Feb. 8, 1872] 



NATURE 



289 



" Philosophical Transactions " for 1862, the memoir on the 

 rigidity of the earth is fully printed, and immediately 

 following it is another designated " dynamical problems 

 regarding elastic spheroidal shells and spheroids of in- 

 compressible liquid." The conclusions arrived at in the 

 first are essentially and admittedly dependent on the 

 investigations presented in the second. Not long after 

 they were published I gave my best attention to the study 

 of both, and it soon appeared to me that the problems 

 treated in the second could have no physical bearing on 

 the question of the earth's structure. The very title of 

 this memoir partly reveals its character in this respect. 

 In order to apply the results obtained in this memoir to 

 the earth, it is supposed to be a spheroidal homogeneous 

 elastic shell filled with incompressible lluid ; whereas in 

 such an inquiry the earth can scarcely be supposed to be 

 otherwise than a heterogeneous solid envelope containing 

 a fluid whose properties are not inconsistent with those of 

 fluids coming under our notice. Under this form I have 

 treated the hypothesis in the " Philosophical Transac- 

 tions " for 1851, and also in subsequent publications. 



Incompressibility is not a property of any known fluid ; 

 and Neumann, when referring in his comprehensive 

 treatise on geology to the influence of pressure in pro- 

 moting the density of the interior parts of the earth, 

 expresses what is very generally admitted among philoso- 

 phical geologists as well as physical inquirers, when he 

 says that " fluid bodies are endowed with far more com- 

 pressibility than solids."* Hypotheses are often indispen- 

 sable in physical inquiries where we are proceeding from 

 the known to the unknown, but there are two conditions 

 to which they should conform ; first, they should be 

 capable of verification by a comparison of the results to 

 which they lead with those of observation, and secondly, 

 they should not contradict established physical laws or 

 the known properties of matter, unless the contradiction 

 is specially explained and fully accounted for. The 

 second of these conditions is clearly violated when the 

 internal fluid of the earth is supposed to differ froin all 

 known fluids by being supposed to be incompressible. 

 And this violaiion is especially flagrant when the solid 

 matter enclosing the incompressible fluid is supposed to 

 be at the same time elastic and therefore compressible, 

 and when, moreover, the line of reasoning adopted as to 

 the earth's internal structure pointedly depends upon 

 these assumptions as to the properties of its fluid and 

 solid portions. Sir William Thomson endeavours to 

 prove, by a process of rt-diictio ad al'surJiim that the 

 interior of the earth is for the most part or altogether 

 solid ; in other words, he supposes the interior to be fluid, 

 and then tries to show that the tidal actions produced in 

 this fluid by the sun and moon must cause oscillations in 

 the crust which have not been observed. He may justly 

 claim to have proved that the earth does not consist of 

 an elastic solid envelope enclosing a mass of the ideal 

 substance called an incompressible liquid, but he has not 

 proved the point which he intended to establish, namely, 

 the absence of an interior fluid nucleus endowed with the 

 properties commonly attributed to fluids. He also sup- 

 poses throughout his investigations, in the same manner 

 as was supposed by Mr. Hopkins, that the transition from 

 the solidity of the shell to the fluidity of the nucleus is 

 not gradual but abrupt. Those who maintain the validity 

 of the hypothesis of the interior fluidity of the earth are 

 far from holding this opinion. On the contrary, all 

 observations hitherto made on the materials of the earth 

 lead to the conclusion that the solid shell is so constituted 

 as to present first a superficial coating whose mechanical 

 properties we can partly ascertain by direct experiment ; 

 secondly, a mass whose density and rigidity probably 

 increase with the depth from the outer surface ; thirdly, 

 an interior coating in which the cft'ccts of pressure are 

 resisted by those of teinperature, and where an imperfectly 



* Lehrbuch der Geologic, i. p. 268, 2nd edition. 



fluid and pasty mass is in contact at one side with the 

 solid shell, and on the other with the more perfect fluid. 

 This mass should be manifestly much more yielding and 

 compressible than the perfectly solidified shell ; for if 

 compression tends to increase the rigidity of solid matter, 

 the middle division of the shell, as just described, should 

 be more rigid than its superficial portion, and very much 

 more rigid than the interior pasty mass. The work 

 performed by small changes of shape in the fluid nucleus 

 due to the action of exterior disturbing bodies should thus 

 be expended partly in producing small variations of 

 density among the compressible strata of which it is com- 

 posed, and partly in changing the shape of the yielding 

 matter of the inner surface of the shell. The deformations 

 of a shell consisting of homogeneous elastic matter, such 

 as steel acted upon by exterior forces, must be the 

 resultants of all the elementary deformations among its 

 particles summed up or integrated. It would behave 

 somewhat like a vibrating bell ; but such is not the 

 behaviour to be expected in a mass of discontinuous and 

 heterogeneous materials. Vibratory motions in such 

 bodies are for the most part extinguished by interferences, 

 or their amplitudes are at least very much reduced. 



If the conclusions deduced by M. Perrey of Dijon from 

 his voluminous labours so often referred to by Mr. Mallet 

 in his Reports on Earthquakes, be correct, soine connection 

 between these disturbances and the phases of the moon 

 seems to be established which may be due to such com- 

 paratively feeble vibratory actions. Sir William Thomson's 

 conclusions rightly interpreted show that the constitution 

 of the fluid nucleus and the nature of the materials of the 

 shell must be essentially different from what he supposes 

 in order to establish these conclusions. A person who 

 never saw a railway train might as justly reason as to the 

 impossibility of travelling in it at high rates of speed, by 

 demonstrating that the shocks experienced by perfectly 

 rigid carriages connected w-ithout any compressible 

 arrangements would be too great for travellers to endure, 

 if not too great for the pennanent integrity of the 

 carriages themselves. In assuming the incompressibility 

 of the fluid nucleus for the purposes of his indirect de- 

 monstration of the rigidity of the earth. Sir William 

 Thomson makes a pctitio priiicipii nearly as vital as 

 shocks incident to influence of buffers in reasoning on the 

 the omission of the railway carriages. 



I am at a loss to know where any warrant was found 

 for affixing the property of incompressibility to the 

 supposed fluid nucleus of the earth ; and those who main- 

 tain the hypothesis of the interior fluidity of the earth are 

 entitled to repudiate an assumption fastened on that 

 hypothesis not only in opposition to evidence derived 

 from experiments on fluids, but in direct contradiction to 

 the arguments employed by them in discussing the 

 question of the earth's structure. 



Henry Hennessv 



THE LANDSLIPS AT NORTHWICH 



IN the " Notes " of the number of Nature, for Jan. 25, 

 I find one referring to the landslips at Northwich in 

 Cheshire, by mistake called Nantwich. As the descrip- 

 tion given of these landslips and their cause is scarcely 

 accurate, your readers may like to see a short account 

 of them. 



Northwich is the great centre of the Cheshire salt 

 trade. The manufacture is principally carried on now at 

 Northwich and Winsford, both towns lying in the valley 

 of the Ri\-er Weaver, though formerly Nantwich was 

 engaged in this trade, and Middlewich still continues so 

 to be. The position of the latter is indicated by its name, 

 it lying between Northwich and Nantwich. The salt is 

 found lying in two beds, called the upper and lower 

 rock salt. The first bed is met with in the neighbour- 

 hood of Northwich at the depth of about forty yards, and 



