because the wind relative to the wave acts like an opposing wind. 

 When (T > V, the vertical particle velocity and the normal wind force 

 component are out of phase by a difference of tt, and energy is given 

 off from the wave to the air. In this case the wave motion is slowed 



down. 



There will be no objection to considering the action of this 

 normal pressure component r^^, and H. Jeffreys [l8] took it into ac- 

 count as the main source of wave energy. He assumed the effective 

 pressure component to be proportional to the product of the density 

 of the air p',the square of wind velocity relative to the wave 

 velocity (v - O" ) , and the slope of the wave profile c)7/dx. These 

 assTomptions are the most plausible which may be made with regard to 

 the action of normal wind force components as considered here, but 

 some uncertainties still exist when going into details. For example, 

 there is the question of the accurate definition of the difference 



v^ - ar i where v^ is the wind velocity immediately at the sea sur- 

 o ' o 



face, and questions about the so called "sheltering coefficient" 

 imder different conditions (wave form). 



However, it seems that there are more difficulties encountered, 

 and opinions differ when the action of tangential wind stress compon- 

 ents is considered. If we assume that the wind blows over a general 

 wave profile which may be considered really as an ideal "smooth" sur- 

 face in the hydrodynamic meaning of this word, then only viscosity 

 stresses would act. The work done by these viscosity stresses would 

 probably be small compared with the effect of normal pressure com- 

 ponents. This perhaps may be the reason that Jeffreys did not take 

 into account a transfer of energy by tangential stress and considered 



51 



