May 12, 1899.] 



SCIENCE. 



669 



its heat oontinued in an endless cycle, violates the 

 principles of natural philosophy in exactly the same 

 manner, and to the same degree, as to believe that a 

 clock constructed with a selt-winding movement may 

 fulfil the expectations of its ingenious inventor by 

 going forever." 



It was only by sheer force of reason that 

 geologists have been compelled to think 

 otherwise, and to see that there was a defi- 

 nite beginning, and to look forward to a 

 definite end of this world as an abode 

 fitted for life. 



§11. It is curious that English philoso- 

 phers and writers should not have noticed 

 how Newton treated the astronomical prob- 

 lem. Playfair, in what I have read to you, 

 speaks of the planetary system as being 

 absolutely eternal, and unchangeable ; hav- 

 ing had no beginning and showing no signs 

 of progress towards an end. He assumes 

 also that 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 over- 

 looked Newton's counterblast to the plan- 

 etary 'perpetual motion.' Newton, com- 

 menting on his own ' First Law of Motion,' 

 says, in his terse Latin, which I will en- 

 deavor 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 planetai-y system. 



§ 12. I shall now, without further j^ref- 

 ace, explain, and I hope briefly, so as not 

 to wear out your patience, some of the ar- 

 guments that I brought forward between 

 1S62 and 1869, to show strict limitations 

 to the possible age of the earth as an abode 

 fitted for life. 



Kant* pointed out in the middle of last 



* In an essay first published in the Konigsberg 

 NadiricJiten, 1754, Nos. 23, 24 ; 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 appears in Vol. VI. of Kant's Collected 

 Works, Leipzig, 1839 : Untersuchung der Frage : 



century what had not previously been dis- 

 covered by mathematicians or physical as- 

 tronomers, 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 dis- 

 covery in natural philosophj' seems to have 

 attracted very little attention — indeed to 

 have passed quite unnoticed — among 

 mathematicians and astronomers and nat- 

 uralists, until about 1840, when the doc- 

 trine of energy began to be taken to heart. 

 In 1866, Delauuay suggested that tidal re- 

 tardation of the earth's rotation was prob- 

 ably the cause of an outstanding accelera- 

 tion of the moon's mean motion reckoned 

 according to the earth's rotation as a time- 

 keeper found by Adams in 185.3 by correct- 

 ing 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 time- 

 keeper, in the course of a century, would 

 get 22 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 



Ob die Erde in ihrer Umdrehung um die Achse, 

 wodurch sie die Abwechselung des Tages und der 

 Nacht hervorhringt, einige Veriinderung seit den 

 ersten Zeiten ihres Urspunges erlitten habe, welches 

 die Ursache davon sei, und woraus man sioh ihrer 

 versichern kiinne? welche von der Koniglicben 

 Akademie der Wissenschaften zu Berlin zum Preise 

 aufgeueben worden, 1754. 



* ' 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 com- 

 municated to the Glasgow Geological Society, Feb- 

 ruary 27, 1868. 



