difference

U bend of mercury, the differential pressures push either side of the mercury and floats are used either side, like the wheel barometer, to give an output to a meter.
Sounds simple, its not, they are huge great chambers that have to be sealed, to cope with the high pressures of the water, or later the isolation fluid.
Shown is the differential amplifier sometimes called the diff. amp., u bend and various other things.

Points to note are;

1) The drain at the base to draw the mercury

2) The two taps either side (one showing) which are connected to the isolation chambers.

3) The two output pulley at the lip of the cylinders

Hence the input is the fluid from the isolation chambers, this is the differential input. The action of;

Well, we now have a mechanical output, in this case the pulleys that rotate and drive through a cord the meters at the top. This now leads us to the mechanical calculator than needs a time base, clock, a way of recording the pressure and the differential pressure and how are we going to get to volumetric measurement. Pressure proportional to flow, and flow /second = volume. Time for the pencil and paper and have a guess.