Two identical cylindrical vessels are kept on the ground and each contain the same liquid of density $\rho$. The area of the base of both vessels is $S$, but the height of liquid in one vessel is $x_1$ and in the other is $x_2$. When both cylinders are connected through a pipe of negligible volume very close to the bottom, the liquid flows from one vessel to the other until it comes to equilibrium at a new height. The change in energy of the system in the process is: