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OF CYLINDRICAL VAULTS,

The most extraordinary cylindrical vaults will befound in bridges. Augustus Csesar built one # of mar-ble over the river Nera at Narni , which is now partlydestroyed; it joined two hills on the road from Rome to Loretto, and consisted of four arches of greatwidth; it was remarkable for its height and theexcellence of the workmanship, it afforded a levelroad from the top of one hill to the other. Of the modeof the construction of the wonderful t bridge builtover the Danube by Trajan , and described by Dion Cassius , some doubt has arisen, from an observationof Montfaucon, from the supposed representation ofit on the Trajan Column, by which it would appearto have been of wood. Gibbon, from the authorityof Reimar ad Dion from Marsigli , vol. v. p. 273,is of that opinion. On a medal struck by Constantine(Oisel Num. Sel. xxiv. 4.) the bridge is represented ofstone. Gauthey says the nascent parts of the archstill exist on some of the piers.

The stone bridge t over the Tagus at Alcantara ,built by that Emperor , and that of Augustus atNarni , in some measure confirm the account of Dion

* The chord of the great arch of this bridge, which was con-structed without cement, was 139 feet. See Martinelli, 1676,Descr. de ponti.

t The chords of the arches of the bridge built over the Danube by Trajan were 180 feet each ; the height of the bridge 187 feet.

X The chord of the middle arch of the stone bridge built byTrajan over the Tagus at Alcantara is 101 feet, and the height ofthe bridge is 143 feet. See Voy. Pit. De L’Espagne.

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