Consider then a range of possible "significant" periods (de- 
pending upon the analyst) and the multiplication of the "significant" 
height of the pressure record by the possible amplification factors. 
Then the quantity 
2 
ea 4 
1/2 
= ] 
2 
[(E, 4 /[cosh( I(w4,H))) E 
Pmax 
is a ratio which represents roughly the value obtained by the correct 
method divided by the value obtained by the erroneous method. If 
the ratio were one then the error would not be apparent; if it is 
sreater than one then the part after the decimal point represents the 
percentage error referred to by Snodgrass [1951]. Table 19 gives 
some of the ratios which can result from the assumption of various 
significant periods. 
Thus for this depth, which is quite shallow compared to most 
depths at which pressure recorders have been installed, if the pres- 
sure record were given any "significant" period greater than &.6 sec- 
onds, then there would be a considerable error in the computation of 
the "significant" height of the free surface. At greater depths and 
for differently shaped pressure power spectra the errors would be 
different and there is no hope of consistency in the old methods of 
analysis. Note that the power lost above 4.8 seconds would serve 
only to increase the error if it were included. Also note that the 
filtering nature of the pressure recording method always tends (given 
a widely variable power spectrum) to give too large a sicnificant 
period to the free surface record and too small an amplification 
factor by the old methods. 
oO 
