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after its construction by the University of California in 1947, was tested in 600 

 feet of water at Drakes Bay and near the Farallon Islands (Chinn, 1949). There 

 was considerable difficulty in making the installations, but records were obtained 

 at both places. Tide effects were filtered from the record by a slow leak in the 

 static pressure chamber. 



The records obtained did not show pressure variations that could be re- 

 lated to surface waves. The pressure fluctuations were random, without any 

 evident periodicity. Frequently, readings remained above, or below the center 

 line of the chart (indicating an increase or decrease in pressure respectively) 

 for as long as 15 minutes. The readings were erratic and it seemed impossible 

 that the indicated changes in pressure could be caused by waves. 



The explanation given for these records was that the random variations 

 were due to pressure changes in the instrument air dome, caused by tempera- 

 ture fluctuations. Temperature changes of only one degree, or less, can cause 

 the sort of variation recorded by this instrument. 



Tsunami Recorder (Scripps Institution of Oceanography) - The tsunami record- 

 ers developed by the Scripps Institution of Oceanography were installed in shal- 

 low water, where pressure fluctuations of waves of all periods were present. 

 Both the short period wind wave and tide pressure fluctuations were removed by 

 means of a tuned hydraulic filter and the remaining long period waves were re- 

 corded by a pen recorder. 



The Mark I (2-stage) tsunami recorder operated at the end of the Scripps 

 pier from January to August, 1948. This instrument (which is sensitive to 

 waves with frequencies lying between those of the wind-generated swell and the 

 astronomic tides) revealed, during times of high wind waves, the presence of ir- 

 regular oscillations of several minutes period. These waves were called "surf 

 beat", as they are related to fluctuations in the height of incoming waves. 



Waves of 15 to 30-minute periods were also observed and tentatively 

 correlated with meteorological disturbances. These storm "seiches" were 

 largely obscured by the relatively high surf beat. Accordingly, it was decided 

 to retune the circuit to lower frequencies, shifting the period of peak response 

 from 15 to 45 minutes (Mark II, 3-stage). 



Two Mark II instruments (Munk, 1948) have been installed, one on the 

 end of the Scripps pier and the other at the end of the municipal pier at Ocean- 

 side, California. They differ in minor details of construction, but are designed 

 for identical responses. 



MEASUREMENT OF HEIGHT AND PERIOD OF ORDINARY GRAVITY WAVES 



Numerous instrunnents have been developed throughout the world for re- 

 cording the height and period of ordinary gravity waves of the ocean. More ac- 

 curately, these instrunaents record the time history of the water surface eleva- 

 tion at a point; in the case of pressure-recording type wave gages the surface 

 elevation time history is measured indirectly by recording the variations of 

 pressure at a point below the surface. The wave height and period are defined 

 in terms of the surface variation at this point. Wave height is determined by 

 the amplitude of the surface elevation variation and the wave period is deter- 

 mined by the length of time between successive maximums of the surface eleva- 

 tion. 



For most engineering applications the measurement of the surface at a 

 point provides a sufficient description of the sea surface. For many special 



