By invoking the thin wall approximation , assuming r >> t, (i) the circumferential stress σ is spatially uniform in the material and (ii) we do not distinguish between the inner and outer radius r . A free body diagram on a hemisphere of the vessel (see Fig. ) balances two opposing forces in the direction normal to the plane on which the hemisphere sits, arising from: (a) the circumferential stress in the material and (b) the pressure of the gas on the inner walls, giving: