rebuilding London Bridge. 33 



tects very wisely abandoned round-headed arches, and adopted 

 pointed arches. Their arches surmounted, were composed of two 

 arcs of a circle, their arches depressed, were composed of two 

 arcs of an ellipse ; the latter kind did not prevail until the time of 

 Henry VII. It is manifest, that these arches greatly increase the 

 summering of the vertex stone. Ammanati, at the bridge of the 

 Holy Trinity at Florence, to his great honour, living at the revival 

 of Roman architecture, adopted a pointed arch of Henry the Vllth's 

 time, with that style of architecture. 



Having derived that the thickness of the arch at the vertex being 

 of an uniform thickness directly from the strength of the material, 

 and the weight to be borne, this result leads to the consideration 

 of another practice of the Gothic architects, not less wise than that 

 just mentioned, by which the great cost, and the great strain and 

 compression, and consequent variation of the form, of the wooden 

 centring is materially reduced ; and the abutments are subjected 

 to less thrust and weight, viz., by using ribs, or what in Roman 

 architecture are called double arches. By this practice, the wooden 

 centring became a mere skeleton, or in the mason's phrase " a 

 horse," on which they constructed another skeleton, or the sub- 

 arches, which they then keyed, and on the stone skeleton as a cen- 

 tring, they then laid the arched ceiling or floor, adopting the same 

 course, as from necessity modern bridge builders do in erecting 

 iron and wooden bridges. The Gothic architects also, in order to 

 save the walling and stuffing in the spandrills, and to bring up the 

 arch to the road, and consequently to reduce weight and expense, 

 sometimes pierced the bearing piers by arches longitudinally, to 

 which the cut-waters became abutments, and took their share of 

 pressure. Perronet imitated this arrangement at the bridge of 

 St. Maxence. Others have pierced the bearing piers transversely, 

 with ceils. Perronet gave grace to the elevation of the bridge of 

 Neuilli by splaying the angles of his ellipses to an arc of a circle, 

 by what have obtained the name of Coi nes de Vache. 



It would be in vain, in the present state of the science and art of 

 bridge building, to urge, the adoption of such of these excellent 

 inventions, as apply in the case, and also to advocate a due 



Vol. XVI. D 



