158 ,THE WATERS OF THE EARTH 



but does not move forward. If the water moved along 

 with the waves, it would be next to impossible to propel a 

 boat against the direction of the wave movement. 



That it is possible to generate wave movement without 

 the particles themselves moving along with the wave is 

 seen when a field of grain is bending before a gentle wind. 

 The troughs and crests move one after the other across the 



OCEAN WAVES 



field but the heads of grain simply vibrate back and forth. 

 The crest of a water wave, however, is often blown forward 

 by the wind and thus a drift in the direction of the wind is 

 established at the surface. 



When great waves are raised by the wind at sea, there 

 is danger that the mighty crests may be blown forward 

 and engulf a ship. To calm the waves ships sometimes 

 pour " oil on the troubled waters." The oil spreads out 

 in a thin film over the water and forms a " slick " which 



