THE THEORY OF FISHER. 



199 



The alternation of the sign begins with the second and the steady decline 

 in numerical value continues throughout. 



A convenient unit of time for exhibiting the changes produced by con- 

 duction in the temperature-curve is the interval T required to reduce the 

 amplitude of the first component by 1 per cent, which with the same con- 

 stants as before, for the surface-stratum, would be about 1^ billion years. 

 The successive changes in the temperature-curve are shown in table 6, 

 abbreviated from an extended computation covering an epoch of 35 such 

 intervals, the number of terms of the series included ranging from 100 for 

 the earliest entries to 5 for the latest. The unit of temperature is the initial 

 temperature at the center. 



Table 6. 



Here, as in the previous case, the earlier- stages of the conduction are 

 marked by the division of the mass into two zones, an inner where the 

 temperature falls and an outer where it rises. The spherical surface of 

 boundary between these lies initially at a depth of not quite 1,200 miles, 

 and as the conduction progresses rises toward the surface. Its passage 

 through any particular horizon marks the attainment of the maximum tem- 

 perature which occurs at that depth; while its ultimate coalescence with 

 the free surface, simultaneous with the occurrence of the maximum surface 

 gradient, indicates the final establishment of a downward trend of temper- 

 ature throughout the body. This decline during the further progress of the 

 conduction is in the outer portions first accelerated and later retarded, 

 and is ultimately everywhere of a character more and more closely approx- 

 imating to the simple type represented by the first component alone. 



Table 7 gives numerical data, with the epochs reckoned from the begin- 

 ning of the conduction: 



