Water falling from one level to a lower one is used to drive machines like the water wheel and the turbine. The difference in height between the highest and the lowest level is called the head. The amount of work produced per pound of falling water is proportional to the head. Water power can be produced in this way from many natural sources, such as waterfalls and dammed rivers. Where no natural sources are available, an artificial reservoir can be made. When energy is plentiful, it is used to pump water into the reservoir; the water is then available as a power source to drive turbines when energy becomes scarce.
In driving certain industrial hydraulic machines an apparatus called an accumulator is employed to supply high power for short periods of time. One type consists essentially of a cylinder enclosing a piston loaded with weights. When water is slowly pumped into the cylinder, the piston and weights are forced up to a position where they are held. When they are released, they force the water out of the cylinder rapidly, providing the machine with hydraulic power.