I AM much obliged to Mr. Wright for his correction (NATURE, July 8, p. 40). I had made a search in English and Continental books on curves and concluded that this spiral had been overlooked as a curve. But it appears to have been recognised in connexion with the spherical pendulum. Prof. Lamb in his “Dynamics,” p. 288, as I now find, refers to the curve as “a kind of elliptic spiral,” and Dr. Besant describes it as “an ellipse gradually shrinking in size.”