8 THE AGE OF THE EARTH 



length of our day. Huxley explains the reason with his 

 usual lucidity : ' That this must be so is obvious, if one 

 considers, roughly, that the tides result from the pull 

 which the sun and the moon exert upon the sea, causing 

 it to act as a sort of break upon the solid earth.' x 



A liquid earth takes a shape which follows from its 

 rate of revolution, and from which, therefore, its rate of 

 revolution can be calculated. 



The liquid earth consolidated in the form it last 

 assumed, and this shape has persisted until now, and 

 informs us of the rate of revolution at the time of con- 

 solidation. Comparing this with the present rate, and 

 knowing the amount of lengthening in a given time due 

 to tidal friction, we can calculate the date of consolidation 

 as certainly less than 1,000 million years ago. 



The argument is fallacious, as many mathematicians 

 have shown. The present shape tells us nothing of the 

 length of the day at the date of consolidation ; for the 

 earth, even when solid, will alter its form when exposed 

 for a long time to the action of great forces. As 

 Professor Perry said in a letter to Professor Tait : 3 ' I 

 know that solid rock is not like cobbler's wax, but 1,000 

 million years is a very long time, and the forces are 

 great.' Furthermore, we know that the earth is always 

 altering its shape, and that whole coast-lines are slowly 

 rising or falling, and that this has been true, at any rate, 

 during the formation of the stratified rocks. 



This first argument is dead and gone. 3 We are, indeed, 

 tempted to wonder that the physicist, who was looking 

 about for reasoning by which to revise what he con- 

 ceived to be the hasty conclusions of the geologist as to 

 the age of the earth, should have exposed himself to 

 such an obvious retort in basing his own conclusions 

 as to its age on the assumption that the earth, which 

 we know to be always changing in shape, has been 



1 Anniversary Address to Geol. Soc. 1869. 



2 Nature, January 3, 1895. 



3 It must not be forgotten, however, that this argument and those 

 ■which follow it have done good work in modifying the unreasonable 

 demands of geologists a quarter of a century ago. 



