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TRANSPORTATION 



placement of 7,063,000 cubic feet and a total lift of 

 472,940 pounds. The lift capacity of this balloon equals 

 the weight of a steam locomotive or five loaded 

 freight cars. 



All balloons depend upon the principle of buoyancy. 

 In the preceding topic you learned that water trans- 

 portation was made possible by the principle of buoy- 



1 nternational News Photos 



FIG. 518. THE Hindenburg 



ancy. That study taught you that whenever a body 

 is placed in a liquid it is pushed up by a force which 

 is equal to the weight of the liquid displaced by it. If 

 the body weighs more than the liquid it displaces, it 

 sinks, while if the body weighs the same as the liquid 

 it displaces, it floats. 



In the case of dirigible balloons the same principle 

 holds true. Balloons and dirigibles are known as 

 lighter-than-air craft ; that is, they are able to float in 



the air because the air displaced weighs as much as 

 the balloon and its load. 



Dirigible balloons are very heavy and must there- 

 fore be built large to displace sufficient air to weigh 

 as much as the ship and its load. Water weighs 62.4 

 pounds per cubic foot, while air weighs only about 

 1^4 ounces per cubic foot. The gas capacity of some 

 of the modern dirigibles will give a comparison of 

 their sizes: 



The Graf Zeppelin 3,708,000 cubic feet 



The Hindenburg 7,063,000 cubic feet 



In the early experiments with balloons they were 

 inflated with warm air or the gases from smoldering 

 straw. These gases served their purpose until they 

 cooled ; then the balloon would no longer have buoy- 

 ancy enough to hold'it in the air. As early as 1783 

 Professor Charles filled a balloon with hydrogen, the 

 lightest gas known. This balloon soared over Paris 

 and traveled a distance of nearly twenty miles. Since 

 this early attempt hydrogen has been used extensively 

 in balloons and dirigibles, but is being replaced, es- 

 pecially in the United States, by helium, which, 

 though heavier, is not explosive. Many dirigibles have 

 met disaster through the explosion of the hydrogen 

 gas. Helium is found issuing from the ground with 

 other natural gases in certain wells in Oklahoma and 

 Texas. It is obtained from this gas by refining. Fig- 

 ure 519 shows a section of a dirigible illustrating how 

 the gas is contained inside the outer fabric. 



What is the history of heavier-than-air devices? 

 The foundations for the first power flight by a heavier- 

 than-air craft in 1903 were made between 1887 and 

 1903. It is interesting to note that Sir Hiram Maxim 

 in England and Professor S. P. Langley in America 

 were both at work between 1890 and 1900 on pre- 

 liminary experiments dealing with air flight. Both 

 built planes which, although not practicable for flight, 

 did much to further the progress of this infant science. 

 In 1896 Otto Lilienthal, a German, was killed while 

 experimenting with a glider in Germany. The account 

 of this accident in the newspapers came to the notice 

 of Orville and Wilbur Wright, then living in Day- 

 ton, Ohio. They became interested in air flight and 



Courtesy Goodyear Tire and Rubber Company 



FIG. 519. CROSS SECTION OF A MODERN DIRIGIBLE 



