8

OF CYLINDRICAL VAULTS.

over a river from one mountain to another, havingone arch the chord of which is 600 feet, may exist.The widths of the bearing piers, which supported thisextraordinary production of Perronet, are only 14feet; the lateral pressure being wholly communicatedto the abutments. *

It is impossible here not to call to the recollectionof the enlightened antiquary, that unknown butwonderful arch, above the vaulting and between thetowers of Lincoln Cathedral t; the diameter of thecircle of curvature of this arch at the vertex is 262feet, and the height of the key-stone there 11 inches :and also to that in the church of the Jesuits at Nismesstill more extraordinary, for the diameter of the circleof curvature at the vertex of it is 565 feet, and theheight of the key-stone there 25 inches. These ex-amples suggest that the voussoirs of arches withproper abutments may be reduced, and the chords ofarches increased, beyond what the most skilful andhardy architects have attempted, t

* Since 1812, when these observations were published by theSociety of Antiquaries , Waterloo-bridge, a very magnificent work,has been erected under the direction of Mr. Rennie; the arches areellipses 120 feet transverse, and 32 feet semi-conjugate; conse-quently, the diameter of the circle of curvature at the vertex is 225feet. The bearing piers are 20 feet wide; the bridge is built ofgranite : the height of the key-stone of each arch is 60 inches.

f The horizontal extrados of the arch between the towers,of Lincoln Cathedral is 28 feet in length, 11 inches thick in themiddle, 20 inches thick at one extremity, and 28 inches at theother. The extrados is about 15 inches wide; it resembles awooden beam rather than a stone arch composed of voussoirs.

J Perronet, in reasoning on the due height of a key-stone at thevertex of an arch, from which the ratio of the other parts and thepiers are determined, took into account the ability of the stone to