CXvi PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



to discover the mode in which nature worked in its internal labo- 

 ratory, displays great ingenuity. In fact, geology has now arrived at 

 such a stage that nothing short of a complete explanation of the great 

 problem of the earth's history in all its relations, whether purely 

 chemical, physical, or organic, will satisfy the philosophical geologist. 



Geologists are all aware that the recognition, as an incontrovertible 

 fact, of the increase of temperature in descending from the surface 

 towards the centre of the earth, in whatever way the experiments may 

 have been made, whether in Artesian borings, or in the shafts of mines, 

 or by the examination of hot springs, was considered a certain proof 

 of the existence of a source of internal heat, due to the retention of 

 a portion of the original heat of the planet when in a state of igneous 

 liquidity, by some part of the internal nucleus. "When this theory was 

 first propounded, many objections were urged in opposition to the 

 fact itself, but when it had been satisfactorily proved that no acci- 

 dental circumstances could account for the increase of temperature, the 

 question appeared settled, and the theory was adopted without further 

 hesitation. One remarkable circumstance had, however, been noticed, 

 namely, that the rate of increase varied in different places, or in 

 different deposits ; and this fact has suggested, to our great physical 

 geologist and former president, an eocperimentum cruris, to which he 

 has subjected the theory. It is this, — that according to the ordinary 

 law of the distribution of heat, the progressive increase of tempera- 

 ture from above downwards should be in an inverse proportion to the 

 conducting power of the strata bored or sunk through. After a care- 

 ful experimental examination of the subject, he comes to the con- 

 clusion that the rate of increase does not conform to this law, and, 

 hence, that some other mode of explanation must be sought for, the 

 fact remaining, as before, uncontroverted. It is hard to abandon a 

 favourite theory so pregnant in deductions of great interest ; and there 

 can be no doubt that both the experiments and reasonings of Mr. 

 Hopkins will undergo a rigorous examination ; but should their accu- 

 racy be fully established, it will be better that we should be put 

 upon a new scent and an active hunt, rather than be allowed to 

 repose in an unwarranted confidence that we had solved so great a 

 problem in the earth's history. 



Another able investigator of the physical conditions of the earth 

 is Mr. H. Hennessy, F.R.S., who, in a recent paper on the Physical 

 Structure of the Earth, has brought forward additional developments 

 and illustrations of the views he had already put forward in other 

 publications. He quotes a result at which he arrived by a mathe- 

 matical examination of the figure of a spheroid acted on by the 

 abrasion of water, in order to show that the earth could not have 

 acquired its present figure by the action of its watery envelope alone, 

 as maintained by some distinguished philosophers. 



Assuming, therefore, the earth to have been in a state of fluidity 

 from intense heat, he proceeds to consider the way in which, accord- 

 ing to known mechanical and physical laws, such a fluid mass may 

 have passed into the actual condition in which we find our planet. 

 He observes that the increase of density of the strata of the fluid in 



