16

OF GROINED VAULTS.

Trans. June, I76O, and which had been adopted inconsequence of the sinking of one of the piers ofWestminster Bridge, is very ingenious, and appearsparticularly applicable, in many cases, where circum-stances require .a number of small arches.

Perronet, in the Bridge of Saint Maxence, on theriver Oise, adopted another mode of reducing the sizeof the piers, which was by supporting his arches oncolumns instead of single piers ; so that there are open-ings longitudinally through the bridge, by which thesize of the supports are reduced by a mass of materialequal to the intercolumniations. The parts of thearches over the intercolumniations are supported byarches something resembling the inverted concaveconoidal vault of King’s College Chapel , Cambridge ,piercing the great arches, but the weight, nevertheless,is spread on the whole surface of what would havebeen the base of the pier. This method may beconsidered as an introduction to groined vaults inthe arches of bridges; a practice which would bemore securely followed by adopting the Gothic man-ner of arcs doubleaux : the theorist will perceive, insuch a practice, a greater approximation to equili-bration than by any other method j and the practicalbuilder will in the precautions necessary to obtainadequate abutments to the arches piercing the greatarches, give to the cutwaters a duty which will equa-lize their pressure to that of the other parts of thepier. Timidity, and the want of a full precedent, ra-ther than any reasonable objection, has preventedthose improvements which Perronet’s plan has a ten-dency to produce. — The Baths of Caracalla and