Also 7–9 æquilibrium. [a. L. æquilībrium, f. æquus equal + lībra balance.]
1. In physical sense: The condition of equal balance between opposing forces; that state of a material system in which the forces acting upon the system, or those of them that are taken into consideration, are so arranged that their resultant at every point is zero.
A body is said to be in stable equilibrium, when it returns to its original position after being disturbed; in unstable when it continues to move in the direction given to it by the disturbing force; in neutral, when it remains stationary in its new position.
1660. Boyle, New Exp. Phys.-Mech., i. 36. The pressure on all hands being reduced as it were to an Æquilibrium.
1697. Phil. Trans., XIX. 446. The Fluids, pressing equally and easily yielding to each other, soon restore the Æquilibrium.
1796. Atwood, Floatation, ibid. LXXXVI. 51. 1st. The equilibrium of stability…. 2dly. The equilibrium of instability…. 3dly…. The equilibrium of indifference.
1830. Sir J. Herschel, Stud. Nat. Phil., 222. Thereby to maintain equilibrium.
1838. Jas. Grant, Sk. Lond., 299. Young people, amusing themselves with the popular exercise of trying how fast they could run down [the hill] without losing their equilibrium.
1860. Mill, Repr. Govt. (1865), 6/1. A government so situated is in the condition called in mechanics unstable equilibrium, like a thing balanced on its smaller end.
1879. trans. De Quatrefages’ Hum. Spec., 4. In the crystal once formed, the forces remain in a state of stable equilibrium.
b. Equilibrium of temperature: see quot.
1794. J. Hutton, Philos. Light, etc. 91. The supposed state of a perfect equilibrium, or equal temperature among bodies.
1871. B. Stewart, Heat, § 12. Two bodies may be said to be in a state of equilibrium of temperature with each other when if shaken together they neither change their state with respect to heat, nor [etc.].
2. The state of equal balance between powers of any kind; equality of importance or effect among the various parts of any complex unity.
1677. Govt. Venice, 80. So to balance their [the Spaniard and the French] Power, as to keep both in an Equilibrium.
1741. C. Middleton, Cicero, I. (ed. 2), Pref. p. xxxviii. They [the two Gracchi] had in great measure overturned that æquilibrium of power in the Republic, in which it’s peace and prosperity depended.
1773. Observ. State Poor, 80. A destruction of the equilibrium of population, by a defection of inhabitants from one county to another.
1840. Macaulay, Ranke, Ess. 1851, II. 144. The war which followed was a war for the equilibrium of Europe.
1871. R. F. Weymouth, Euph., 4. Of such equilibrium and symmetry in antithesis every page of the Euphues furnishes examples.
b. The condition of suspense or uncertainty produced by equality in the force of opposing influences; neutrality of judgment or volition.
1685. Sharp, Doubting Conscience, 4. There is an end of the Doubt or Æquilibrium.
1754. Edwards, Freed. Will, I. i. 4. Where there is absolutely no preferring or chusing, but a perfect continuing Equilibrium there is no Volition.
1794. Paley, Evid., III. viii. (1817), 372. That indifferency and suspense, that waiting and equilibrium of the judgement.
1876. E. White, Life in Christ, I. vii. 74. This is the infidelity of persons … who pass their lives in a state of equilibrium or indifference.
c. Well-balanced condition of mind or feeling.
1608. J. King, Serm. St. Mary’s, 26. Salomon a man in the perfit æquilibrium and stablest state of his age.
1874. Farrar, Christ, II. 45. In this outward activity, she lost the necessary equilibrium of an inward calm.
1875. Hamerton, Intell. Life, X. ix. 384. It is best to preserve our minds in a state of equilibrium.
3. The Lat. in æquilibrio ‘in equilibrium’ appears usually with anglicized spelling (equi-). (The Latin ablative in this phrase was formerly sometimes treated as an Eng. word; hence such expressions as in perfect equilibrio.)
1650. Bulwer, Anthropomet., 229. When the body is erect and in equilibrio.
1683. Pettus, Fleta Min., I. (1686), 150. Weigh it … until the Ballance standeth even in Æquilibrio.
1709. Prior, Poems, Ladle, 45. Is it in equilibrio, If deities descend or no? Then let the affirmative prevail, As requisite to form my tale.
1755. Gentl. Mag., XXV. 164/2. Being at the vernal equinox in an equilibrio.
1798. T. Jefferson, Writ. (1859), IV. 231. The fate of Sprigg’s resolutions seems in perfect equilibrio.
1868. Rogers, Pol. Econ., xv. (ed. 3), 209. These bills … may exactly balance between country and country. In such a case, the trade between the two countries is said to be in equilibrio.
4. attrib.
1874. Knight, Dict. Mech., Equilibrium-valve … having a pressure nearly equal on both sides.
1880. Haughton, Phys. Geog., iii. 92. The point of equilibrium temperature, which is the Fahrenheit zero.
