readable

The Differential amplifier . At the top you have the input pipes, then the chambers, with output pulleys at the rear. At the base of the cylinder you have a drain pipe. The differential amplifier is made up of a u bend filled with mercury. Works like a mercury barometer except instead of one side being open to atmospheric pressure, one end is connect to the high pressure, of the Ventura pipe and the other to the low pressure side.
As the flow changes so does the differences between the two pressures. I.e as it goes up both do, by the same amount but the difference widens. Clever trick is to use the difference. As the difference changes so the mercury is pushed from one side of the U bend to the other. Small floats bob on the top of the mercury pools and these are mechanically linked to a wheel (pulley), which rotate depending on their level.

Top view of one of the output pulleys. This is then attached to the meters above and relates to the differential Ventura water pressures. As such this rotation is proportional to flow.
Each meter was said to be custom made, in fact the system was modular, customised by the throttle ratio.