# Trouble deriving hydrostatic pressure equation

I'm having a difficulty deriving hydrostatic pressure. To be specific, I don't understand this image from Wikipedia:

How does the fluid below act on the below surface? Is it the normal force? Or something else?

And doesn't the buoyancy force act on the volume of fluid too?

another image:

• In the figure you have shown, you actually are determining the buoyant force. It is the upward force due to pressure from below minus the downward force due to pressure from above. This net force supports the weight of the fluid in-between. – Chet Miller Feb 4 '17 at 13:28

Thus to maintain equilibrium, one can say that the differential pressure, dp is going to be such that it balances the weight of the layer, thus Adp = gdm where 'dm' is the mass of the layer, on dividing by the volume of the cylindrical cross section, $\frac{dp}{dh} = -g\rho$ which is probably the formula you were trying to derive.