The moving power employed is two small steam-engines turning a shaft, on each cud of trhich is a large iron wheel whereon the guide-chains rest. The peripheries of these wheels are cast with sockets fitted to the links of the chains, so that w hen the bridge is put in motion by the steam engines, it is moved in the reverse direction of, and with the same velocity as the wheels. The.ends of the chains have balanee-weights attached to them, which rise or fall as the tension of the chains becomes more or less.
A similar bridge has been established at Portsmouth, and plies between that place and Gosport.
this article we may also mention Portable Bridges, which are easily taken to pieces, and as readily put together again. N. Couplet speaks of a bridge of this kind, 200 feet long, carried by .10 men.
Writers on architecture have bestowed considerable atten tion or the subject of bridge-building, which is justly esteemed as one of the most noble and striking specimens of human art. The earliest of these is Alberti, a native of Florence, who flourished about the middle of the 15th century ; he has given several judicious precepts, which, with little alteration, were afterwards laid down by Palladio, Serlio, and Seamozzi. The best of these rules are likewise by Goldman and Baukhurst, as well as by Hawkesmoor, iu his Ifistu•y of London Bridge.—M. Gautier has written a
large volunie ou loidges, ancient and inodeni. M. lielidor has treated on this subject, in his „Architecture Ilydraulique ; as has M. Parent, in his Evsuis et Reeherelths Matbematiques, iii.—De la Hire, too, has touched on it, in his Traith de ilechanique.—Pcrronet has given the result of his expe rience in a magnificent work, which has acquired him great credit in Franee.—Bosset has given an excellent. treatise on bridge-building, in the Henwires de r.leadelnie.-11egemortes published, in 1771, an account of the bridge built by 'din over the Allier, at Monlins.—In 1760, Mr. Rion published a work entitled, Short Principles fur the _I Irchilecture Bridges ; and Mr. Semple has given some excellent prac tical remarks in his Treatise on Building in 'rater, published in 177G. Other writers on the construction and principles of arches and bridges, are Muller, Labelve, Atwood, Emerson, and Dr. Hutton.
When a bridge is constructed of stone, and arched over, it requires, in the act of building, to be supported upon a mould, called a centre ; the construction of which is shown under the CENTRE.