Stewart. — Address. 3 



Sixty years ago the design of bridges adhered, with few 

 exceptions, to the arch or suspension type. But stone or 

 brick arches were going out, and designs in cast or wrought 

 iron were coming in. The suspension type had been tried for 

 railway-work and found unsuitable without such application 

 of stiffening as led it practically to partake quite as much of 

 the girder type as of suspension. The disastrous breakdown 

 of the Dee Bridge, near Chester, in which a deep cast-iron 

 girder was reinforced in a rather unscientific manner by 

 malleable-iron ties, led to the abandonment of cast-iron for 

 all but very small spans, and even for such it has long dis- 

 appeared. With the last of the " forties " came the tubular 

 bridges of Conway and Britannia ; but with the succeeding 

 great Victoria Bridge over the St. Lawrence at Montreal this 

 design may be said to have been abandoned. The design is 

 not economical, really very much the reverse, but the mathe- 

 matical investigations necessary to its evolution bore imme- 

 diate fruit in the inception and development of the open-girder 

 pattern, which in some of its many modifications has become 

 the standard of all long and short span bridges the design of 

 which is not governed by aesthetic traditions. It will be 

 readily granted that it is the size of the span, and not the 

 mere length of the structure, that stamps the importance of a 

 bridge, and the advance of engineering in this particular in 

 fifty years cannot be more forcibly illustrated than by the 

 comparison put forth by Sir Benjamin Baker — that is to 

 say, the span of the Britannia Bridge, 460 ft., is to that of 

 the Forth Bridge, 1,710 ft., as a newly born babe is to a life- 

 guardsman. The bridge now in progress over the St. Lawrence 

 at Quebec is to have a span 90 ft. longer than that of the Forth, 

 and the designs of the proposed great suspension-bridge over 

 the Hudson, between New York and Hoboken, show a span 

 of 2,700 ft., or 60 ft. more than half a mile. 



In direct contrast to bridges are tunnels, and in this line 

 an enormous advance has been made, not only in the magni- 

 tude of the works, but in the facility and certainty w T ith which 

 operations can be carried out under all circumstances, even to 

 driving under the Thames at Blackwall with only a few feet 

 of mud between the water and the lining of the tunnel. 

 Driving railway-tunnels for miles under cities like London or 

 Glasgow is now such an every-day occurrence as to call for 

 no remark. During the last half-century the Mont Cenis 

 Tunnel, seven miles and a third, and that of the St. Gothard, 

 nine miles and a quarter, have been constructed, and at the 

 present time the Simplon is being pierced by twin tunnels of 

 twelve miles and a half in length. 



Turning to railways, the principal departure seems to be 

 in the direction of application to steep and mountainous 



