70 GREAT ACHIEVEMENTS IN BRIDGE BUILDING 
America. The principle of the modern cantilever was 
known and applied to bridge construction by the inhabitants 
of India and by our own North American Indians. 
Any survey of the progress of bridge building must of 
necessity fellow closely the devclopmert of the materials of 
engineering construction. As the possibilities of one 
material for bridge construction were exhausted, furtlier 
development was contingent upon the employment of a 
stronger, more durable or more aesthetically — correct 
material. The four principal materials have been wood, 
stone, metal (iron or steel), and concrete (plain or rein- 
forced), and as each one has been applied to bridge con- 
struction, new types of structure have been evolved in 
order that the new material might be used to the greatest 
advantage The achievements of the bridge builder cannot 
_ therefore be judged as a whole, a wooden structure against 
a stone, or a steel or a concrete one, for example, for com- 
parisons are possible only between structures of the same 
material. 
In the realm of wooden construction, bridges remark- 
able for their boldness and the originality displayed in their 
design, have been built from prehistoric times down to the 
present, or, perhaps, till half a century ago. Caesar’s 
bridge, built over the Rhine at Bonn in 55 B.C. ; Trajan’s 
remarkable arch bridge over the Danube, built in 104 A.D., 
containing a score of wooden arch spans of possibly 170 feet 
in length, were both achievements of magnitude in their 
time. Probably the high water mark of boldness in wooden 
_ bridge construction was reached in 1760 when two village 
carpenters, the Grubenmann brothers, built over the Lim- 
mat River at Baden a bridge containing a span of 366 feet 
in the clear, never equalled before or since in this material. 
It did not stand very long, however. but was destroyed by 
Napoleon in the campaign of 1799. 
Stone, from its permanency, strength and beauty as a 
material of construction, was impressed into the service of 
