the situation is still undefined outside of some possibly 
larger time interval. 
Consider a wave record, say twenty minutes long. Is it 
possible to pick some functional representation for7) (t) which 
will coincide with the wave record for the twenty minutes over 
which our attention is concentrated? Many functional represent= 
ations are so obviously inadequate that they will not even be 
considered, but for other functional representations it is not 
immediately obvious that they do or do not apply. 
As a start consider a wave record which is not too irregu- 
lar.* Such a wave record might appear as sketched in Plate IV. 
The essential feature of the record for this part of the discussion 
is that there are groups of high waves and that between the groups 
of high waves there are time intervals where the amplitude of 
the disturbance of the free surface is small compared to the ampli- 
tude near the center of the group. These groups of high waves 
will simply be referred to as "wave groups." 
One way to analyze the actual wave record would be by the 
Significant wave method of analysis as defined by ereranue and 
‘tank [1947]. Suppose that the significant height and period are 
ten feet and eight seconds. Now Sverdrup and ifunk carefully state 
that the significant wave does not behave like a classical wave, 
yet in many applications it is tacitly assumed that the free sur- 
face at a point in relatively deep water can be represented by 
equation (3.1) of Plate IV where in this case A = 5 and T = 8. 
*Irregular wave records will be discussed very much later. 
a eae 
