Transportation Water and Air 279 



made to rise higher by throwing out sand ballast that is carried 

 in bags for the purpose. To make the balloon descend, gas is 

 released from the bag by means of a valve. In fact, the valve 

 acts automatically if the gas pressure gets too high. In case a 

 rapid descent is necessary, a pull on a cord attached to a ripping 

 panel arranged in the top of the balloon bag will rip the balloon 

 open and allow the gas to escape. 



As a rule, balloons carry trail ropes which touch the ground 

 long before the balloon reaches it, and act as a drag, or brake, 

 upon its motion. 



Except in regulating altitude, the navigator of a balloon has 

 comparatively little control over it. It floats with the wind, con- 

 sequently seeming motionless to the rider, except for its tendency 

 to spin around. Owing to this motion, compasses are practically 

 useless for determining direction. 



The Dirigible. With the invention of the gas motor, experi- 

 ments were quickly undertaken to determine the possibility of 

 building balloons which could be guided in any desired direction, 

 and which could even be driven against the wind. As a result the 

 modern dirigible has been developed. Dirigibles are long, pointed 

 balloons usually carrying their covering on light aluminum frames, 

 well braced to stand the pressure of the wind. The making of 

 frames that are both light and very strong has been the main 

 difficulty in dirigible construction. These covered frames are 

 either themselves filled with gas or contain many separate com- 

 partments filled with gas. This gives them buoyancy, or float- 

 ing power. Beneath the immense gas bag are attached the cabins 

 for the crew and the big motors which supply power. There are 

 often several motors which develop tremendous power. The 

 dirigible airship compares favorably in length and bulk with 

 ocean liners. The United States Navy dirigible Macon was 785 

 feet long. Many striking records have been made by dirigibles, 

 including a trip around the world in August, 1929, by the Graf 

 Zeppelin, a German passenger dirigible. 



The Airplane. Just as man has worked to invent a ship that 

 will float in air, so he has striven for a long time to imitate 

 the birds in flight through the air. Early attempts at airplanes 



