Transportation on Land 



257 



Other modern roads are built of concrete, asphalt or brick. 

 The concrete, or cement, road is made of a mixture of broken 

 stone and sand, bound together by a stone powder, called cement. 

 When these materials are properly moistened and laid they set and 

 form a solid rock-like pavement that will support very heavy loads. 

 Asphalt roads are constructed by spreading layers of asphalt over 

 a concrete base. This material is usually made firm and smooth by 

 steam rollers. Modern brick pavements consist of bricks laid on 

 edge on a sand cushion over a concrete base. Wood blocks are 

 sometimes used in place of bricks because they deaden the noise of 

 traffic. 



Bridges. In like manner, science has made possible great 

 steps forward in bridging streams on the line of roads or trails. 



Brown Brothers. 



THE GEORGE WASHINGTON BRIDGE 



A great modern suspension bridge across the Hudson at New York City. 



The first bridges were made of single logs, or were woven out 

 of heavy vine growth. Later, crude log structures, roughly fas- 

 tened together, were thrown across small streams. When the 

 science of mathematics showed how to compute the strains in 

 beams, so that bridges could be built to stand the weights they 

 were supposed to carry, it was not long before simple bridges of 

 framed timbers were developed. 



By degrees wood gave way to iron, capable of supporting 

 heavier weights and covering greater spans. When iron gave way 

 to steel, spans hundreds of feet in length could be built, or steel 

 wire cables could be constructed to hold up the enormous weights 

 of the modern suspension bridges. All these developments were 



