virtu (virto)

[Also vertu; = It. virtù, ver IMG:content-1879.png:[blocks in formation] Sir M. Hale , Knowledge of Christ Crucified. 3. In mech., as usually understood, po ible and infinitesimal: but this meaning seems to have arisen from a misunderstanding of the original phrase virtual velocity, first used by John Bernoulli, January 26th, 1717, which was not clearly defined as a velocity at all, but rather as an infinitesimal displacement of the point of application of a force resolved in the direction of that force. The principle of virtual velocities is that, if a body is in equilibrium, the sum of all the forces each multiplied by the virtual velocity of its point of application is, for every po ible infinitesimal displacement of the body, equal to zero. The epithet appears to have been derived from an older statement that when, by means of any machine, two weights are brought into equilibrium, the velocities are inversely as the weights; so that virtual would here mean practical, as in def. 1.- Virtual coefficient. See coeficient. -Virtual cognition ( notitia virtual is ), the implicit existence in the mind of a concept of another, without special attention to this secondary concept. The term is due to Duns Scotus. - Virtual difference. See difference. Virtual displacement, an infinitesimal arbitrary displacement, e entially the same as a virtual velocity. Virtual focus, in optics, a point at which the lines of a pencil of rays would meet if sufficiently produced, al though the rays themselves do not actually reach it. See focus, 1. - Virtual head. See head. - Virtual image, in optics, an apparent image; an image which has no real existence. See under lens, mirror. - Virtual moment of a force. See moment. - Virtual monopoly. See monоро ly. Virtual quantity. Same as intensive quantity (which see, under intensive ). - Virtual resistance. See resistance, 3.- Virtual velocity. See def. 3.