﻿Chemistry and Physics. 397 



prevalence of metamorphic action (which must have taken place 

 at depths of not many miles, if so much), all agree in demonstrat- 

 ing that the rate of increase of temperature downwards must 

 have been much more rapid, and in rendering it probable that 

 volcanic energy, earthquake shocks, and every kind of so-called 

 plutonic action, have been, on the whole, more abundantly and 

 violently operative in geological antiquity than in the present 

 age ?" 



Now, while I entirely agree with the general conclusion of Sir 

 William Thomson, it is not unimportant to indicate a possible 

 flaw in the argument. The flaw will only be acknowledged as 

 possible by those who agree with the previous criticism on the 

 argument from tidal friction. 



The present argument as to the date of the consolidation of the 

 earth reposes on the hypothesis that the earth is simply a cooling 

 globe, and there are reasons why this may not be the case. The 

 solidification of the earth probably began from the middle and 

 spread to the surface. Now is it not possible, if not probable, 

 that, after a firm crust had been formed, the upper portion still 

 retained some degree of viscosity ? If the interior be viscous, 

 some tidal oscillations must take place in it, and, these being sub- 

 ject to friction, heat must be generated in the viscous portion ; 

 moreover the diurnal rotation of the earth must be retarded. 

 Some years ago, in a paper on the tides of a spheroid, viscous 

 throughout the whole mass {Phil. Trans., part ii, 1879), I esti- 

 mated the amount and distribution of the heat generated whilst 

 the planet's rotation is being retarded and the satellite's distance 

 is being increased. It then appeared that on that hypothesis the 

 distribution of the heat must be such that it would only be possi- 

 ble to attribute a very small part of the observed temperature 

 gradient to such a cause. Now, with a probable internal consti- 

 tution for the earth in early times, the result might be very differ- 

 ent. Suppose, in fact, that it is only those strata which are 

 within some hundreds of miles of the surface which are viscous, 

 whilst the central portion is rigid. Then, when tidal friction 

 does its work the same amount of heat is generated as on the 

 hypothesis of the viscosity of the whole planet, but instead of 

 being distributed throughout the whole mass, and principally 

 towards the middle, it is now to be found in the more superficial 

 layers. 



In my paper it is shown that with Thomson's data for the con- 

 ductivity of rock and the temperature gradient, the annual loss 

 of heat by the earth is 1/260,000,000 part of the earth's kinetic 

 energy of rotation. 



Also, if by tidal friction the day is reduced from D hours to 



D hours, and the moon's distance augmented from 77 to 77 earth's 



radii, the energy which has been converted into heat in the pro- 



. / D \ 2 / 1 1 \ 



cess is I — —1— 8 -84(——-=r- times the earth's kinetic energy 



of rotation. 



Am. Jour. Scl— Third Series, Yol. XXXII, No. 191 —November, 1886. 

 25 



