218 SEE— TEMPERATURE, SECULAR COOLING [April 20 



melting temperature of basalt is less than 1688° Fahr., the smaller 

 value for V obviously is to be preferred. Thus the assumed surface 

 temperature of 2000° Fahr. seems sufficiently high, and our most 

 probable age of the earth calculated in this way is 



/ CI vy 

 t=\- I = 8,302,210 years. (21) 



Accordingly, we conclude from this Fourier-Kelvin formula that the 

 age of our encrusted planet can scarcely exceed 10 million years, 

 which accords very well with the duration inferred from the 

 theory of the sun's heat (cf. ^.A^., 4053) • 

 § 10. The Age of the Earth's Consolidation Calculated by Fisher's 



Method. 



The Rev. O. Fisher has developed another method for calculating 

 the age of the earth which we shall now explain. It is in the main 

 independent of Lord Kelvin's procedure based on Fourier's equation 

 for the rate of increase of temperature downward, and has some 

 advantages over it. Fisher's method is treated in Chapter vL, and 

 also in the Appendix, to the second edition of his ** Physics of the 

 Earth's Crust." He established the following formulae : 



(22) 



= Zfif'^^MX, for inert substratum, in which f = o. 



k=fM 1/4/^, (24) 



In these equations /x is a function differing but little from unity ; M 



is the definite integral I ^-"W^; k the coefficient of conductivity 



taken to be 400, as in Lord Kelvin's work ; k is the thickness of the 

 earth's crust; ^ the surface rate of augmentation of temperature 

 downward ; \ is the latent heat of molten rock measured in terms of 

 the amount of heat required to raise one cubic foot of the rock 



