  We expand the equations used in MacDonald's 1964 theory and Fourier analyze
the tidal variations of the height at one point on the Earth surface, and also
the tidal potential at such point. It is shown that no intrinsic law is
relating the lag of the tide components to their frequencies. In other words,
no simple rheology is being intrinsically fixed by MacDonald's equations. The
same is true of the modification proposed by Singer(1968). At variance with
these two cases, the modification proposed by Williams and Efroimsky (2012) fix
the standard Darwin rheology in which the lags are proportional to the
frequencies and their model is, in this sense, equivalent to Mignard's 1979
formulation of Darwin's theory.
