And the pressure, we assume that the pressure has to be undisturbed pressure. So, you have to now remove the column or the structure from your mind. So, when a wave is propagating, you try to find out the pressure at this location and at this location, that is what we are exactly trying to do. We should recollect the problem which we have done under wave hydro dynamics. One or two problems, we looked at the phase at which, the velocity is maximum or the acceleration is maximum etcetera. So, we consider this location and try to find out, what would be the pressure and that in terms of coordinates is given as minus a, that is on the negative side. This is at x equal to minus, but any particular elevation z.