Eroica,

Are you on slightly dodgy ground with this one? Thomsonś calculations failed for two reasons - 1- he didnt know about radioactivity and 2- he also didnt know about convection within the molten core. The radioactivity bit explains about 60% of the problem and is the principal fault and was the reason that Thomson conceded defeat in 1905.

However I think youŕe getting at the fact that Thomson based his calculations on conduction (Fourier theory) and radiative cooling, neglecting the possibility that the interior of the earth would be molten and therefore convective.