It has been clearly demonstrated that the rectal cooling curve does not obey Newton's Law, which is exponential. The first success in modelling rectal cooling mathematically was achieved by Marshall and Hoare [1]. An amendment was made to the simple exponential curve which led to a good mathematical model, exhibiting the three main sections of rectal cooling, i.e. lag, linear and quasi-exponential. The resultant method of postmortem interval estimation required a knowledge of the body mass and height. The present study has led to a totally different amendment to Newton's Law, which provides a means of postmortem interval estimation from body temperature data only. The derivation of the method, with a background on Newton's Law follows.