242 PROGRESS OF SCIENCE IN THE CENTURY. 



his first shell into the camp of the geologists, which 

 he has not since ceased to bombard. From that date 

 the history has been this, — the physicists have calcu- 

 lated out certain limits; the geologists have agreed 

 that they do not require eternity, but yet much more 

 than the physicists will grant them; there has been 

 much criticism of data and calculations and some 

 reconsideration on both sides; of late the biologists 

 have also insisted on being heard. 



(a) Physical Arguments. — The chief arguments 

 of the physicists as to the age of the earth are based 

 (1) on the downward increase of terrestrial temper- 

 ature, (2) on the retardation of the earth's angular 

 velocity by tidal friction, and (3) on the limitation 

 of the sun's age. Lord Kelvin began by declaring 

 that the age of the earth must be more than twenty 

 millions of years, and less than four hundred mil- 

 lions ; but he subsequently cut down his maximum to 

 the former minimum, and Professor Tait would not 

 allow even half as much. In one of his last utter- 

 ances on the subject. Lord Kelvin states " it was more 

 than twenty and less than forty million years, and 

 probably much nearer twenty than forty." * 



That the physicists are far from being agreed 

 among themselves may be inferred from the frank 

 statement of Professor George Darwin: ^' At pres- 

 ent our knowledge of a definite limit to geological 

 time has so little 'precision that we should do wrong 

 to summarily reject any theories which appear to 

 demand longer periods of tiine than those which now 

 appear allowable/' f 



(h) Geological Arguments: From the rate of 



deposition of roch-formirig materials. — Ever since 



Hutton published his observations and reflections on 



* Pres. Address Victoria Institute for 1897. Phil, Ma§., 

 January, 1899. 



t Rej). Brit, Ass., 1896, p. 518. 



