In the first edition of "On the origin of species" (1859), Darwin had estimated that the erosion of the Sussex Weald must have taken 300 million years. Both he and Lyell were surprised when the physicist William Thomson (Lord Kelvin) said the Earth could not be as old as they thought. He made the calculation based on how long it must have taken the Earth to cool down to its present temperature, given a starting point of 2,000C. Kelvin's result was based on the idea that the near-surface geothermal gradient reflects the conductive cooling of the solid Earth.