For the manometer shown below, write an equation that governs the pressure difference from point $1$ to point $4.$ The equation should be a function of the densities of the liquid and the gas as well as the heights z$2,$ z$3$ and z$4.$ You can assume that the density of the liquid is MUCH greater than the gas. Also, you can assume that z$3-$z$2>$ z$4($defining the direction of flow$)$