’s at a point vary when we change our mesh-system. In the majority of cases they also vary from place to place throughout the same mesh-system. If, however, the meshes are always orthogonal at their points of intersection,
vanishes at every point of the surface. If the
and
lines always have the same slant over the surface and are always equally spaced, the three
’s will remain constant throughout; their values being given by 1,