For a "swell" record with a narrow band width such as those 
shown in Chapter 9, the fallacy of the method does not produce too 
important a discrepancy between the theoretically computed values 
of the surface quantities and the observed surface quantities, but 
for a "sea" record, such as those shown in the appendix to part 
one, the procedure effectively ignores a large part of the high 
end of the power spectrum. The surface "significant" height (or 
"characteristic" height) in "sea" conditions is always observed to 
be greater than the value predicted erroneously from the pressure 
record, and the surface "significant" period, (or "characteristic" 
period), were it also measured, would be found to be lower than the 
"significant" period (or the "characteristic" period) of the pressure 
record. 
Thus the fact that "wave heights computed from these equations 
have been shown by several observers to be from six to twenty-five 
percent too low" is not at all surprising. The error is not a con- 
sistent error in that it varies from record to record depending on 
the power spectrum and in that it varies as a function of the depth 
of the pressure recorder. If the basic theory referred to in the 
last paragraph of the quotation is the theory which accepts as a 
basic definition the definition of wave period at the start of the 
quotation, then these considerations have shown wherein the error 
of the theory lies. , 
On the other hand, if the basic theory referred to in the last 
paragraph of the quotation is the theory of purely sinusoidal waves 
with one discrete period, then that basic theory is correct and the 
theory has been misapplied to a pressure record which is not a purely 
se] 
