The Berkeley-Hartley method is a technique for measuring the osmotic pressure of a solution.
The apparatus consists of two concentric tubes.
The inner tube is made of a semipermeable membrane with two end-tubes. It contains the solvent (usually water).
The outer tube contains the solution.
Osmosis of water from the inner tube to the solution causes the level of fluid in the capillary indicator tube to fall.
A piston applies external pressure to the outer tube to oppose the osmotic pressure and keep the level in the capillary tube stationary.
This pressure, as read from a manometer or pressure gauge, is equal to the osmotic pressure.