I30 A TEXTBOOK OF OCEANOGRAPHY 



Seafaring men generally say that in stormy weather the fourth 

 or fifth wave is the highest, and that this wave is foUowed by 

 one or two of less height, which gives a favourable opportunity 

 of putting the ship about should that be necessary. 



When waves are first formed by the wind they are short 

 and steep, but if the wind continue to blow in the same 

 direction for some time their length and height increase, but 

 the periodicity decreases until a balance of forces is produced. 

 In calm weather the first hint of a breeze is given by the 

 darkening of the sea surface. The growth of waves from the 

 smallest capillary waves to the highest waves has been treated 

 mathematically by Airy* and others. When waves hâve once 

 been formed the wind has greatest effect on their crest, which 

 it tends to drive faster than the main body of the wave, and so 

 causes the wave to break. In deep water waves hâve no 

 motion of translation, but on reaching shallow water their 

 troughs are retarded, so that they break and rush forward with 

 considérable force; thèse are the " breakers " of the coast. 



The growth of waves is hindered by foreign bodies in the 

 water, such as mud or sand in suspension, ice, seaweed, oil, 

 heavy rain, bail, or sleet. Particles of ice, or sand, or mud in 

 suspension increase the viscosity and so hinder wave-forma- 

 tion. The gigantic seaweeds of high southern latitudes 

 {Macrocystis pyrifera), which flourish on rocky coasts, exer- 

 cise a remarkable effect in stilling the waves, so much so that 

 in Kingston (South Australia) an open bay has been made a 

 safe anchorage. 



■* " Waves and Tides." 



