72 Lord Kelvin on the Age of the 



Natural Philosophy seems to have attracted very little 

 attention, — indeed to have passed quite unnoticed, — among 

 mathematicians, and astronomers, and naturalists, until about 

 1840, when the doctrine of energy began to be taken to 

 heart. In 1866, Delaunay suggested that tidal retardation 

 of the earth's rotation was probably the cause of an out- 

 standing acceleration of the moon's mean motion reckoned 

 according to the earth's rotation as a timekeeper found by 

 Adams in 1853 by correcting a calculation of Laplace which 

 had seemed to prove the earth's rotational speed to be uni- 

 form *. Adopting Delaunay's suggestion as true, Adams, in 

 conjunction with Professor Tait and myself, estimated the 

 diminution of the earth's rotational speed to be such that 

 the earth as a timekeeper, in the course of a century, would 

 get 22 seconds behind a thoroughly perfect watch or clock 

 rated to agree with it at the beginning of the century. 

 According to this rate of retardation the earth, 7,200 million 

 years ago, would have been rotating twice as fast as now : 

 and the centrifugal force in the equatorial regions would 

 have been four times as great as its present amount, which 

 is 2J9 of gravity. At present the radius of the equatorial 

 sea-level exceeds the polar semi-diameter by 21-J- kilometres, 

 which is, as nearly as the most careful calculations in the 

 theory of the earth's figure can tell us, just what the excess 

 of equatorial radius of the surface of the sea all round would 

 be if the whole material of the earth were at present liquid 

 and in equilibrium under the influence of gravity and centri- 

 fugal force with the present rotational speed, and £ of what 

 it would be if the rotational speed were twice as great. 

 Hence, if the rotational speed had been twice as great as 

 its present amount when consolidation from approximately 

 the figure of fluid equilibrium took place, and if the solid 

 earth, remaining absolutely rigid, had been gradually slowed 

 down in the course of millions of years to its present speed 

 of rotation, the water would have settled into two circular 

 oceans round the two poles : and the equator, dry all round, 

 would be 64*5 kilometres above the level of the polar sea 

 bottoms. This is on the supposition of absolute rigidity of 

 the earth after primitive consolidation. There would, in 

 reality, have been some degree of yielding to the gravitational 

 tendency to level the great gentle slope up from each pole to 

 equator. But if the earth, at the time of primitive consolida- 



* ' Treatise on Natural Philosophy ' (Thomson and Tait), §830, ed. 1, 

 1867, and later editions ; also ' Popular Lectures and Addresses/ vol. ii. 

 (Kelvin), ' Geological Time,' being a reprint of an article communicated 

 to the Glasgow Geological Society, February 27th, 1868. 



