﻿398 Scientific Intelligence. 



From these data it results that the heat generated in the length- 

 ening of the day from twenty-three to twenty-four hours is equal 

 to the amount of heat lost by the earth, at its present rate of loss, 

 in 23,000,000 years. 



Now if this amount of heat, or any sensible fraction of it, was 

 actually generated within a few hundred miles of the earth's sur- 

 face, the temperature gradient in the earth must be largely due to 

 it, instead of to the primitive heat of the mass. 



Such an hypothesis precludes the assumption that the earth is 

 simply a cooling mass, and would greatly prolong the possible 

 extension of geological time. It must be observed that this view 

 is not acceptable unless we admit that the earth can adjust itself 

 to the equilibrium figure adapted to its rotation. 



It seems also worthy of suggestion that our data for the aver- 

 age gradient of temperature maybe somewhat fallacious. Recent 

 observations (Challenger Expedition) show that the lower stratum 

 of the ocean is occupied by water at near the freezing tempera- 

 ture, while the mean annual temperature of the earth's surface, 

 where the borings have been made, must be at least 30° higher. 

 It does not then seem impossible that the mean temperature 

 gradient for the whole earth should differ sensibly from the mean 

 gradient in the borings already made. 



The foregoing remarks have not been made with a view of 

 showing Sir William Thomson's argument from the cooling of the 

 earth to be erroneous, but rather to maintain the scientific justice 

 of assigning limits of uncertainty at the very least as wide as 

 those given by him. Professor Tait (" Recent Advances in Phys- 

 ical Science," 1885) cuts the limit down to 10,000,000 years; he 

 may be right, but the uncertainties of the case are far too great 

 to justify us in accepting such a narrowing of the conclusion. 



The third line of argument by which a superior limit is sought 

 for the age of the solar system appears by far the strongest. 

 This argument depends on the amount of radiant energy which 

 can have been given out by the sun. 



The amount of work done in the concentration of the sun from 

 a condition of infinite dispersion may be computed with some 

 accuracy, and we have at least a rough idea of the rate of the 

 sun's radiation. From these data Sir William Thomson concludes 

 (Thomson and Tait, " Natural Philosophy," Appendix E) : — 



" It seems, therefore, on the whole most probable that the sun 

 has not illuminated the earth for 100,000,000 years, and almost 

 certain that he has not done so for 500,000,000 years. As for the 

 future, we may say, with equal certainty, that inhabitants of the 

 earth cannot continue to enjoy the light and heat essential to 

 their life for many million years longer unless sources now un- 

 known to us are prepared in the great storehouse of creation." 



This result is based on the value assigned by Pouillet and 

 Herschel to the sun's radiation. Langley has recently made a 

 fresh determination, which exceeds Pouillet's in the proportion of 



