Transportation on Land 289 



The arch bridge, represented by the steel arch over the East 

 River at Hell Gate, New York, is built of materials that resist crush- 

 ing forces. 



The cantilever is like the truss, supported not at the ends but at 

 the middle. 



REVIEW QUESTIONS 



1. Why do we need bridges? 2. Describe primitive types of 

 bridges. 3. What is meant by (a) crushing force? (6) tensile force? 

 (c) crushing strength? (d) tensile strength? 4. Show how a simple 

 girder supports a load. 5. What advantage has the truss over the 

 girder? 6. Why is the frame of a bicycle made of steel tubes rather 

 than of steel rods? 7. (a) Mention an instance of the use of a sus- 

 pension bridge. (6) How is the weight supported? 8. (a) What 

 materials are usually employed in such construction? (6) Describe 

 the railway bridge across the Menai Straits. 



9. (a) How does the principle of the arch bridge differ from that of 

 the suspension bridge? (6) What is the chief feature of the arch 

 bridge? (c) Of what materials is it usually built? 10. (a) What is 

 the chief feature of a cantilever bridge ? (6) In what other uses is 

 the same principle employed? 11. Indicate the compression and 

 tension members of the arch bridge, Fig. 89. 



SUGGESTIVE QUESTIONS 



1. When you try to split wood across the grain, what sort of strength 

 or resistance do you encounter? 2. Why have some kinds of wood 

 better crushing resistance than others ? Which kind is better for bridge 

 structures? 3. (a) What types of bridges depend almost entirely 

 upon the crushing strength of materials? (6) What types depend 

 mostly upon the tensile strength of materials? (c) Will a bridge that 

 depends upon both kinds of strength be better than one that depends 

 chiefly upon one kind? Explain. 



4. Indicate the compression and tension members of (a) a simple 

 girder bridge, (6) a truss bridge, (c) a suspension bridge, (d) an arch 

 bridge, (e) a cantilever bridge. 



5. Give instances where the same principles of compression and 

 tension members are used, as in the frame of a bicycle, of an auto- 

 mobile, and of a house. 



VI. STEAM AND THE LOCOMOTIVE 



124. Early locomotives. The story of the application of 

 steam to locomotives for land transportation is full of romance. 



