In the application of hydraulic power to machine tools great advances have been made. It has become a system, in which Tweddle of England was a pioneer. The great force of water pressure combined with comparatively slow motion constitutes the basis of the system. Sir William Fairbairn had done with steam what Tweddle and others accomplished with water. Thus the enormous force of men and the fearful clatter formerly displayed in these huge works where the riveting of boilers was carried on can now be dispensed with, and in place of the noisy hammer with its ceaseless blows has come the steam or the hydraulic riveting machine, which noiselessly drives the rivet through any thickness of metal, clinches the same, and smooths the jointed plate. The forging and the rolling of the plates are performed by the same means. William George Armstrong of England, afterward Sir William, first a lawyer, but with the strongest bearing toward mechanical subjects, performed a great work in the advancement of hydraulic engineering. It is claimed that he did for hydraulic machinery, in the storage and transmission of power thereby, what Watt did for the steam engine and Bessemer did for steel. In 1838 he produced his first invention, an important improvement in the hydraulic engine. In 1840, in a letter to the _Mechanics’ Magazine_, he calls attention to the advantages of water as a mechanical agent and a reservoir of power, and showed how water pumped to an elevated reservoir by a steam engine might have the potential energy thus stored utilised in many advantageous ways. How, for instance, a small engine pumping continuously could thus supply many large engines working intermittently. In illustration of this idea he invented a crane, which was erected on Newcastle quay in 1846; another was constructed on the Albert dock at Liverpool, and others at other places. These cranes, adapted for the lifting and carrying of enormous loads, were worked by hydraulic pressure obtained from elevated tanks or reservoirs, as above indicated. But as a substitute for such tanks or reservoirs he invented the _Accumulator_. This consists of a large cast-iron cylinder fitted with a plunger, which is made to work water-tight therein by means of suitable packing. To this plunger is attached a weighted case filled with one or many tons of metal or other coarse material. Water is pumped into the cylinder until the plunger is raised to its full height within the cylinder, when the supply of water is cut off by the automatic operation of a valve. When the cranes or other apparatus to be worked thereby are in operation, water is passed from the cylinder through a small pipe which actuates the crane through hydraulic pressure. This pressure of course depends upon the weight of the plunger. Thus a pressure of from 500 to 1,000 pounds per square inch may be obtained. The descending plunger maintains a constant pressure upon the water, and the water is only pumped into the cylinder when it