Consider a spherical vessel having four cylindrical tubes A, B, C and D each fitted with air-tight
frictionless pistons of areas of cross section A, A/2, 2A and 3A, respectively, as shown in above figure. The vessel is filled with an incompressible liquid such that there is no air between the liquid and the pistons.
If the piston A is pushed with a force F, the pressure on the piston and the liquid in the vessel is pA = F/A. It is seen that the other three pistons are pushed outwards. To keep these pistons at their respective original positions, forces of F/2, IF and 3F, respectively are required to be applied on pistons B, C and D respectively to hold them. Then, the pressures on the respective pistons are
PB = \(\frac{F/2}{A/2}\) = F/A, PC= 2F/2A = F/A and
pD = 3F/3A = F/A
∴ pA = pB = pC = pD = F/A
This indicates that the pressure applied is trans-mitted equally to all parts of liquid. This proves Pascal law.