SECT. 5] 



WINO WAVES 



687 



discrepancies of at least 16% under some circumstances (Fig. 10) and these 

 appear to be too large to be explained by experimental error. This is a most 

 important problem from the engineering point of view. 



An important potential source of error in pressure -type wave meters was 

 brought to our attention by R. L. Wiegel : the orbital motion of the water due 

 to waves is disturbed by the presence of the measuring head, and considerable 

 dynamic pressures can be set up. The design of the measuring head must be 

 such as to minimize these. 



Pressure-type wave meters may be self-contained (e.g. Valembois, 1955) but 

 these have been found to be unsatisfactory, since even the best instruments 

 develop faults, and, on recovery, it is occasionally found that enough records 

 have been lost to spoil a year's recordings. Electrical measuring heads designed 

 to be as simple and reliable as possible and connected to shore by a cable are, 

 therefore, preferred : if anything goes wrong, it is usually immediately obvious 

 and can probably be quickly corrected. Ways of measuring pressure electrically 

 are also numerous, and the reader is again referred to the summaries of wave 

 recording methods by Snodgrass (1951) and Draper (1961) and to papers in the 

 Proceedings of the First Conference on Coastal Engineering Instruments (1955). 



B. Wave Recording on the Deep Sea 



The wave recorders described above are designed for mounting on a fixed 

 platform. On the deep sea no such platform is accessible and different tech- 

 niques are necessary. 



FLAT BUOY 



WATER LEVEL UP 

 POLE IS MEASURED 



SUP-FACE OF EQUAL 

 PRESSURE 



Fig. 11. Suspended pressure -meter and pole-and-drogue-type wave recorders. 



One class of recorders uses the still water below the action of the waves as a 

 reference. A drogue (for example, a large horizontal flat plate) may be suspended 

 from a pole-type surface buoy designed so that its lift changes as little as 

 possible as the waves pass (Fig. 11). The drogue thus effectively prevents 

 vertical motion of the buoy, and the wave motion relative to this may be 

 measured by one of the techniques described above. Alternatively, a wide flat 



