Consequently, for purposes of illustration, a form for the power 
spectrum has been assumed. The assumed form of the power spectrum 
has been plotted below the part of the figure just discussed. It 
represents what mead to be known about the power spectrum at the 
source before the power spectrum at any other point of observation 
can be forecasted. The power spectrum at the source is given by 
the dash dot curve. 
The band width determined above has been used to multiply the 
power spectrum at the source by the square cornered filter in 
order to find the power spectrum at the point and time of obser- 
vation. The square cornered filter is given by equation (7.49), 
and to apply it to the given power spectrum set the forecasted 
power spectrum equal to zero outside of the segment described above 
and set it equal to the power spectrum at the source inside of the 
segment described above. The heavy solid lines show the effect of 
applying the square cornered filter to the power spectrum at the 
source. 
The forecasted power spectrum is an instantaneous power spect- 
rum, and in terms of our original definition, it has no meaning. 
However, if a wave record taken at x = 200 km from twenty hours 
minus ten minutes to twenty hours plus ten minutes is analyzed for 
its power spectrum, it might be expected that something like the 
above pattern, except for slight smoothing at the edges, would be 
obtained because the filter function is a slowly moving function of 
time. 
The remaining power spectra show the forecasted power spectra 
for various x's and various times. For a fixed x, as time increases, 
Erie ee 
