1. The discussion of the motion of bodies of varying mass recently revived by Mr. A. S. Ramsey's article in the Gazette for July 1941 has produced many sidelights on the momentum method of treating these problems, but not one of your correspondents has suggested the alternative approach by the method of energy. This method was shown to me (and, if I am not greatly mistaken, also to Mr. Ramsey) by one of the ablest, if least known, of the last generation of Cambridge teachers of mathematics, William Welsh of Jesus, but it does not yet seem to have reached the textbooks. It has one great advantage over the momentum approach in that it forces attention on the physical assumptions involved, the slurring of which is the main cause of the difficulty experienced by many students in applying the equations of momentum.