18 THE KIGHT HON. LOED KELVIX, G.C.V.O., ON 



dilations in the theory of the earth's fi_c;ure can tell ns, just 

 what the excess of equatorial radius of the surface of the 

 sea all round would be if the wliole material of the earth 

 were at present liquid and in equilibrium under the in- 

 fluence of gravity and centrifugal 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 con- 

 sohdation 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 Avould 

 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 sup- 

 position 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 consolidation, had been rota- 

 ting twice as fast as at present, or even 20 per cent, faster 

 than at present, traces of its present figure must have been 

 left in a great preponderance of land, and probably no sea 

 at all, in the equatorial regions. Taking into account all 

 uncertainties, whether in respect to Adams' estimate of the 

 rate of frictional retardation of the earth's rotatory speed, 

 or to the conditions as to rigidity of the earth once consoli- 

 dated, we may safely conclude that the earth was certainly 

 not solid 5,000 million years ago, and was probably ]iot solid 

 1,000 milhon years ago.* 



§ 13. A second argument for limitation of the earth's age, 

 which was really my own first argument, is founded on the 

 consideration of underground heat. To explain a first rough 

 and ready estimate of it I shall read one short statement. 

 It is from a very short paper that I communicated to the 

 Royal Society of Edinburgh on the 18th December, 1865, 



* "The fact that the continents are arranged along meridians, rather 

 than in an equatorial belt, affords some degree of proof tliat the consoli- 

 dation of the earth took place at a time when the diiu-nal rotation ditfei'ed 

 Ijut little from its present value. It is probable that the date of consoli- 

 dation is considerably more recent than a thousand million years ago." 

 Thomson and Tait. Treatise on Natural Philosophy, 2ud Edition, 1883, 

 § 830. 



