RAYLEIGH1 was not quite satisfied in having to obtain his equation for speed of wave energy by indirect means, from consideration of 'interference groups'and 'variation in wave-length', and he searched for an alternative method; but the one he devised was also indirect, so that his result still rested on mathematical hypothesis, and was devoid of any obvious explanation. It seems to have remained in this state until Sverdrup and Munk2 recently produced a simple description of what happens to the energy in deep water, and the present purpose is to amplify their description, and show why the speed of the energy of sea waves ranges from a half to the whole of the wave-speed, according to the value of the depth-length ratio, D/L.