The resistance of an elastic membrane to a pressure acting on its surfaces depends on the primary resistance of the membrane and on its stretching. By stretching the resistance increases and the bulging produced by a pressure is reduced; by relaxation the resistance decreases and the bulging produced by a pressure is increased. A membrane of primarily low resistance is stretched more extensively by a given force than a membrane with primarily high resistance, and its gain of resistance is higher than that of the primarily more resistant membrane. In strong stretching the resistances of the two membranes may be about the same, while they may be greatly different when the membranes are relaxed. If the pressure acting on the membrane is changing too, increasing during stretching and decreasing during relaxation, so that during stretching of the membrane both pressure and resistance increase, while during relaxation of the membrane both