set so that sea level is recorded as zero pressure; thus, an increase 

 in wave height increases the pressure, and a decrease in wave height 

 decreases the pressure. A wave trace is obtained when these changes 

 in pressure are recorded continuously. The wave trace may be studied 

 to obtain its statistical properties by considering each individual wave. 

 It also may be studied by considering it an example of a time series, 

 and the energy spectrum of the wave profile can be obtained by an 

 electronic analyzer or a high-speed digital computer. 



2. Accelerometer 



This is a device which is used on some oceanographic instru- 

 ments to measure the vertical component of acceleration that the sea 

 surface imparts to an object floating on it. Since the vertical displace- 

 ment of the object usually is required, the output must be integrated 

 twice. 



3. Capacitance-type wave gauge 



This is a gauge which utilizes the principle that the capaci- 

 tance of a rod varies as the depth of immersion is varied. For example, 

 a dielectric- coated metal rod when placed in water acts as a variable 

 capacity to the water, the capacity being linearly dependent upon the 

 depth of immersion providing that the coating is uniform. A contin- 

 uous measurement of the changes in capacity provides the wave-form 

 of any disturbance on the water surface as it passes the rod. 



4. Resistance-type wave gauge 



This type of gauge utilizes the principle that the resistance of a 

 rod varies as the depth of immersion. For example, the step-resist- 

 ance gauge consists of a series of exposed electrodes mounted verti- 

 cally on a staff. As the wave passes and covers more electrodes, 

 the total gauge resistance decreases. Thus, measurement of the 

 wave-form is reduced to the measurement of a varying resistance. 

 Some gauges use a continuous length of wire as the sensing element, 

 but the principle remains the same. 



C. INSTRUMENT TYPES 



Three types of wave measuring instruments are discussed in this 

 section: bottom pressure instruments, floating wave gauges, and fixed 

 wave gauges. A few instruments are discussed in detail, whereas 

 others are described briefly; still others merely are cited in the 

 literature. Many types of wave measuring instruments are in use, 



VII- 3 



