formula. EDfe ) is given by equation (11.5) and nt x iS given by 
a 
equation (11.6). Ey u Ds for z not zero, is always less than E(p ) 
point for point. Emax is always less than Enax* 
There is always some depth below which the variation of the 
pressure caused by the passage of a short period wave overhead is 
undetectable due to the design of the pressure recorder. For ex- 
ample a five foot high wave with a five second period produces a 
pressure variation of only one one hundredth of a foot at a depth 
of 125 feet. This variation is essentially undetectable. Any 
variation in the power spectrum at the surface under the conditions 
described above is undetectable for all » greater than 2nr/5. 
These arguments also follow in a slightly modified way for 
pressure recorders located in shallow water (see Chapter 12). 
Ewing and Press [1949] have commented on the problem of the inter- 
pretation of pressure records, and their explanation is correct in 
that the correction must be applied to the whole power spectrum as 
indicated in equation (11.4). 
Everything that has been said about records of the sea surface 
is true also about pressure records. The probability distribution 
of points of a pressure record is Gaussian. An equation similar to 
equation (7.33) can be written for the pressure distribution simply 
by substituting P(t,) for 7 (t,) and E for E,ax° In addition, 
pmax 
the pressure record contains less of the non-linear effects which 
cause an asymmetry of the distribution for the free surface. 
The potential function and the velocity field 
Given the pressure field and equation (2.7) and (2.9), then 
by the methods of equation (11.1) and (11.2), the potential function 
