T. Mellard Reach— The Age of the World. 147 



To Sir Wm. Thomson is due the merit of first attempting to 

 apply the physical, or what I prefer to call the mechanical method, 

 to the determination of the age of the Earth. Three ways have 

 been suggested by him for doing this ; but it is only to the one on 

 which he most relies — founded on the increase of temperature 

 proved by experiment to take place from the surface downwards — 

 that I propose to refer. The calculation is based upon the assump- 

 tion that the whole mass of the Earth was at one moment at an 

 incandescent heat estimated at 7000° Fahr. warmer than our present 

 surface temperature ; that it was at an initial moment at this 

 uniform temperature throughout, from which time it has since been 

 uniformly cooling through being exposed to the present surface 

 temperature. By a mathematical process, the experimental elements 

 of which depend upon the average thermal conductivity of the 

 materials of the Earth, and the average augmentation of heat down- 

 wards, he arrives at the conclusion that it is improbable more than 

 100 millions of years have elapsed since the surface of our globe 

 became habitable. 



Now the geologist neither denies nor affirms that the Earth was 

 once an incandescent mass. By analogy it is probable at the present 

 time the heat goes on increasing downwards until incandescent 

 matter is reached : but it cannot be experimentally proved ; though 

 volcanic action now existing in various parts of its surface points to 

 the same conclusion. It is also not an improbable further deduction 

 that at one time the globe was incandescent from the centre to the 

 surface. Arguing from an hypothesis of this nature, and recon- 

 structing the globe upon it, is what the cosmogonists used to do; but 

 it is a method alien to geology, which aims at interpreting the his- 

 tory of the Earth by an examination of its rocks. 



Now, for a calculation such as this one suggested by Sir Wm. 

 Thomson to command assent, it must be based upon very accurate 

 experiment and observation, that the material points on which it 

 hinges maybe first proved beyond doubt. He says,^ " Whatever the 

 amount of such efi"ects is at any one time, it would go on diminishing 

 according to the inverse proportion of the square roots of the times 

 from the initial epoch. Thus, if at 10,000 years we have 2° per 

 foot of increment below ground, 



at 40,000 years we should have 1° per foot 



160,000 „ „ \° 



4,000,000 „ „ ^° 



100,000,000 „ _ „ ^° . " . . •" 



If, therefore, through any error in the calculation, arising from 

 incorrect inference from experiment, the increment of temperature 

 should be not S'^ but 4° per foot at the end of the first 10,000 years,* 

 the times would be multiplied fourfold, and would stand thus : 



^ Treatise on Natural Philosophy, Thomson and Tait, p. 721. 



2 Prof. Helmholtz has calculated, from the rate of cooling of lavas, that the 

 earth in passing from 2000° to 200° C. must have taken three hundred and fifty 

 millions of years ! Dana, commenting upon this, says : " But the temperature when 

 the Archaean Time (Laurentian and Huronian periods) ended was probably not over 

 38° C. (100° Fahr.)." Dana's Manual of Geology, second edition, p. 147. 



