Curiously, neither Dr. Mandl, nor any other advocate of breath-control, seems to have read an article by Sir Charles Bell dealing with this same action, the closing of the glottis against a powerful exhalation. This paper, "On the Organs of the Human Voice," was read before a meeting of the London Philosophical Society on February 2, 1832. Dr. Bell dispels all the mystery concerning the closure of the glottis, and the holding of the breath against a powerful contraction of the expiratory muscles. He points out that this action occurs in accordance with the law of the distribution of pressure in a fluid body, commonly known as Pascal's law of fluid pressures. Pascal's law is stated as follows:--"Pressure exerted anywhere upon a mass of fluid is transmitted undiminished in all directions, and acts with equal force on all equal surfaces, and in a direction at right angles to those surfaces." (Atkinson's _Ganot's Physics_, 4th ed., New York, 1869.) The hydraulic press furnishes the familiar illustration of this law. Two vertical cylinders, one many times larger than the other, are connected by a pipe. The cylinders are fitted with pistons. Both the cylinders, and the pipe connecting them, are filled with water, oil, air, or any other fluid; the fluid can pass freely from one cylinder to the other, through the connecting pipe. Suppose a horizontal section of the smaller cylinder to measure one square inch, that of the larger to be one hundred square inches. A weight of one pound on the smaller piston will balance a weight of one hundred pounds on the larger. If a downward pressure of one pound be exerted on the smaller piston, the larger piston will exert an upward pressure of one hundred pounds. Conversely, a downward pressure of one hundred pounds, exerted on the larger piston, will effect an upward pressure of only one pound on the smaller piston. A type of the hydraulic press is presented by the chest cavity and the larynx, considered as one apparatus. This fact is illustrated in the following quotation: "If a bladder full of water be connected with a narrow upright glass tube, heavy weights placed on the bladder will be able to uphold only a very small quantity of liquid in the tube, this arrangement being in fact a hydraulic press worked backwards. If the tube be shortened down so as to form simply the neck of the bladder, the total expulsive pressure exerted by the bladder upon the contents of the neck may seem to be very small when compared with the total pressure exerted over the walls of the bladder upon the whole contents." (_A Text Book of the Principles of Physics_, Alfred Daniell, London, 1884.)