"periods". The "periods" are designated by T. T and so on. In such 

 a wave record it is theoretically wrong to' eqdate these observed 

 "periods" with the period of a simple harmonic progressive wave, 



B. Visual "Period" Observations 



1. Theory of the "Period" Distribution 



The probability distribution function of the time intervals between 

 successive wave crests is not known. Rice (1944) has given a formula 

 that gives the mean of this unknown distribution in terms of an integral 

 which involves the spectrum of the waves. Apparently none of the higher 

 monnents is known. 



Even if the distribution of these "periods" were known, it would 

 still tell us very little about the true spectral periods in the exact 

 mathematical sense of the word. In addition, the loose interchange of 

 "periods" and periods in theoretical work leads generally to invalid 

 resiilts. 



2. Method of Observation 



The observed statistical distribution of the "periods" and the 

 average "period" for a given state of the sea are nevertheless useful 

 values which can be obtained by the use of a stop watch in visual 

 observations. Recommended procedures for observing the "periods" 

 are given in Pierson, Neumann, and James (1955). A foam patch or a 

 floating object can be used as a reference point. Two observers 

 working as a team can make the observations more rapidly and 

 efficiently. 



C. Calculation Of The Average "Period" In Terms Of Theoretical 

 Spectra 



1 . Method of Calculation 



Neumann (195 3) has shown good reason to believe that the spectrum 

 of a sea grow^s irom high frequency to low^ frequency with increasing 

 duration or fetch. The spectrum of a given state of the sea is given by 



[A(yLi)]'= c e-2g'AV ^°' H- > hi (24) 



where yx- is a function of the wind velocity and either the fetch or the 

 duration, and v is the wind velocity. 



32 



