Prof, W. Thomson on the Secular Cooling of the Earth. 3 



this suggestion may yet be taken up, and may prove to some 

 extent useful; but the disturbing influences affecting under- 

 ground temperature, as Professor Phillips has well shown in a 

 recent inaugural address to the Geological Society, are too great 

 to allow us to expect any very precise or satisfactory results. 



5. The chief object of the present communication is to esti- 

 mate, from the known general increase of temperature in the 

 earth downwards, the date of the first- establishment of that 

 " consistentior status," which, according to Leibnitz's theory, is 

 the initial date of all geological history. 



6. In all parts of the world in which the earth's crust has 

 been examined, at sufficiently great depths to escape large influ- 

 ence of the irregular and of the annual variations of the super- 

 ficial temperature, a gradually increasing temperature has been 

 found in going deeper. The rate of augmentation (estimated at 

 only YYo^h of a degree Fahr. in some localities, and as much as 

 T jth of a degree in others, per foot of descent) has not been 

 observed in a sufficient number of places to establish any fair 

 average estimate for the upper crust of the whole earth. But 

 j 1 ^ th is commonly accepted as a rough mean ; or, in other words, 

 it is assumed as a result of observation that there is, on the 

 whole, about 1° F. of elevation of temperature per 50 British 

 feet of descent. 



7. The fact that the temperature increases with the depth 

 implies a continual loss of heat from the interior, by conduction 

 outwards through or into the upper crust. Hence, since the 

 upper crust does not become hotter from year to year, there 

 must be a secular loss of heat from the whole earth. It is pos- 

 sible that no cooling may result from this loss of heat, but only 

 an exhaustion of potential energy, which in this case could 

 scarcely be other than chemical affinity between substances 

 forming part of the earth's mass. But it is certain that either 

 the earth is becoming on the whole cooler from age to age, or 

 the heat conducted out is generated in the interior by temporary 

 dynamical (that is, in this case, chemical) action. To suppose, 

 as Lyell, adopting the chemical hypothesis, has done*, that the 

 substances, combining together, may be again separated electro- 

 lytically by thermo-electric currents, due to the heat generated 

 by their combination, and thus the chemical action and its heat 

 continued in an endless cycle, violates the principles of natural 

 philosophy inexactly the same manner, and to the same degree, 

 as to believe that a clock constructed with a self-winding move- 

 ment may fulfil the expectations of its ingenious inventor by 

 going for ever. 



8. It must indeed be admitted that many geological writers 

 * Principles of Geologv. 

 B2 



