Transportation on Land 



285 



steel. The tubular form makes the machine lighter and, with 

 the triangular arrangement of its tubes, stronger than a solid 

 steel frame of the same weight. The bicycle frame is really 

 a bridge, with the two axles as its supports. The same sort 

 of form is sometimes used in bridges. The tubular form gives 



FIG. 87. Cantilever bridge. Tubular members. Length of two main spans 

 is 1710 feet each. Identify the compression members ; the tension members. 



strength and lightness, an important factor where the spans 

 are long. An example of such is the bridge across the Menai 

 Straits for the London and Northwestern Railway. Tubular 

 framework is combined with different types of bridge construc- 

 tion, such as the cantilever (Fig. 87) and the arch. 



122. The suspension bridge. Another type of bridge is 

 the suspension. It is susperided very much as the hammock 

 from two supports. Tall, massive towers furnish support 

 for heavy wire cables. The ends of the cables are held fast 



