In a recently published book, The Aerodynamics of Propulsion, by Küchemann and Weber, there is included a brief discussion of heat flow in an air-air heat exchanger. The treatment given rests on an assumption which appears to be generally accepted, but which seems to the present writer to be physically unsatisfactory and to yield at least one misleading result. It is the purpose of the present note to show that a solution of the heat transfer equations can be obtained using an alternative assumption which seems physically more acceptable.