III. VISUAL "PERIOD", "WAVE LENGTH", AND "SPEED" OBSER- 

 VATIONS 



A. Definition of Terms 



Part of the difficulty in making wave observations and wave record 

 analyses lies in the loose interchange between theory and practice of 

 two distinctly different meanings of the word, "period". 



A period of a simple harmonic progressive wave is a number with 

 a precise mathematical meaning. A true period will be underlined in 

 this paper, and it will be designated by the symbol, T. 



The time interval between two successive characteristic points 

 in a wave record, such as the wave crests or the zero up- crosses, is 

 not a period in the exact mathematical sence since a wave record is not 

 periodic. These time intervals will be called "periods". A wave record 

 has many different "periods". A simple sine wave has only one period. 

 "Periods" in this sense will have quotation marks around them. The 

 individual "periods" will be designated T. as they are enumerated in 

 an observation or from a wave record; and the average "period", that 

 is the average of all of the observed "periods", will be called T. 



Similarly, wave length , (L), and "wave length", (L), will be discussed. 

 For additional discussion of these terms, see Pierson (1954) and Pierson, 

 Neunaann, and James (1955). 



Figure 5 illustrates the analysis of a wave record for its various 



Figures. The Definition of the "Periods" in a Wove Record. 



31 



