waves everywhere would be the same. Waves at the forward edge 
of a storm can be thought of as being in a steady state along 
that forward edge if [A,( py Seni and [A( ie do not change with 
time during the duration of the storm. Since the methods of 
analysis to be presented can be applied to time intervals which 
are short compared to the duration of the storm and to areas which 
are small compared to the dimensions of the storm, the methods 
are valid in the analysis of actual wave records at the edge of 
a storm or over a wave generation area. 
The methods of analysis which will be presented are also valid 
in the area of wave decay. The filters described in Chapters 7 and 
9 are slowly varying functions of time. If [ace ae is analyzed 
from a wave record thirty or forty minutes long, the wave system 
will be so slowly varying that the methods will be valid. If 
anu) ]- is determined over an area of the sea surface thirty 
or forty miles on a side, and for thirty or forty minutes, the 
wave system will be so slowly varying that the methods will be 
valid. 
An analogy to electronic practices might clarify the situation. 
An engineer designs a radio to operate on 60 cycle AC current. 
The design of the power supply is based mainly on the formula 
E=E sin 2rt/l. For nearly all practical purposes, the fact 
that the radio is turned on or off can be ignored, and the fact 
that the voltage is actually given by an equation like equation 
(5.1) is not important. 
Similarly, the amplification sections of a radio are treated 
as if they were amplifying constant musical notes. A small enough 
SOs 
