record as a function of time as observed at a fixed point. How- 
ever, it tells us very little about what to expect for times 
outside of the interval in which the analysis was performed. 
There is also the difficulty that the wave groups as defined 
above do not seem to be really persistent phenomenon, that is, 
there is no mean time, T , which separates the wave groups. 
Time series analysis 
A fourth method of analysis, which has not been illustrated 
in Plate IV, can be carried out with the aid of the more recent 
concepts of time series analysis. These concepts will be dis=- 
cussed in Chapter 7 where it will be shown that they form the 
most realistic method of wave analysis. In this method of analy- 
sis, equation (3.10), for example, has a most convenient inter- 
pretation. 
The problem of variable direction 
When 7 (t) has been represented as a function of time by 
one of the methods described above, the problem of representing 
the short crested appearance of the sea surface is still unsolved, 
and the representation of 7 (t) is not enough to yield the com- 
plete solution. More information is needed to solve the short 
crested wave problem. The exact information needed will be de= 
scribed from Chapter 8 onward in this paper. 
Ge doleue mand Ppl ano ib ac ment aphueesehapters 
The first three methods of analysis are all inadequate. 
Various simple models which have the properties of the second and 
third models and which have infinitely long crests will be con- 
sidered mathematically in Chapters 4, 5, and 6. When these models 
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