72 GREAT ACHIEVEMENTS IN BRIDGE BUILDING 
George Stephenson, to devise and carry to completion the 
two colossal tubular girder bridges at the Britannia Rock 
and over the Conway River. This movement was a retro- 
grade one, as was shown later. Few tubular bridges were 
ever built because of their enormous expense and suscepti- 
bility to the corrosive effect of locomotive gases. The 
largest one ever built, the Victoria Tubular Bridge at Mon 
treal, was replaced eleven years ago by a medern truss 
bridge. 
In America, Stephenson’s difficulty had been met and 
largely overcome by the able and versatile John A. 
Roebling, who, in 1855, successfully completed a suspen- 
sion bridge of 821 feet span over the Niagara gorge for the 
heaviest existing railway traffic. Subsequent development 
showed that except for the longest spans and moderate 
loadings, the simple truss or the cantilever span is to be 
preferred. Following Roebling’s achievement at Niagara 
came his Cincinnatti Suspension Bridge of 1,057 span, and 
the great Brooklyn Bridge at New York. This magnificent 
structure of 1,595% feet span, has been surpassed but little 
in the twenty-six years since its completion. Its carrying 
capacity has been exceeded by bridges of more recent date 
built over the East River and elsewhere, but only two 
bridges in the world contain greater spans—the Forth Canti- 
lever Bridge and the Williamsburg Suspension Bridge at 
New York. The latter has a central span of 1,600 feet, 
the longest suspension span in existence. . 
For spans of great length carrying the heaviest railway 
traffic, rigidity demands a stiffer structure than the suspen- 
sion bridge, and the massive cantilever of the present day 
is the result. The great Forth Bridgeisof thistype. Two 
of its spans, each 1,710 feet long, hold the world’s record 
for length. A few years before its construction the first 
Tay Bridge had collapsed with great loss of life, and the 
engineers of the Forth Bridge, Sir John Fowler and Sir 
Benjamin Baker, exercised especial care and vigilance in 
