Transportation Water and Air 273 



invented and now has taken the place of paddle wheels on vessels 

 of large size. In screw propelled boats, a great metal shaft 

 extends from the engine through the back, or stern, of the boat 

 below the water level. To this shaft, on the outside of the boat 

 in the water, the screw is attached. It consists of two or more 

 metal blades somewhat resembling the blades of an electric fan. 

 When the shaft is revolving, the screw blades, set at an angle, 

 drive the ship forward by their pressure against the water. 



Vessels now use oil as well as coal for fuel, to generate 

 steam and sometimes electricity with which to drive the pro- 

 pellers. Very small boats or launches often use gasoline or electric 

 motors. Many of the newest and finest types of steamships are 

 driven by steam turbines. In the steam turbine, series of blades 

 are connected with the propeller shaft within the vessel's hull 

 and, of course, revolve with it. These blades alternate with fixed 

 blades on a casing that surrounds the shaft. Both sets of blades 

 increase in size from the point nearest the boiler to the end of 

 the turbine. Steam is admitted at the -small end and, by its pres- 

 sure on the curved blades of the shaft, tends to revolve it and 

 so turns the propeller. The steam is then deflected to the station- 

 ary blades of the casing and is again deflected to the next series 

 of shaft blades and so on. As the steam gradually expands, it 

 is given a larger and larger surface upon which to exert 

 pressure. 



Steam turbines develop tremendous power and very high speed 

 over long continued periods. Thus far, turbines have not been 

 invented that will reverse direction. Consequently, great ships 

 using the turbine system usually carry several turbines for forward 

 motion and other turbines for reversing direction. 



The development of the iron and the steel industries led to the 

 making of vessels from these metals. Such ships are larger, 

 stronger, and are capable of carrying much greater cargoes than 

 wooden ships. 



The question naturally arises, "Why does a metal ship float?" 

 Any object will float if it weighs less than an equal volume of 

 water. A cubic foot of water weighs 62 l / 2 pounds. A cubic 

 foot of wood weighing 50 pounds will float, because it is not 



