distance in the direction of progress of the wave (fig. U) . Con- 

 sequently, there is a mass transport in the direction of progress 

 of the wave. The mass transport velocity ( u' ) at the sea surface 

 is expressed by the formula, 



u' = (^H)2 C 



Figure U- Orbital motion during two wave periods 

 of a water particle in a deep-water wave of moderate 

 or great height. In two wave periods the forward dis- 

 placement equals 2u'T. 



The velocity is aporeciable for high, steep waves but is 

 very small for low waves of long period. Mass transport in 

 waves has received little attention in previous work because 

 in most practical applications it is sufficient to consider 

 the water -oarticles as moving in circles regardless of the 

 wave height. In order to understand the growth of waves 

 through wind action, however, it is necessary to take the 

 mass transport velocity into account. 



