Final answer:
The phenomenon described is Pascal's Principle, which states that pressure changes in an enclosed fluid are transmitted undiminished throughout the fluid.
Step-by-step explanation:
The phenomenon where the pressure in a sphere filled with liquid surrounded by a thin membrane is measured by the counter pressure which just flattens the membrane is known as Pascal's Principle. This principle, named after French philosopher and scientist Blaise Pascal, states that a change in pressure applied to an enclosed fluid is transmitted undiminished to all portions of the fluid and to the walls of its container. Because of this, when pressure is applied to any part of the enclosed fluid, it is felt equally at every point within the fluid.