An accumulator was water under pressure by a very large piston. The piston top was usually filled with gravel. It increased the pressure.
Basics: Water ~30 foot high gives you 1 bar, 1 atmosphere (the weight of the atmosphere at sea level), which is 14.7 lb/squ inch at the bottom of the 30 foot. To get 100 ib/sq inch you would need a tower well over 200 foot high.
Put in a piston with weights on, the pressure is raised and the tower need only be about 30 foot high. The piston also stabilises the pressure distribution. It also takes out peaks and dips when assisting a pump. A bit like the pressure expansion vessel on your heating boiler - that assists the pump delivery to the radiators and avoids cavitation when fitted just before the pump.
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