In Mr. Fearn's article on this subject in the February number of the Journal, in which a solution is given in the form of a Fourier series, a reference is made to the unsatisfactory results of an attempt to use double differentiation of the displacement curve. It is generally recognised that such a process is fundamentally inaccurate, and analytical methods, either exact or approximate, are usually given in books dealing with mechanisms. For mechanisms of the piston-connecting rod type, in which the higher harmonics of the motion of the driven member diminish very rapidly, two harmonics in all generally give all the necessary accuracy, both for displacement and acceleration. Mr. Fearn's solution is of this type, though he includes a fourth harmonic (see e.g., the expression for ρ at the top of page 138).