56 REPOKT — 1864. 



to its temperature. Such results do uot agree witli the general mass of facts 

 presented by geological inquiries, and we have, moreover, no geological evidence 

 of the past existence of considerable gi'oups of fossil aerolites. Had such bodies 

 fallen upon the sun during its passage through space in much gi-eater quantities 

 formerly than now, we should expect to find some corresponding accimiulations in 

 the stratified deposits which form the outer crust of the sim's satellite, our eaiih ; 

 and their apparent absence shows that it is scarcely safe to use Mayer's theory for 

 explaining such climatic changes as those under consideration. The varying 

 position of the earth in its orbit around the sun was then referred to ; and the 

 author concludes that, in the present state of the question, our only reliable 

 evidence is negative, or, in other words, the variations in the earth's position in 

 its orbit are not proved to be sufficient to account for great changes in its climate. 

 The author referred briefly to the proofs of the stability of the axis of rotation of 

 the earth as eliminating a possible cause of changes of climate, and to the commu- 

 nications he published on the subject in the Proceedings of the Royal Society fir 

 1852, and in the ' Athenaeum ' for September 1860. On the last occasion the same 

 subject was treated by the Astronomer Royal, and with precisely the same results 

 as those previously established by the author. The modes of accounting for the 

 obseBved increase of heat in approaching the earth's interior imply a different 

 order of increase or decrease of its influence during former geological epochs. The 

 hypothesis of an incandescent nucleus of fused matter contained in a shell whicli 

 had solidified from the fluid has been objected to on various gxounds. These 

 objections, when fully examined, tend to confirm the probable truth of the hypo- 

 thesis. Thus, in 1857, Mr. Hopkins communicated to the Association an account 

 of his experiments on the conductive powers of different rocks. He also compared 

 the conductivities so foimd ■\\'ith the rate of increase of temperature in descending 

 mines excavated out of rocks such as had been made the subject of experiment. 

 It seemed to follow that no connexion was traceable between the increase of 

 interior temperature and the conducting-power of different parts of the earth's crust. 

 The conclusion was hence dravra that the supposition of a cooling central mass of 

 matter in the earth was inconsistent with observation. The author now ventures 

 to reproduce the remarks he then made on Mr. Hopkins's comjuunication, as these 

 remarks have never hitherto appeared in a printed form. 



The author described the discontinuous and broken structure of the greater part 

 of the rock-formations constituting the earth's crust. The propagation of heat 

 throughout such a mass could not follow the same simple laws as in a continuous 

 unbroken homogeneous solid. He showed that the rate of transmission of heat 

 would be much slower than even the extremely slow rate assigned by mathematical 

 analysis to a continuous mass. The amoimt of heat passing outwardly to the earth's 

 surface through the dry rocks and soil would thus be generally so small as to pos- 

 sess no geological importance. By studying the actual conditions of the interior 

 rocky masses rendered accessible by mines, we shall arrive at more valuable and 

 trustworthy conclusions relative to terrestrial heat. The mines from which most of 

 our facts regarding interior temperatm'e have been collected are provided with exten- 

 sive hydraulic apparatus for draining off the rapidly accumulating water. Many of 

 our facts have been deduced from observing the temperature of artesian wells bored 

 through water-bearing strata. Thermal springs, like those of Bath, would imdoubt- 

 edly arise wherever we could bore downwards to similar depths. The constant per- 

 colation of superficial moisture tends to saturate the joints and fissures among the 

 lower rocky beds as well as the more porous and permeable overlying strata. In 

 limited masses of liquid, heat is propagated upwards, not by conduction, but by the 

 far more energetic process of convection ; and thus, while each piece of rock is 

 bathed by water, it acquires a different temperature from what it would possess 

 under the imaginary conditions of continuity and drj'ness. We should thus be led 

 to expect no kind of definite connexion between the rate of increase of temperature 

 and the conducting properties of the dry stone, although we might fauly expect to 

 trace such a connexion between the permeability of rocks to moisture and the 

 distribution of temperature in borings made through great depths. A consequence 

 of primary geological importance appears to the author to be deducible from these 

 considerations. The consequence alluded to is deduced from the probable past 



