148 T. Mellard Reade—The Age of the World. 



At 40,000 years 2° per foot 



40,000 



years 2° 



160,000 



1° 



4,000,000 



1° 



100,000,000 



1 " 



5J 2 5 



400,000,000 



„ -aV" 



,600,000,000 



1 o 



JJ 10 



So, if we assume the increment at 1° in 100 feet, and there are to 

 my mind as good reasons to do so as to assume 1° in 50 feet, the 

 observations being, as I shall show, so contradictory and inconclusive, 

 it would in this case bring up the age of the world to 1600 millions 

 of years. These values may hold good with regard to a homogeneous 

 mass of uniform conductivity, but not in the case of a globe built 

 up of strata of varying conductivities. Let us, for example, con- 

 sider how the eifect would be varied in a very simple case. 



First, we may assume a globe, the mass of which below a certain 

 depth is of unknown materials and unknown conductivity, — as is 

 really the case with the Earth, but that it is a reservoir of heat 

 which it transmits to a surface layer of rock 3500 feet deep. — having 

 a conductivity of -006,^ and that in 100 million years it has cooled 

 down so that this surface stratum possesses an increment of -ro of 

 a degree of heat per foot. 



Secondly, let us assume a similarly unknown globe having a 

 surface layer of rocks also 3500 feet deep, but composed of two 

 equal layers, the external one having a conductivity of "003, and 

 the inner layer -009, the average conductivity of the two strata 

 would be '006, the same as before ; but as the increment of heat 

 varies inversely as the conductivity, with the same flow of heat, the 

 outer layer would increase in temperature at the rate of ^ of a 

 degree per foot, and the inner one -vV of a degree per foot. Thus 

 the average rate of increase of temperature with the same loss of 

 heat taking place would be -g-g- of a degree per foot.^ The conductivity^ 

 averaging the same as in the first example, and the increment of heat 

 being greater, the inferred age of the globe would be less than 100 

 million years. 



But, as we have already assumed, only the same loss of heat taking 

 place, the actual age of the globe, if its constitution in other 

 respects were the same, in both cases would appear to be more 

 correctly estimated at the same figure. Should, however, the calcu- 

 lation be based upon the rate of increase of the outer layer and the 

 average conductivity of the two layers, a practical error that might 

 readily occur in estimating the increment of heat in borings, and 

 the average conductivity of the materials of the Earth within our 

 reach, the age of the Earth would be inferred as only 25 million 

 years.^ It is, however, quite evident that all these conclusions 



^ I adopt in all cases the British Association Unit of Conductivity, viz. centimetre — 

 gramme — second. 



^ This is arrived at by calculating the total increase of heat downwards in hoth 

 cases and dividing by 3500 feet the total thickness of the strata. 



3 In these illustrations, and my reasoning throughout, for the sake of simplicity I 

 omit all consideration of specific heats, as they do not aifect the line of argument 

 adopted, except that in nature they further complicate the thermal effects. 



I 



