AGE OF THE EARTH AS AN ABODE FITTED FOR LIFE. 343 



wliicli is, as nearly as most careful calculations in the tlieory 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 centrifugal force with the present rotational speed, and 

 one-fourth 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 kilometers above the level of the polar sea bottoms. 

 This is on the supposition of absolute rigidity of the earth after primi- 

 tive 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 rotating 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 retarda- 

 tion of the earth's rotatory speed, or to the conditions as to rigidity of 

 the earth once consolidated, we may safely conclude that the earth was 

 certainly not solid 5,000 million years ago, and was probably not solid 

 1,000 million 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 under- 

 ground 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 com- 

 municated to the Eoyal Society of Edinburgh on the 18th of December, 

 1865, entitled " The Doctrine of Uniformity in Geology briefly refuted." 



"The 'Doctrine of Uniformity' in geology, as held by many of the 

 most eminent of British geologists, assumes that the earth's surface 

 and upper crust have been nearly as they are at present, in temperature 

 and other physical qualities, during millions of millions of years. But 

 the heat which tee Imow, by observation, to be now conducted out of the 

 earth yearly is so great that if this action had been going on with any 

 approach to uniformity for 20,000 million years the amount of heat lost 

 out of the earth would have been about as much as would heat by 

 100° 0. a quantity of ordinary surface rock of one hundred times the 

 earth's bulk. This would be more than enough to melt a mass of sur- 



' '' The fact that the continents are arranged along meridians, rather than in an 

 equatorial belt, affords some degree of proof that the consolidation of the earth took 

 place at a time when the diurnal rotation differed but little from its j)resent value. 

 It is probable that the date of consolidation is considerably more recent than a 

 thousand million years ago." Thomson and Tait, Treatise on Natural Philosophy, 

 second edition, 1883, ^ 830, 



