slowly over time intervals of the order of four hours. In the 
Same sense that sin t can be approximated in the neighborhood 
of t = 0 by t, it will be possible to represent wave records 
obtained at a certain time by the functions which will be studied. 
The more rapidly the sea surface characteristics change, the 
less valid some of the techniques described here will be. 
The nature of available data 
Ocean waves are recorded by two methods at the present time. 
Either the actual height of the free surface is recorded at a 
fixed point as a function of time, or the pressure at some depth 
below the free surface and at a fixed point is recorded as a 
function of time. Neither method is sufficient to determine com- 
pletely the actual space and time distribution of the free sur- 
face, the pressure, and the fluid motions. By a sufficient number 
of simplifying assumptions, it is possible to draw a few con- 
clusions about the distribution in space and time of the above 
properties. 
The actual height of the free surface is frequently measured 
on the open ocean by an upright graduated pole with a large disk 
on the bottom to damp out the motions of the pole. On.the end 
of piers or at fixed installations such as oil drilling structures 
in the Gulf of Mexico as reported by Glenn [1950], it is possible 
to use the instrument developed by the Beach Erosion Board and 
described by Caldwell [1948]. In either case a record is ob- 
tained of the height of the free surface as a function of time 
at a fixed point. Or in terms of the equations employed in 
this paper, 7] = 1(x,,¥,92 = 0,t) is known. 
Fey We 
