PROF. H. STANLEY ALLEN has retired from the chair of natural philosophy in the University of St. Andrews. His life in St. Andrews has been one of quiet painstaking work characterized by thoroughness in everything he undertook. His highest quality was a constant striving for the clearest, the best, way in which he could present a point. A simple example is his treatment of entropy. He has stated that in his opinion it is necessary that the beginner in a scientific subject should be given at the outset some familiar mental picture. He then points out that momentum is a vector quantity depending on the first power of the velocity, and remarks that therefore it is not likely to be a suitable analogy for entropy. It has to be remembered that Kelvin's definition of entropy makes it vary directly with the heat and inversely with the temperature. He then points out that on the basis of the kinetic theory of gases, not only the square of the velocity is involved, but also the time taken by one molecule to pass over the space in the near neighbourhood of any other. He finally shows that Callendar's identification of the energy with the form ½QV2 gives agreement. Prof. Allen has spent twenty years of his life in St. Andrews, and the sympathy of everyone who knows him will go out to him in his recent great loss through the death of his wife. All will entertain the hope that the greater freedom from routine labour consequent on his retirement may enable him to devote more time directly to the service of the science in which he takes his delight.