Transportation Water and Air 287 



The first practical steamship was built in 1807. The early ones 

 were driven by paddle wheels, the later ones by propellers, or metal 

 blades, whose rapid revolution drove the ship forward by water 

 pressure. 



Some immense ships today are driven by steam turbines, in 

 which the generated steam expands against movable and fixed 

 blades in a casing surrounding the propeller shaft. 



The invention of iron and of steel ships led to the building of 

 great and strong vessels. Such ships float because their weight is 

 less than the weight of an equal volume of water. 



The stability of ships, or their ability to keep from overturn- 

 ing, is secured by width, or beam, by fixed ballast, by weighting 

 the keel, by air chambers, or by centerboards. 



The safety of ships has been greatly increased by strength of 

 structure, by modern instruments of navigation, by charts of the 

 coast lines and harbors, by lighthouses and buoys, and by wireless 

 communication. 



Early air navigation was by balloons, or bags of cloth filled 

 with hot air. These rose because the heated air was lighter 

 than the outside air. Later, coal gas and hydrogen gas were 

 used but were found dangerous because they were very in- 

 flammable. Today in the great airships non-inflammable helium 

 gas is coming into use. 



Balloons can go higher by dropping out ballast, or lower by 

 releasing gas. They cannot be driven or guided. 



The dirigible developed from the balloon. It is a long, pointed 

 balloon, often built around a light metal frame. Motors and 

 cabins are attached to the frame and suspended beneath the bal- 

 loon. Dirigibles may be driven at high speeds and guided on a 

 course. 



Airplanes are the result of years of effort to enable man t6 

 fly. They followed long studies of flights of kites and birds. 

 Langley's model a miniature machine was the first plane to fly 

 under power. The Wright brothers made the first practical air- 

 plane. 



Modern airplanes are run by gas motors having six or more 

 cylinders. Some planes carry several motors. 



