io THE AGE OF THE EARTH 



inasmuch as Professor 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 by the attitude taken up 

 by Lord Salisbury in 1894. Professor 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 com- 

 mands anything approaching universal 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 assumptions, 

 and to inquire whether they would bear the weight of 

 such far-reaching conclusions. Before stating 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 

 increases, on the average, i° F. for every fifty feet of 

 depth near the surface everywhere, he concluded that 

 the earth would have occupied not less than twenty, 

 nor more than four hundred, million years in reaching 

 its 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 homogeneous earth an earth 

 which conducts heat better internally than it does 

 towards the surface, Professor 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 capacity for heat of 

 unit volume, implies a longer age than that conceded 



