The method used to write down the preceding solution is based on the well-known analogy of one-dimensional heat flow with the flow of an electric current in a simple transmission line having only series resistance and parallel capacitance. Thus when a / ' temperature wave," propagated from the heat source, arrives at a point where the thermal properties change abruptly, part of the wave is reflected at, and part passes through the discontinuity. The respective wave strengths are determined by easily calculated reflection and transmission coefficients. The temperature at a point is then made up of all the contributions from all the possible wave paths. Due to dissipation effects, in general, the longer the path length, the smaller the contribution. For the problems where this method is particularly useful, the transient temperature for "short" times only is required and all but a few terms are negligible. This method is fully described in reference 2 for many different types of heat source and for many different boundary conditions. A simpler description of the method is also given in reference 3 for a somewhat more restricted range of boundary conditions. The solutions given by Wassermann in reference 1 lack terms of the form F(0 — 0) and are thus incomplete for 6 > 6.