. . . my entire failure to catch the object of your paper. For I seem to gather that you don’t object to Lord Kelvin’s mathematics. Why, then, drag in mathematics at all, since it is absolutely obvious that the better conductor the interior in comparison with the skin, the longer ago must it have been when the whole was at 7000°F [Kelvin’s assumed melting point for rocks]: the state of the skin being as at present? I don’t suppose Lord Kelvin would care to be troubled with a demonstration of that.

Tait appears to have missed the point entirely. Since no one at the time could tell with any certainty what the conditions at great depths really were, what one assumed for the purpose of the calculation was a matter of mere conjecture. Perry’s intention was simply to show that if one made a different assumption from Kelvin’s about the Earth’s interior—that at the bowels of the Earth heat was transported more easily than in the Earth’s outer skin—then calculations based on physical principles could be made compatible with the old age that the geologists and biologists were requiring. Kelvin’s blunder was in not realizing that the latitude allowed by the existing observations could introduce a much larger uncertainty into his estimated age than he was willing to acknowledge.

In his response to Tait, Perry attempted to be polite, noting, “You say I am right, and you ask my object. Surely Lord Kelvin’s case is lost, as soon as one shows that there are possible [emphasis added] conditions as to the internal state of the earth which will give many times the age which is your and his limit.” In language probably reflecting the admiration of a former assistant, he then added, “What troubles me is that I cannot see one bit that you have reason on your side, and yet I have been so accustomed to look up to you and Lord Kelvin, that I think I must be more or less of an idiot to doubt when you and he were so ‘cocksure.’ ”

Figure 10