While the idea was fascinating, turning it into an actual number for the age of the Earth was extremely tricky. Kelvin himself admitted, “It is impossible, with the imperfect data we possess as to the tides, to calculate how much their effect in diminishing the earth’s rotation really is.” Nevertheless, Kelvin felt that even the mere fact that one could place a limit on the age of the Earth, no matter how uncertain, was sufficient to refute the uniformitarian notion of an inconceivably vast time. Referring to his own numerical estimate of a 22-second retardation per century in the Earth’s rotation period, he concluded, “[Whether] the earth’s lost time is 22 seconds, or considerably more or less than 22 seconds, in a century, the principle is the same. There cannot be uniformity. The earth is filled with evidences that it has not been going on for ever in the present state, and that there is a progress of events toward a state infinitely different from the present.”

Disappointingly for Kelvin, the estimate based on the Earth’s spin rate did not last for very long, at least not in any quantitative way. As fate would have it, none other than George Howard Darwin, Charles Darwin’s fifth child, showed the argument to be useless for an age estimate. George was a physicist with considerable mathematical dexterity. He attacked the problem of the spinning Earth with infinite patience and attention to detail. In a series of papers published mainly between 1877 and 1879, the younger Darwin was able to demonstrate that, contrary to Kelvin’s expectation, the Earth could continue to gradually change its shape even as its rotation rate was slowing down. This was a consequence of the fact that even a solidified Earth was not completely rigid. The bottom line was unequivocal. Darwin showed that given the many uncertainties about the Earth’s interior, there was no reliable way to calculate the planet’s age from its spin.

Needless to say, Charles Darwin was delighted to discover that his own son managed to “stagger” the great Kelvin, and he exclaimed, “Hurrah for the bowels of the earth and their viscosity and for the moon and for the Heavenly bodies and for my son George.”

But George Darwin’s work did not affect Kelvin’s main claims—it only established that Kelvin’s third argument (concerning the Earth’s rotation) could not be used to support the value of the estimate of the age of the Earth. There was another sense, however, in which Darwin’s work was revealing. It showed that even the august Lord Kelvin was not infallible. As we shall see in the next chapter, this may have helped to open the door for further criticism.

To describe the age-of-the-Earth controversy as a battle to the death between physics and geology would be a mistake. While certainly there was tension along disciplinary lines, Kelvin saw himself so much in the mainstream of British geology that in his address at the Glasgow Geological Society meeting in 1878 he did not hesitate to declare, “We, the geologists [emphasis added], are at fault for not having demanded of the physicists experiments on the properties of matter.” This “flexible” self-identification reflected the less compartmentalized scientific world of the nineteenth century. Victorian scientists freely attended meetings of societies that formally represented other branches of science. Rather than a dispute between disciplines, therefore, the age-of-the-Earth debate was largely a clash between Kelvin and the doctrine of some geologists.