So, this will be, this is a elevation up to which, at which the pressure is going to act. So, you need to consider that elevation that pressure exerted at that elevation, you need to consider. And having consider the pressure at that elevation, you carry out the integration multiply it. So, once you carry out the integration that alone will give you the force and multiplying it by the (( )) is the x bar in this case, you will get the forces due to the or moment due to the vertical forces. So, these two when added, you get the total moment as given by equation 25.