288 Our Surroundings 



Airplanes are held aloft by air pressure against the under 

 side of the wings, which are usually concave. The convexity 

 of the upper side tends to shed wind pressure and to cause a 

 partial vacuum above them. Airplanes are guided by vertical rud- 

 ders and are headed up or down by raising or lowering horizontal 

 tail planes. 



Monoplanes have one supporting wing, or plane, biplanes two, 

 and triplanes three. 



Ordinary airplanes have landing gear fitted with wheels. Sea- 

 planes, or hydro-airplanes, carry pontoons, or boat-like floats, 

 which support them on the water. 



Airplanes are rapidly finding a place in the transportation field. 

 There are already a number of regular passenger lines. The 

 United States is rapidly extending a very successful airmail 

 service. 



FACT AND THOUGHT QUESTIONS 



1. Outline briefly the development of water craft from early times until 

 now. 



2. Why does a metal ship float? 



3. Name several advantages of a steamship over sailing vessels. 



4. Define stability. State four ways in which a ship's stability is assured. 



5. Why is the cargo of a ship fastened in place so it cannot shift? 



6. Define (a) propellers, (b) steam turbines. 



7. Suggest some advantages of the propeller over the paddle wheel. 



8. Complete this statement: The invention of the made possible 



sea voyages out of sight of land. 



9. Why is it necessary for a sea captain to take the "position" of his ship 

 at fixed intervals? 



10. Why do ships tend to follow regular highways? What conditions 

 govern the choice of these? 



11. What is a submarine? How is it submerged and raised? 



12. Why is it safer to travel by sea now than it was a century ago? 



13. Why cannot a submarine go far below the surface of the water? 



14. Why does a balloon mount faster when it first leaves the ground than 

 when it is some distance up? When does it stop rising? 



15. What is a dirigible? What advantages has it over the balloon? 



16. What gases are used in balloons and dirigibles? 



17. What is the safest and best gas thus far used in airships, and why? 



18. State how airplanes are lifted, guided and propelled. 



19. Can an airplane fly directly into the air? Why? 



