220 SEE— TEMPERATURE, SECULAR COOLING [April 20, 



obviously the proper one. !For the substratum is shown to move only 

 under the throes of an earthquake, and no circulatory movement of 

 lava exists even just beneath the crust. Hence we adhere to the 

 result there obtained, and must consider the significance of the small 

 age of the earth. It will be seen that for a thickness of 22 miles, the 

 age of the earth's consolidation is almost exactly the same as that 

 reached by the use of Lord Kelvin's formula. 

 § II. Remarkable Agreement of the Times Since the Consolidation 



of the Globe as Concluded from these Two Methods. 



The result found by Lord Kelvin's method rests upon the observed 

 rate of increase of temperature downward, namely 1° Fahr. for 51 

 feet, which is about the same as the value used by Fourier nearly a 

 century ago, and not improved upon by the deep borings made in 

 recent years. It also rests upon the assumed surface temperature of 

 2000° Fahr., which probably is comparatively near the truth. Is it 

 purely an accidental coincidence that with these data one is led by the 

 Fourier- Kelvin formula to an age of 8,302,210 years, while by 

 Fisher's formula, depending on the thickness of the crust essentially 

 verified by earthquake phenomena, one finds the almost identical age 

 of 8,248,380 years ? Moderate variations of the data might derange 

 this excellent agreement somewhat, but probably no change of the 

 constants within admissible limits would produce extreme discordance 

 in the resulting ages of the earth. It seems therefore difficult to 

 escape the conclusion that these figures really approximate the true 

 age of our encrusted planet. At least the period since the consolida- 

 tion is of the order of ten million years. 



Different investigators will naturally form different estimates of 

 the age of the earth as found by the several methods of approxima- 

 tion ; but it is difficult to see how the larger values formerly current 

 can be justified by physical research based on the propagation of heat 

 involved in the secular cooling of the globe. The writer has not the 

 geological learning requisite for the use of the methods based on 

 sedimentary rocks and their deposits of organic remains, but it seems 

 very doubtful if these methods can lay claim to even approximate 

 accuracy ; and to most minds the conclusions drawn from the physical 

 methods will naturally carry much greater weight. 



Some considerations, however, based on the probable average 



