SECT. 5] WIND WAVES 685 



recording digitally on 5-hole teleprinter tape which can be fed directly to 

 computing machines, though an analogue record is nearly always taken at 

 the same time as a check. Several of these are in use, but have not been des- 

 cribed in print. Digital recording allows very high resolution, and to take 

 advantage of this there is a tendency towards f.m. measuring heads. These 

 produce a frequency dependent on the wave variable being measured : the 

 number of cycles in a fixed time is counted electronically, and this count 

 recorded digitally. Such systems do not depend on cable characteristics, and a 

 useful resolution of as high as 10~6 of full scale can be obtained. In the most 

 advanced wave recording system of this type which has so far been described, 

 the counted number is printed out and has later to be punched on cards by 

 hand (Munk et ah, 1959). ^ Though designed for studying low-frequency ocean 

 waves, it has produced some fascinating results concerning swell (Munk and 

 Snodgrass, 1957). 2 



Most practical methods for measuring waves at the present time measure 

 the height of the water surface above a fixed point, or some related quantity. 

 They are thus non-directional recorders and allow^ the calculation of the non- 

 directional wave spectrum, that is, taking no account of direction of travel of 

 the waves. Measurement of the directional spectrum is more difficult, but 

 methods are being developed, and, in particular, that starting with measure- 

 ments of the pitch, roll and heave of a small buoy can be regarded as established 

 for practical use (see below). 



A . Wave Recording from a Shore Station 



The measurement required is the height of the surface of the water above a 

 fixed point. Where a suitable structure, such as a pier, is available or can be 

 built, there are several types of instrument which can be used. 



The most straightforward consists of a small float in a tube which has long 

 vertical slots cut in it, so that it acts merely as a guide and the water level 

 inside is always the same as that outside (Wemelsfelder, 1955). The movement 

 is recorded mechanically. Electrical methods of measuring the surface elevation 

 are numerous, but most fall into three classes: (1) the water closes a series of 

 contacts on a vertical pole ; (2) the resistance between two wires varies with the 

 water level up them ; (3) the capacitance to earth of an insulated wire, passing 

 vertically through the water surface, varies with the water level. For examples 

 of these, the reader is referred to surveys of wave recording instruments by 

 Snodgrass (1951) and Draper (1961), and to the Proceedings of the First 

 Conference on Coastal Engineering Instruments (see references). 



In many cases it is either not possible or not economic to build a structure 

 suitable for such instruments, and the measuring head has to be laid on the sea- 

 bed. In these circumstances, an inverted echo-sounder has sometimes been 



1 It is understood that a tape-punch has since been fitted. 



2 A detailed account of digital techniques is given by Cartwright, Tucker and Catton 

 (in press). 



