A long-standing branch of engineering that is part of the more general field of fluid dynamics. It is concerned with the use of the properties of water and other liquids (as well as gases), at rest or in motion, to perform mechanical tasks using the principles of hydrostatics and hydrodynamics. In engineering, the hydraulic principle of fluid pressure transmitting a small force over a small area to engender a larger force over a larger area has been applied, for example, to the control of vehicle braking systems. More recently, it is has been used to predict weather patterns and to simulate the control of traffic flow. In the past, hydraulics was drawn upon as source of metaphors for understanding the workings of the mind or brain, as can be found in the writing of René Descartes (1596-1650). The same tactic was evident in the use made by Sigmund Freud (1856-1939) of 19th century hydraulics to bolster his theory of the libido (i.e., the symbolic release of libidinal tension through dreaming) and the pleasure principle (i.e., the hydraulics of unpleasure-avoidance through pleasure). Later, the ethologist Konrad Z. Lorenz (1903-1989) divined the same source to illustrate his theory of motivation, as did the behavioral physiologist Erich von Holst in presenting his ideas about the dynamical control and coordination of rhythmical movements. Thus, the language of hydraulics has played a significant role in the construction of theories concerning the regulation of behavior, giving way in the second half of the 20th century to those derived from computer-based analogies and pattern formation in open systems. It continues, however, to play role in how we verbally communicate our emotions, with the use of such expressions as ‘letting off steam’ and ‘running out steam’.
See Fluid mechanics, Metaphor, Open system, Pleasure-pain pleasure