154 TRANSACTIONS OF THE [mAY 23, 



The latest advance in what some would perfer to call the 

 ''evolution^' of the bridge, is the cantilever system. Though 

 designs were made, and the system strongly urged upon the 

 engineering profession of this country nearly twenty years ago, 

 for spans exceeding the possible limits of the straight girder, 

 yet it was not until Mr. C. C Schneider designed, and the Cen- 

 tral Bridge Works, of Buffalo, constructed, under his super- 

 vision, the Niagara Cantilever Bridge, that the merits of this 

 system for very long spans became fully appreciated. 



The system has spread with great rapidity since the Niagara 

 Cantilever Bridge was completed, the Forth Bridge, now being 

 erected in Scotland on this principle and destined for railway 

 traffic, having the extraordinary span of seventeen hundred feet, 

 a greater length than that of the New York Suspension Bridge. 



A cursory glance at the development of the art of bridge con- 

 struction thus reveals a series of interesting facts : 



First — That masonry arch-bridges, from their simplicity, 

 elegance, permanence, and strength, have in all ages been the 

 most favored forms of construction for highways, and that the 

 use of iron and steel has in no way changed this popular favor, 

 except where the employment of these materials is either favor- 

 able to economy or to the introduction of larger spans. For 

 railway-bridges, other considerations favor also the use of iron 

 and steel over the use of stone. 



Second — The introduction of cast-iron arches, though favor- 

 able to architectural elegance of design, has not resulted in any 

 permanent useful developments. 



Third — Timber bridges have in all ages been regarded as 

 temporary structures Avhicli were to be replaced sooner or later 

 by others more permanent in chai'acter. 



Fourth — The springing up and immense development of the 

 railway systems of the world within the last sixty years, and the 

 concurrent progress made in the metallurgy of iron and steel, 

 have given rise to entirely new problems in bridge building and 

 to a new branch of the engineering profession. 



Fifth — The demands of railway traffic for direct lines which 

 must surmount every obstacle either by tunnels or bridges, the 

 heavyweights transported, and the great speed of trains required. 



