The resultant force is therefore in the direction of the steepest pressure-gradient, and this is normal to the surface of equal pressure; for equilibrium to exist in a fluid the lines of force must therefore be capable of being cut **orthogonally** by a system of surfaces, which will be surfaces of equal pressure.

Every circle through the limiting points cuts all circles of the system **orthogonally**; 5.

If we project both polyhedra **orthogonally** on a plane perpendicular to the axis of the paraboloid, we obtain two figures which are reciprocal, except that corresponding lines are orthogonal instead of parallel.

To define the moment of a force about an axis HK, we project the force **orthogonally** on a plane perpendicular to HK and take the moment of the projection about the intersection of ilK with the plane (see 4).

Projecting **orthogonally** on a plane perpendicular to the central axis we obtain two reciprocal figures.