It wasn’t surprising, then, that in 1868, when Kelvin delivered an address before the Geological Society of Glasgow, he chose as the target for his acrimonious criticism the first text that had brought the principle of uniformitarianism (formulated by James Hutton) to the attention of a wide audience. This was the 1802 book Illustrations of the Huttonian Theory of the Earth by the Scottish scientist John Playfair. From this book, Kelvin cited the following stunning passage, which to him represented the epitome of the orthodox opinion of the geologists of the day:

How often these vicissitudes of decay and renovation have been repeated it is not for us to determine; they constitute a series of which, as the author of this theory [Hutton] has remarked, we neither see the beginning nor the end, a circumstance that accords well with what is known concerning other parts of the economy of the world . . . in the planetary motions where geometry has carried the eye so far both into the future and the past, we discover no mark either of the commencement or the termination of the present order. It is unreasonable, indeed, to suppose that such marks should exist anywhere [emphasis added]. The Author of nature has not given laws to the universe which, like the institutions of men, carry in themselves the elements of their own destruction. He has not permitted in His works any symptoms of infancy or of old age, or any sign by which we may estimate either their future or their past duration. He may put an end, as He, no doubt, gave a beginning to the present system, at some determinate time; but we may safely conclude that the great catastrophe will not be brought about by any of the laws now existing, and that it is not indicated by anything which we perceive.

Kelvin’s reaction to this excerpt was merciless. “Nothing,” he said, “could possibly be further from the truth.” Explaining again his argument in layman’s terms, he noted:

The earth, if we bore into it anywhere, is warm, and if we could apply the test deep enough, we should, no doubt, find it very warm. Suppose you should have here before you a globe of sandstone, and boring into it found it warm, boring into another place found it warm, and so on, would it be reasonable to say that that globe of sandstone has been just as it is for a thousand days? You should say, ‘No; that sandstone has been in the fire, and heated not many hours ago.’ It would be just as reasonable to take a hot water jar, such as is used in carriages, and say that that bottle has been as it is for ever—as it was for Playfair to assert that the earth could have been for ever as it is now, and that it shows no traces of a beginning, no progress towards an end.

To strengthen his argument further, Kelvin decided not to rely just on his old reasoning about the Earth and the Sun. He came up with yet a third line of evidence, based on the Earth’s rotation around its axis. The concept itself was ingenious and easy to understand. An initially molten Earth would have assumed, due to its spin, a slightly oblate shape: more flattened at the poles and more bulging at the equator. The faster the initial spin, the less spherical the resulting shape. This form, Kelvin inferred, would have been preserved upon the Earth’s solidification. Precise measurements of the deviation from sphericity could therefore be used to determine the original rate of rotation. Since tides caused by the Moon’s gravity were expected to act like friction and slow down the rotation, one could estimate how much time was required to reduce the initial spin rate to the present one rotation every twenty-four hours.