ROTATION AND SHAPE OF THE EARTH 9 



unable to alter Its equatorial radius by a few miles 

 under the action of tremendous forces constantly tending 

 to alter It, and having 1,000 million years in which to do 

 the work. 



With this flaw in the case it is hardly necessary to 

 insist on our great uncertainty as to the rate at which 

 the tides are lengthening the day. 



The spectacle presented by the geologist and biologist, 

 deeply shocked at Lord Kelvin's extreme uniformitarian- 

 ism in the domain of astronomy and cosmic physics, is 

 altogether too comforting to be passed by without remark ; 

 but in thus indulging in a friendly tu qiioque, I am quite 

 sure that I am speaking for every member of this Section 

 in saying that we are in no way behind the members of 

 Section A in our pride and admiration at the noble work 

 which he has done for science, and we are glad to take 

 this opportunity of congratulating him on the half-century 

 of work and teaching — both equally fruitful — which has 

 reached its completion in the present year [1896]. 



The second argument is based upon the cooling of 

 the earth, and this is the one brought forward and 

 explained by Lord Salisbury in his Presidential Address. 

 It has been the argument on which perhaps the chief 

 reliance has been placed, and of which the data — so it 

 was believed — were the least open to doubt. 



On the Sunday during the meeting of the British 

 Association at Leeds (1890), I went for a walk with 

 Professor Perry, and asked him to explain the physical 

 reasons for limiting the age of the earth to a period 

 which the students of other sciences considered to be 

 very Inadequate. He gave me an account of the data 

 on which Lord Kelvin relied in constructing this second 

 argument, and expressed the strong opinion that they 

 were perfectly sound, while, as for the mathematics, 

 it might be taken for granted, he said, that they were 

 entirely correct. He did not attach much weight to the 

 other arguments, which he regarded as merely offering 

 support to the second. 



This little piece of personal history is of interest, 



