670 



SCIENCE. 



[N. S. Vol. IX. No. 228. 



been four times as great as its iDresent 

 amount, which is -t\-^ of gravity. At pres- 

 ent the radius of the equatorial sea-level 

 exceeds the polar semi-diameter by 21-J- kilo- 

 meters, which is, as nearly as the most 

 careful calculations in the theory of the 

 earth's figure can tell us, just what the ex- 

 cess 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 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 rota- 

 tional speed had been twice as great as its 

 present amount when consolidation from 

 approximately the figure of fluid equilib- 

 rium took place, and if the solid earth, re- 

 maining absolutely rigid, had been gradu- 

 ally slowed down in the course of millions 

 of years to its present speed of rotation, 

 the water would have settled into two cir- 

 cular oceans round the two poles ; and the 

 equator, dry all rovind, would be 64.5 kilo- 

 meters 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 

 fdst as at present, or even 20 per cent, 

 faster than at present, traces of its present 

 figure must have been left in a great pre- 

 ponderance 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 

 frlctional retardation of the earth's rotatory 

 speed, or to the conditions as to the rigidity 

 of the earth once consolidated, we may 

 safely conclude that the earth was cer- 

 tainlj' 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 considera- 

 tion of undergrouud 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, 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 Geolo- 

 gists, assumes that the earth's surface and upper 

 crust have been nearly as they are at present in tem- 

 perature, and other physical qualities, during millions 

 of millions of years. But ilic heat which we know, 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° C, a 

 quantity of ordinary surface rock of 100 times the 

 earth's bulk. This would be more than enough to 

 melt a mass of surface rock equal in bulk to the 

 whole earth. No hypothesis as to chemical action, 

 internal fluidity, effects of pressure at great depth, or 

 possible character of substances in the interior of the 

 earth, possessing the smallest vestige of probability, 

 can justify the supposition that the earth's upper 

 crust has remained nearly as it is, while from the 

 whole, or from any part, of the earth, so great a 

 quantity of heat has been lost. ' ' 



§ 14. The sixteen words which I have 

 emphasized in reading this statement to you 

 (italics in the reprint) indicate the matter- 

 of-fact foundation for the conclusion as- 

 serted. This conclusion suf&ces to sweep 

 away the whole system of geological and 

 biological speculation demanding an ' in- 



* 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 present value. It is prob- 

 able that the date of consolidation is considerably 

 more recent than a thousand million years ago." — 

 Thomson and Tait, ' Treatise on Natural Philosophy, ' 

 2d ed., 18S3, | 830. 



