S78, entitled " Roads and Bridges act." Private bridges are those erected and main tained under contracts authorized by private acts of parliament. Sec ROAD.