114 THE OCEAN 



density superimposed, whether the difference 

 of density be due to saUnity or temperature, 

 we may have slow travelling waves occurring 

 at the surface of separation. Observations 

 in the ocean show the existence of such 

 progressive waves, but again there is the 

 difficulty that we must have observations 

 taken continuously for a lengthy period in the 

 same position before we can be sure of the 

 nature of these waves. Their period also 

 depends on the difference of density of the 

 liquids above and below the surface of separa- 

 tion, and the smaller this difference the slower 

 will be the velocity of the waves, and for a 

 given disturbing force the greater the ampli- 

 tude. These waves may be caused by cur- 

 rents in either the bottom or the surface 

 waters, just as waves on the surface of the sea 

 are caused by wind currents in the atmos- 

 phere. 



Dead Water. — ^When a layer of compara- 

 tively fresh water several feet in thickness 

 rests on a salt layer, a passing ship gives rise 

 to a boundary-wave, such as above referred 

 to, which may retard or even stop the ship, 

 so that it lies in what is called " dead water." 



