Observations and Measurements of Ocean Waves 37 



record of a wave registering instrument (see below), show that the surface 

 of the ocean is composed of many very irregular wave groups. Due to this 

 fact, it can be expected that the determination of the elements of a wave will 

 give entirely different results depending on which parts of such a wave train 

 are considered and which are not. Figure 22 gives a typical record of a wave 



A AAA 



I / / 



^Interval of 6 sec 



Fig. 22. Registration of a wave train by a wave recorder of the Admiralty. 



train made by the Admiralty's wave recorder. It shows the variations of the 

 ocean surface with time at a fixed point and, therefore, represents the up 

 and downward motion of an object floating at the ocean surface as seen 

 by an observer. This record shows at once the difficulties encountered in the 

 case of visual observation of the average period and height of the waves 

 at this point. In order to get representative values and to be able to obtain 

 comparative values with other observers, it seems necessary to consider 

 particularly such waves which do not change during their travel. These are 

 in the first place the waves which occupy the middle of the individual 

 wave groups. Recent instructions require that the observer should note the 

 period and height of these higher waves in each group, neglecting flat and 

 ill-developed waves {A in Fig. 22) in the intermediate region between wave 

 groups. The average period and height of fifteen to twenty of the higher 

 waves is to be determined. The individual waves do not belong to one group 

 and, therefore, the observations must not necessarily be on successive waves. 



These waves were called "significanf' waves (see p. 87) by Sverdrup 

 and Munk. It is not until a large number of such exact determinations of 

 wave elements will be available that discordances in the results of individual 

 observers and of individual series of observations will disappear. 



Wave-measuring instruments have been built by Froude and Paris 

 (1867, p. 731); they were only used occasionally by scientific expeditions. 

 The principle on which both devices work is the fact that the orbit of the 

 water particles is large in the surface layers, whereas the orbit decreases 

 rapidly with increasing depth. A few metres below the surface it is so small 

 that it can almost be neglected. We can construct a long divided staff which 

 is weighted at the lower end and has a floating anchor. This rod will float 

 without participating in the up and downward movement of the wave motion. 

 We can then measure the variations of the water level on the affixed scale. 

 The Froude instrument has its floating anchor made of a horizontal canvas- 

 covered frame tied to the staff by means of cords. This frame, which lies 

 in almost completely stationary water layers, damps nearly every vertical 



