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THE ACCTJMULATOR.

described), then if the communication is opened between thethree plungers, the weight of the accumulator, which was atfirst supported by two plungers, being now supported by three,the pressure on the water is diminished, and consequently theaccumulator ascends and descends. In Order to raise it againit is only necessary to allow the water to escape from the centralram, when the whole weight becomes supported on the twoplungers as before, and the pressure is consequently increasedand the ram again ascends. A small pump is employed to keepthe accumulator charged.

In order still further to diminish the loss of power entailedby hydraulic rams when raising and lowering heavy weights,Messrs. Clark & Standfield compensate for the varying im-mersion of the ram. When a ram is raised in the ordinarymanner it is evident that as it ascends out of the waterinto the air, it increases in weight, and its balancing powerdiminishes by an amount which is equal to the weight of acolumn of water of its own bulk. Similarly, as the plunger ofan accumulator descends, it loses a weight equal to the bulkof the column of water which it displaces, and both of theseactions concur to diminish the power of the machine more andmore as it approaches the full extent of its stroke. To obviatethis, the load on the accumulator is increased, as its plungerdescends, by a weight of water sufficient to compensate for thevarying immersion of the plunger and of the ram of the press.By this means the dead-weight of the machine itself is counter-poised in every position, and the only power required to workthe machine is that which is requisite to raise the load itself andto overcome friction. By the same means increased power isgiven at the end of the stroke, by adding to the load a greaterweight of water than is required for compensating for thevarying immersion of the rams and plungers. Conversely,decreased power may be given at the end of the stroke by caus-ing weighted tanks or vessels (which form the load of theaccumulator) to descend into water.

Where it is desirable that two or more rams should ascend