Hydraulic Engines
Collier's New Encyclopedia (1921)
engines of which the motive power is water under pressure. In principle they do not differ essentially from steam engines. The water acts by difference of pressure-i. e., it is admitted at a high pressure at the beginning of the stroke, and exhausted at a low pressure at the end of the stroke, thus giving a reciprocating motion to the plunger. They may also be rotary. As water is much more dense than steam, the induction and eduction pipes have to be larger and have no abrupt angles. The eduction valve is required to open promptly at the end of the stroke, and the induction valve must not close till the stroke is completed. In most of the hydraulic engines relief valves or other expedients are used to prevent these concussions. Since hydraulic engines work under much greater pressure they can be made much smaller than steam engines. A common form is the three-cylinder single acting engine; in each cylinder works a plunger; water is admitted by valves behind the plungers and forces them out; at the conclusion of the outstroke the pressure water-supply is cut off, and the exhaust valve opened, allowing the plunger to push the water out of the cylinder on the return stroke, and so on. Hydraulic engines have been introduced in some special industries in large foundries, in the working of cranes and other heavy machinery, and also for draining of mines. Hydraulic machines in a small form are used for many industrial purposes in the operating of printing presses, circular saws, lathes, etc. The water head is obtained from a reservoir or accumulator or, in cities where considerable pressure is constantly supplied, through the water mains.
What Has Changed in the Last Century
When Collier's wrote this in 1921, hydraulic power still meant water "in cities where considerable pressure is constantly supplied, through the water mains" — public high-pressure networks such as the London Hydraulic Power Company's, which pumped mains water at about 750 psi to cranes, lifts and theatre machinery. Those networks are gone: London's closed in 1977, and Tower Bridge, hydraulically operated since 1894, was converted to oil and electric drive in 1976. The reciprocating three-cylinder plunger engine Collier's describes survives mainly in museums, displaced from the 1930s onward by oil hydraulics — axial- and radial-piston pumps and motors running at 3,000–5,000 psi and higher, compact enough to sit inside an excavator arm or an aircraft wing. The physics of the entry is untouched; only the working fluid, the pressures and the machines have moved on, and water hydraulics now survives as a niche choice where oil leakage is unacceptable, as in food processing.
Written for this edition, 2026.