資料來源 : Webster's Revised Unabridged Dictionary (1913)

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.

Moment \Mo"ment\, n. [F. moment, L. momentum, for movimentum
   movement, motion, moment, fr. movere to move. See {Move}, and
   cf. {Momentum}, {Movement}.]
   1. A minute portion of time; a point of time; an instant; as,
      at thet very moment.

            In a moment, in the twinkling of an eye. --1 Cor.
                                                  xv. 52.

   2. Impulsive power; force; momentum.

            The moments or quantities of motion in bodies.
                                                  --Berkley.

            Touch, with lightest moment of impulse, His free
            will.                                 --Milton.

   3. Importance, as in influence or effect; consequence; weight
      or value; consideration.

            Matters of great moment.              --Shak.

            It is an abstruse speculation, but also of far less
            moment and consequence of us than the others.
                                                  --Bentley.

   4. An essential element; a deciding point, fact, or
      consideration; an essential or influential circumstance.

   5. (Math.) An infinitesimal change in a varying quantity; an
      increment or decrement. [Obs.]

   6. (Mech.) Tendency, or measure of tendency, to produce
      motion, esp. motion about a fixed point or axis.

   {Moment of a couple} (Mech.), the product of either of its
      forces into the perpendicular distance between them.

   {Moment of a force}. (Mech.)
      (a) With respect to a point, the product of the intensity
          of the force into the perpendicular distance from the
          point to the line of direction of the force.
      (b) With respect to a line, the product of that component
          of the force which is perpendicular to the plane
          passing through the line and the point of application
          of the force, into the shortest distance between the
          line and this point.
      (c) With respect to a plane that is parallel to the force,
          the product of the force into the perpendicular
          distance of its point of application from the plane.
          

   {Moment of inertia}, of a rotating body, the sum of the mass
      of each particle of matter of the body into the square of
      its distance from the axis of rotation; -- called also
      {moment of rotation} and {moment of the mass}.

   {Statical moment}, the product of a force into its leverage;
      the same as {moment of a force} with respect to a point,
      line, etc.

   {Virtual moment}. See under {Virtual}.

   Syn: Instant; twinkling; consequence; weight; force; value;
        consideration; signification; avail.