Virtual \Vir"tu*al\ (?; 135), a. [Cf. F. virtuel. See Virtue.] 1. Having the power of acting or of invisible efficacy without the agency of the material or sensible part; potential; energizing. Heat and cold have a virtual transition, without communication of substance. --Bacon. Every kind that lives, Fomented by his virtual power, and warmed. --Milton. 2. Being in essence or effect, not in fact; as, the virtual presence of a man in his agent or substitute. A thing has a virtual existence when it has all the conditions necessary to its actual existence. --Fleming. To mask by slight differences in the manners a virtual identity in the substance. --De Quincey. Principle of virtual velocities (Mech.), the law that when several forces are in equilibrium, the algebraic sum of their virtual moments is equal to zero. Virtual focus (Opt.), the point from which rays, having been rendered divergent by reflection of refraction, appear to issue; the point at which converging rays would meet if not reflected or refracted before they reach it. Virtual image. (Optics) See under Image. Virtual moment (of a force) (Mech.), the product of the intensity of the force multiplied by the virtual velocity of its point of application; -- sometimes called virtual work}. Virtual velocity (Mech.), a minute hypothetical displacement, assumed in analysis to facilitate the investigation of statical problems. With respect to any given force of a number of forces holding a material system in equilibrium, it is the projection, upon the direction of the force, of a line joining its point of application with a new position of that point indefinitely near to the first, to which the point is conceived to have been moved, without disturbing the equilibrium of the system, or the connections of its parts with each other. Strictly speaking, it is not a velocity but a length. Virtual work. (Mech.) See Virtual moment, above.

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