340 



THE AGE OF THE EARTH. 



Table No. 4. — Estimated earth tciiiperatwres. Initial excess 1,230'^ C. 10,000,000 years' 

 secular cooling with surface rate of 1'^ F. to oO'G feet of depth. Thermal conduction 

 400 feet- /year. 



TablkNo. .5. — Estimated meltinrj point and depth for the rock diabase expressed in 

 radial earth distance, pressure and melting temperature. 



Table 5 contains a prolonj?ation of Earus's line of melting point and 

 depth for the rock diabase, expressed in radial earth distance n, pres- 

 sure ^> (Laplace's densities), and melting- temperatures, Om- 



THE CHART. 



The chart constituting Plate xviii is constructed to present the pas- 

 sage of certain hypothetical temperature gradients through the upper- 

 most O.OS of the eartirs radius, and the position in the same held of 

 Earus's line nuxrking the melting point in depth of diabase, thus defining 

 the relations of the various distributions of earth-temperature to liq- 

 uidity. The value of the ordinates is each 1,000'^ C. the abscissae, which 

 are platted as etpial in length to the ordinates, rei)reseiit hundredths of 

 radius counting downward from the surface whicli is indicated by the 

 right vertical boundary of the chart. 



Kelvin's applicatiou of the Fourier equation involves an assumed ini- 

 tial excess of temperature, and assigned value of rock conductivity, a 

 given period of secular cooling, and the surface rate of augmentation of 

 earth-temperature. As thus applied to the case of the cooling earth, it is 

 obvious that while the body was of uniform initial heat there would be 

 no augmeutatiou of temperature from the surface dowuwardjor other- 



