364 Introduction to the Study of Science 



and on most types of motor boats. The propeller is much like 

 a ventilating fan, the blades sloping forward so that as they are 

 turned they cut the water in front and force it backward, away 

 from them. This causes the ship to move in a direction oppo- 

 site to that in which the water is forced. In the wake of a 

 screw there is a stream of water opposite to the direction of the 

 ship. If a ship reverses its propellers and moves backward, 

 which way is the water forced ? A ship may be equipped with 

 two propellers, " twin screws," or four, " quadruple screws," 

 as are several great liners. The reason for the increase in num- 

 ber of screws will appear later. 



The steam engine. The engines of the great ocean liners 

 are marvels of mechanical perfection. For more than thirty 

 years triple and quadruple expansion engines have been in use 

 and have been improved in many important ways. The steam 

 is superheated and utilized at high pressures, and much of the 

 energy is taken from it in its course from and to the boilers. 

 Economies in fuel are most important. Liners of an older 

 type consume about 3000 tons of coal every twenty-four hours ; 

 while one of the latest and largest ships afloat gets greater 

 power and a higher rate of speed from the consumption of only 

 1000 tons in the same period of time. This indicates the prog- 

 ress achieved in increasing the efficiency of engines hitherto 

 considered standards of perfection. 



The steam turbine. In the age of Archimedes, Hero in- 

 vented a steam engine, in which a principle different from that 

 of the reciprocating engine was involved. It was made in the 

 form of a turbine and driven by the reaction of the escaping 

 steam (Fig. 129), much as a certain kind of lawn sprinkler is 

 driven by the escaping water. The steam turbine designed by , 

 Sir Charles Parsons and demonstrated publicly in 1897 is a 

 great advance beyond the simple turbine of Hero. Parsons' 

 Turbinia, a ship built to test his turbines, developed a speed 

 or 34 or 35 knots, equivalent to 40 miles per hour. The highest 

 speed previously attained was about 24 knots, Recently 



