There is no linear covariant, since it is impossible to form a symbolic product which will contain x once and at the same time appertain to a quadratic. (v.) is the Jacobian; geometrically it denotes the **bisectors** of the angles between the lines ax, or, as we may say, the common harmonic conjugates of the lines and the lines x x .

Thus if a point of the body be displaced from A to B, whilst the point which was at B is displaced to C, and that which was at C to D, the four points A, B, C, D lie on a helix whose axis is the common perpendicular to the **bisectors** of the angles ABC, BCD.