Along CA and DB, on the same side of MN, take MA' = CA, NB' = DB; and join A'B'.
If BN, CP, DQ, FS, GT are the perpen diculars to AE from the angular points, the ordinates NB, PC,..
Taking the case where the motion is strictly in two dimensions, we may write as the polar equation of the surface at time t r =a cos nB cos pt, (4) where p is given by p2 = (n3_ n)..