630 



SCIENCE. 



[N. S. Vol. IV. No. 96. 



vin's extreme uniformitarianism in the do- 

 main of astronomy and cosmic physics, is 

 altogether too comforting to be passed by 

 without remark ; but, in thus indulging in 

 a friendly tu quoque, I am quite sure that I 

 am speaking for every member of this Sec- 

 tion 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. 



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 Prof. Perry and asked him to 

 explain the physical reasons for limiting the 

 age of the earth to a period which the stu- 

 dents of other sciences considered to be very 

 inadequate. He gave me an account of the 

 data on which Lord Kelvin relied in con- 

 structing this second argument, and ex- 

 pressed the strong opinion that they were 

 perfectly sound, while, as for the mathe- 

 matics, 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, inasmuch as Prof. Perry has now 

 provided us with a satisfactory answer to 

 the line of reasoning which so fully satisfied 

 him in 1890. And he was led to a critical 

 examination of the subject bj?^ the attitude 

 taken up by Lord Salisbury in 1894. Prof. 

 Perry was not present at the meeting, but 



when he read the President's address, and 

 saw how other conclusions were ruled out, 

 of court, how the only theory of evolution 

 which commands anything approaching uni- 

 versal assent was set on one side because of 

 certain assumptions as to the way in which 

 the earth was believed to have cooled, he 

 was seized with a desire to sift these as- 

 sumptions and to inquire whether they 

 would bear the weight of such far-reaching 

 conclusions. Before giving the results of 

 his examination, it is necessary to give a 

 brief account of the argument on which so 

 much has been built. 



Lord Kelvin assumed that the earth is a 

 homogeneous mass of rock similar to that 

 with which we are familiar on the surface. 

 Assuming, further, that the temperature in- 

 creases, on the average, 1° F. for every 50 

 feet of depth near the surface everywhere, 

 he concluded that the earth would have oc- 

 cupied not less than twenty, nor more than 

 four hundred million, years in reaching ita 

 present condition from the time when it 

 first began to consolidate and possessed a^ 

 uniform temperature of 7,000° F. 



If, in the statement of the argument, we 

 substitute for the assumption of a homo- 

 geneous earth an earth which conducts heat 

 better internally than it does towards the 

 surface. Prof. Perry, whose calculations^ 

 have been verified by Mr. O. Heaviside, 

 finds that the time of cooling has to be 

 lengthened to an extent which depends 

 upon the value assigned to the internal con- 

 ducting power. If, for instance, we assume 

 that the deeper part of the earth conducts 

 ten times as well as the outer part, Lord 

 Kelvin's age would require to be multiplied 

 by fifty-six. Even if the conductivity be 

 the same throughout, the increase of density 

 in the deeper part, by augmenting the ca- 

 pacity for heat of unit volume, implies a^ 

 longer age than that conceded by Lord 

 Kelvin. If the interior of the earth be 

 fluid or contain fluid in a honeycomb struc- 



