THE AGE OF THE EARTH. 17 



hope briefly, so as not to wear out your patience, some of the 

 arguments that I brought forward between 18 02 and 1869, 

 to show strict limitations to the possible age of the earth as 

 an abode fitted for hfe. 



Kant* pointed out in the middle of last century, what had 

 not previously been discovered by mathematicians or physical 

 astronomers, that the frictional resistance against tidal cur- 

 rents on the eai-th's surface must cause a diminution of tlie 

 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 out- 

 standing 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 uni- 

 form.! 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 22 seconds behind a thoroughly perfect watch or clock 

 rat.ed to agree with it at the beginning of the century. 

 According to this rate of retardation the earth, 7,200 

 milHon 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 Tj-^-y of gravity. At present the radius of the 

 equatorial sea-level exceeds the polar semi-diameter by 

 21^ kilometres, which is, as nearly as the most careful cal- 



* In an essay first published in the Koenigsbert^ Nachricliten, 1754, 

 Nos. 23, 2.4: ; having been written with reference to the offer of a prize by 

 the Berlin Academy of Sciences in 1754. Here is the title-page, in full, 

 as it ajjpears 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 und der Nacht hervorbringt, 

 einige Veriinderung seit den ersten Zeiten ihres Ursprunges erlitten habe, 

 welches die Ursache davon sei, und woraus man sich ihrer versicheni 

 konne ? welche von der Koniglichen Akademie Der Wissenschaftea zu 

 Berlin zum Preise aufgegeben worden, 1754. 



t T/-;;afisp. on JVatural P/alosophi/ {Thommn and Tait) § 830, ed. 1, 1867, 

 and later editions ; also Popular Lectures and Add7-esses, vol. ii (Kelvin). 

 Geological Time heing a reprint of an article communicated to the Glasgow 

 Geological Society February 27t]i, lH(i8. 



C 



