“…coordinates of the shell (middle) surface. A point in the shell body may then be described by the position vector x(4,, 42, ) = r(4,, 2) + n(4,, 52) where n is the unit normal at the point r(4 1 , 52) of the middle surface (C = 0). The unit vectors Tj = x i/ai with ai = ac(l + /CR), for i = 1, 2, are tangent to the surface coordinate lines k = constant, C = constant, k $ i respectively.…”