Mathematics may be compared to a mill of exquisite workmanship, which grinds you stuff of any degree of fineness; but, nevertheless, what you get out depends upon what you put in; and as the grandest mill in the world will not extract wheat-flour from peascod, so pages of formulae will not get a definite result out of loose data.

Indeed, Kelvin had such an exceptional command of mathematics that it was essentially guaranteed that if he had made any mistake, it would not have been in the actual calculations. It was his set of assumptions that provided the input for those calculations that had to be scrutinized.

The first person who, albeit reluctantly, took a stab at searching for a loophole in Kelvin’s original postulates was Kelvin’s former pupil and assistant, the engineer John Perry. By happenstance, Perry studied engineering under James Thomson, Kelvin’s older brother, but later he spent a year in Kelvin’s Glasgow laboratory. While most of Perry’s scientific output focused on electrical engineering and applied physics, he is perhaps best known today for his brief foray into geology.

In August 1894, Robert Cecil, the Third Marquis of Salisbury, delivered a presidential address at the sixty-fourth meeting of the British Association for the Advancement of Science. Salisbury used Kelvin’s estimate of the age of the Earth (one hundred million years) to argue that evolution by natural selection could not have taken place. As is often the case with messages that are too dogmatic, however, this speech achieved precisely the opposite effect to its intentions, at least with John Perry. Salisbury’s denial of the theory of evolution convinced Perry that there had to be something wrong with Kelvin’s calculations. Impressed by the accumulation of geological and paleontological data, Perry wrote to a physicist friend that “once it became clear to my mind that there was necessarily such a flaw [in Kelvin’s estimates], its discovery was no mere question of chance.”

Perry completed the first version of his investigation of the problem of the cooling Earth on October 12, and during the following weeks, he diligently sent copies of the paper to a number of physicists (including Kelvin) for comments. Respectful even in his criticism, Perry signed his letter to Kelvin “Your affectionate pupil.” While about a half dozen physicists expressed support for Perry’s conclusions, Kelvin himself did not bother to respond. Perry was given a second chance when he was invited to a dinner party at Trinity College in Cambridge, a dinner that Kelvin was also supposed to attend. The opportunity to talk to Kelvin in person was too good to be missed. Perry excitedly described the event to a friend the following day: