ABSTRACT OF PROCEEDINGS. xliii. 



duction of Siemens-Martin steel ; afterwards Thomas suc- 

 ceeded in producing basic steel. Thus the art of the bridge 

 builder was aided by the scientific and practical work of 

 the metallurgist and engineer ; although for many thousands 

 of years iron was only obtainable immediately from the 

 ore, afterwards the metal produced on the hearth took the 

 lead for 400 years. Compared with this the eighty years 

 during which wrought iron ruled supreme, and the short 

 period since it was superseded in its turn, lapse into 

 insignificance. Who will assert how long mild steel in its 

 present form will keep its place? Already aluminium and 

 nickel are being added to it for different purposes, and no 

 doubt further surprises await us in the course of the present 

 century. The lecturer then dealt with the history of 

 girder systems and the theory of bridges, illustrating his 

 remarks by numerous lantern slides of well known bridges, 

 of the plate web, lattice and bowstring girder type, as well 

 as truss-bridges constructed in various parts of the world. 

 The art of the bridge builder in its earlier stages depended 

 more upon judgment than exact theory, and it was not until 

 the investigations of such men as Olapeyron, Castigliano, 

 Mohr and De Saint Venant had been studied that both in its 

 theoretical and practical aspect the art became most pre- 

 cise and exact in every particular. Robert Stephenson, 

 however, actually built the Menai Bridge eight years 

 before De Saint Venant published his theory. Dealing 

 with arches and suspension bridges, the lecturer stated : 

 "that the arch is more economical than the most economical 

 truss, where the site and local conditions are favourable 

 for the construction of suitable abutments at moderate 

 cost." This fact was illustrated by comparing the arch 

 with the bowstring girder. The suspension bridge is the 

 inverted form of the arch bridge, in which the tension in 

 the suspended rib or cables correspond with the compres- 



