Transportation on Water 349 



billiard ball, or other perfect sphere ; for the center of gravity 

 of such objects as these does not change its position however 

 much they are rolled. 



III. ROWING A BOAT 



155. Oars and paddles. The first motive power employed 

 by men in propelling boats was their own strength, aided by 

 the current, poles being used in shallow water, and then paddles 

 or oars. Later animals walking along the banks of a stream or 

 waterway dragged the boat'; and eventually the wind was 

 used for voyages far from sight of land. 



The oar in action is much like the paddle. A paddle is held 

 near the middle with one hand and near the end of the handle 

 with the other. The oar is held by an oarlock which allows 

 both hands to work one oar or two. It brings into play a fuller 

 use of additional muscles, as those of the legs. How are racing 

 shells arranged to utilise the leg muscles of the oarsmen? 



Mechanical advantage of oars. . Oars may be pushed or 

 pulled. Fishermen and rowers of the Venetian gondolas fre- 

 quently push upon the oars. How did the ancient Greeks, 

 Phoenicians, and Romans propel their boats? The oar or 

 paddle is a lever. What other uses of the lever have been 

 studied? One end of the oar is broad to secure greater resist- 

 ance in the water. The oarlock is the fulcrum. The distance 

 from the oarlock to the center of the hand is one arm of the 

 lever, and from the fulcrum to the center of the blade the 

 other arm. It was shown on page 258 that the lever, used in the 

 ways there discussed, gives the mechanical advantage of greater 

 force in overcoming resistance. The lever used as an oar 

 gives the advantage of speed because of the greater length of 

 the resistance arm relative to the length of the force arm. 



Why are long oars used on racing shells? What is the ad- 

 vantage of short oars ? 



Action and reaction. Why does the backward pressure of 

 the blade against the water cause the boat to move forward? 



