Equilib´rium, in statics, the condition when a body is acted on by two or more forces which balance one another. The body may be either at rest or moving with uniform speed in a straight line. In the first case, when the body, being slightly moved out of any position, always tends to return to its position, that position is said to be one of stable equilibrium; when the body, after a slight displacement, tends to move away from its previous position, the body is in unstable equilibrium. If, after displacement, the body tends to remain at rest, its state is one of neutral equilibrium.
Equinoc´tial, in astronomy, the circle in the heavens otherwise known as the celestial equator. When the sun is on the equator, there is equal length of day and night over all the earth: hence the name equinoctial.—Equinoctial gales, storms which have been supposed to take place about the time of the sun's crossing the equator, that is, at the vernal and autumnal equinoxes, in March and September.—Equinoctial points are the two points wherein the celestial equator and ecliptic intersect each other; the one, the first point of Aries, is called the vernal point; and the other, in Libra, the autumnal point. These points move backward or westward at the rate of 50" of arc in a year. This is called the precession of the equinoxes.
Eq´uinox, one of the equinoctial points. The term is also applied to the dates at which the sun passes through them, viz. 21st March and 23rd September, when day and night are of equal length all over the world. See Day; Earth; Equinoctial; Seasons.
Equisetales, a group of Pteridophytes, represented at the present day only by the genus Equisetum (q.v.). It was much more prominent in the Carboniferous flora, in which large woody horse-tails (Calamites) played an important part.
Equise´tum, a genus of vascular cryptogamous plants with hollow jointed stems, type of the group Equisetales, growing in wet places, and popularly called horse-tails.
Eq´uity (Lat. aequus, fair, equal), in English law, the system of supplemental law administered in certain courts, founded upon defined rules, recorded precedents, and established principles, the judges, however, liberally expounding
and developing them to meet new exigencies. While it aims at assisting the defects of the common law, by extending relief to those rights of property which the strict law does not recognize, and by giving more ample and distributive redress than the ordinary tribunals afford, equity by no means either controls, mitigates, or supersedes the common law, but rather guides itself by its analogies, and does not assume any power to subvert its doctrines. The Court of Chancery was formerly in England the especial court of equity, but large powers were by the Judicature Act of 1873 given to all the divisions of the Supreme Court to administer equity, although many matters of equitable jurisdiction are still left to the chancery division in the first instance.—Bibliography: F. T. White and O. O. Tudor, Leading Cases in Equity; C. Thwaites, Student's Guide to Equity.
Equity of Redemption, in law, the advantage allowed to a mortgager of a reasonable time to redeem an estate mortgaged, when it is of greater value than the sum for which it is mortgaged.
Equiv´alent, in chemistry, the number of parts by weight of an element which will combine with or displace 8 parts by weight of oxygen or 1.008 parts by weight of hydrogen.
Eranthis. See Winter Aconite.