F. W. Very— A Cosmic Cycle. 195 



geneous sphere, must be increased in the ratio of 176,868 to 

 100,000, to allow for heterogeneity, according to Lane's theory 

 of the density of the sun's interior, giving a past duration of 

 32,000,000 years ; and as contraction to one-half the present 

 radius, giving density = 11*2 times that of water, would proba- 

 bly end the gaseous sun, a total duration of 36,000,000 years is 

 inferred. 



This computation, like its predecessor, rests upon Pouillet's 

 value of the solar constant, which is too small. Lord Kelvin* 

 had already substituted Langley's value of the solar constant, 

 getting 12,000,000 years for the age of a homogeneous sun, or 

 20,000,000 years, " taking fully into account all possibilities of 

 greater density in the sun's interior, and of greater or less 

 activity of radiation in past ages." 



Lord Kelvin's computation by Fourier's method gives about 

 100,000,000 years for the earth's age,f assuming a conductivity 

 of the terrestrial substance equal to that of most surface rocks, 

 and an initial temperature of 3870° C. If the interior of the 

 earth is highly conductive, cooling must have proceeded to a 

 great depth, and a longer time will have been needed to estab- 

 lish the present distribution of temperature near the surface. 



The mean density of the earth and other facts indicate an 

 inner core of metallic composition. Moreover, the continental 

 relief, as compared with the depth of the oceanic basins, and 

 the contortion of strata by lateral compression, bear witness to 

 a contraction, which proves that cooling of the earth has pro- 

 ceeded to a much greater depth than would be allowable on the 

 supposition of a conductivity equal to that of the surface rock. 

 Hence we have an added argument for an interior conduc- 

 tivity approaching that of metals. A longer duration is to be 

 preferred for this reason. 



Many regions of the earth have been invaded by volcanic 

 action in geologically recent time. This is especially the case 

 in those disturbed districts where most of our mining operations 

 are conducted. The rise of temperature with increasing depth 

 is more rapid in these places than it would be normally. The 

 more rapid the rise of temperature, the shorter is the calculated 

 duration of cooling. The rate of subsidence of regions where 

 sedimentary deposits accumulate, is slow enough for the sub- 

 terranean isotherms to rise into the newly formed rocks, and 

 chemical action perhaps makes them local sources of heat and 

 volcanic action. Some of the deepest wells in rocks which 

 have been undisturbed since Paleozoic times give gradients of 

 temperature much lower than the average. Instead of taking 



* W. Thomson, "On the Sun's Heat,'' ~Roy. Institution of* Great Britain, Jan. 

 21, 1887; Nature Series, Popular Lectures and Addresses, vol i, p. 397, 1891. 

 f W. Thomson, Trans. Royal Soc. Edinburgh, vol. xxiii, p. 1H4, 1862. 



