Letter from Thomas Gold to Dr Donald Menzel, Harvard College Observatory, Cambridge, Massachusetts Says that 'I am sure that the note on the efficiency is basically correct and I am also sure that a more detailed discussion will not change the results very much ... A more detailed discussion of the problem certainly requires an analysis of all the longitudinal and the transverse modes of propagation that occur in the vicinity of a surface in practice in the presence of a stratification of density and velocity of sound. It is possible that such a treatment exists in the literature but I have not found it'. There is obviously no hope of us puzzling all this out right away now. I am therefore in favor of going ahead with what you sent me'. 27 October 1966