over the "surface" (the wave), and f^ a "resistance coefficient," 

 This dimensionless number depends upon certain surface character- 

 istics and its value has to be determined under different conditions. 

 Here, the same question arises for the actual or "effective" wind 

 velocity V . But we may leave this question open at first smd re- 

 gard only the relative distribution of v^ over the rough wavy sur- 

 face (Fig. 11). Then we have to expect that v^ will be greater over 

 the crests than over the troughs, and X^ will be greater over the 

 crests too, even if we assume a constant "resistance coefficient" over 

 the wavy surface. Therefore, energy is also transferred by the tan- 

 gential stress which the wind exerts on the wavy surface. The effect 

 of this drag is to speed up the motion of particles at the wave crests 

 and to slow down the motion of particles at the trough; but the speed- 

 up is greater than the slowdown, so that a net increase in wave energy 

 results not only by "normal pressures" but also by "frictional forces," 



It is seen that the attempt to consider separately the effects 

 of the single wind force components encounters many diffic\ilties and 

 uncertainties, even if these effects may be written in a merely for- 

 mal way. 



Let us assume that TT- on the windward slope of the waves is re- 

 latively greater than on the leeward slope and TT - ^'^ /$x* Then 

 we may write for the distribution of the normal pressure component 

 over the wave profile 



Tjj = Tq + X'cos3t(x - (Tt) (7) 



where ^ represents a constant pressure value over the wave profile. 



If a relatively higher wind velocity over the wave crests is con- 

 sidered, the distribution of wind velocity over the wave may be 



53 



