16 The Sea 
the bay vibrates in three separate parts every 43 minutes. 
A SHIP’S WAKE 
In many respects a ship is like a floating wedge which, in 
order to move, must force itself through the water by the 
work of its screws or the labor of its sails. It takes energy to 
push a ship through the ocean water. Part of this energy goes 
into frictional losses where the water layers slip past the hull; 
x 
TURBULENT WATER /2@_ 
f~ Coo 
STERN WAVES 
ig 
7 Ore 7 LE 
BOW WAVES 
SHIPS WAKE 
more is used up in overcoming the pressure of water that is 
piled up at the stem by the motion of the ship; a good pro- 
portion is also lost in the waves the ship produces, the wake. 
The waves that make up the wake are of two kinds: (1) 
bow waves, extending from the bow of the ship aft, slightly 
concave forward, and (2) stern waves, which are directly 
behind the ship, slightly convex forward, and which move 
