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



the sun is to go on shining forever, and that the earth is to go on 

 revolving round it forever. He quite overlooked Laplace's nebular 

 theory; and he overlooked Kewton's counterblast to the planetary 

 "perpetual motion." Newton, commenting on his own First Law of 

 Motion, says, in his terse Latin, which I will endeavor to translate: 

 " But the greater bodies of planets and comets, moving in spaces less 

 resisting, keep their motions longer." That is a strong counterblast 

 against any idea of eternity in the planetary system. 



12. I shall now, without further preface, explain, and I hope briefly, 

 so as not to wear out your patience, some of the arguments that I 

 brought forward between 1862 and 1869, to show strict limitations to 

 the possible age of the earth as an abode fitted for life. 



Kant^ l^ointed out in the middle of last century, what had not 

 previously been discovered by mathematicians or physical astronomers, 

 that the frictional resistance against tidal currents on the earth's 

 surface must cause a diminution of the earth's rotational speed. This 

 really great discovery in 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 outstanding 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 uniform.^ 

 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 twenty-two 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 o^g of gravity. At j)resent the radius of the 

 equatorial sea-level exceeds the polar semidiameter by 21^ kilometers, 



'In an essay first published in the Konigsberg Nachrichten, 1754, Nos. 23, 24; 

 having been written with reference to the offer of a prize bj^ the Berlin Academy of 

 Sciences in 1754. Here is the title-page, in full, as it appears in Vol. VI of Kant's 

 Collected Works, Leipzig, 1839: Untersuchung der Frage: Ob die Erde in ihrer 

 Umdrehung um die Achse, wodurch sie die Abwechselung des Tages nnd der Naclit 

 hervorbringt, einige Veriindemng seit den ersten Zeiten ihres Ursprunges erlitten 

 habe, welches die Ursache davon sei, nnd woraus man sich ihrer versichern konne? 

 welche von der koniglichen Akademie der Wissenschaften zu Berlin znm Preise 

 aufgegeben worden, 1754. 



2 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 Feb- 

 ruary 27, 1868. 



