Transportation in the Air 401 



tion of a machine that will be perfectly self -balancing, or " auto- 

 stable " as it is described, and which will insure safety to life 

 and to the machine, will contribute greatly to its adaptation to 

 modern transportation. 



SUMMARY 



The two chief types of aircraft are the heavier than air type, and 

 the lighter than air. 



Drifting balloons are of use in determining the conditions of the 

 upper regions of the atmosphere. 



A balloon rises because it displaces a weight of air greater than its 

 own weight. 



The need of more complete control of the balloon in flight led to 

 the invention of the dirigible. 



The development of the heavier than air machines is the result of 

 the labors of many experimenters. The pioneers include Lilienthal, 

 Maxim, Langley, the Wright brothers, and Curtiss. 



The principle of the airplane is the same as that of the kite. 



Stability is secured by the design and construction of the machine, 

 by the action of the pilot, and sometimes by automatic devices. 



The success of the airplane depended chiefly upon the develop- 

 ment of a light and efficient gasoline motor. 



REVIEW QUESTIONS 



I. (a) What are the two chief types of aircraft? (6) How is each 

 sustained? 2. (a) What kind of balloon first carried passengers? 

 (6) For what are hot-air balloons now used? 3. Compare hydrogen 

 and Illuminating gas for filling balloons. 4. How high above the 

 earth's surface have men gone? What conditions of temperature and 

 pressure did they find? 5. What difficulties attend ballooning? 

 6. (a) With what instruments are exploring balloons equipped? 

 (6) How is their safe return provided for? (c) To what height have 

 they gone ? (d) What has been learned by their use about the atmos- 

 phere, as to its pressure and temperature? 7. What phenomena 

 are due to the presence of air at an elevation of fifty miles or more? 

 8. (a) Compare the construction of a balloon with that of a ship. 

 (6) Why do they both float? 9. What is meant by average density? 

 10. What is the difference between a drifting balloon and a dirigible 

 balloon? When did the successful use of dirigible balloons begin? 



II. What advance beyond the use of the balloon was made in air 



