Over the years he refined his arguments and reduced the upper bound by a factor of ten, and in 1897 Thomson, now Lord Kelvin, ultimately settled on an estimate that the Earth was 20–40 million years old.
多年来，他逐渐完善他的论证，将上限降低了十倍；1897年，汤姆森（此时已成为开尔文勋爵）最终得出估计，地球年龄为2000万至4000万年。