224 



NATURE 



[Jan. i8, 1872 



' twelve lunar hours. This would, as we shall see, be the case 

 ' if the geological hypothesis of a thin crust were true. The 

 ' actual phenomena of tides, therefore, give a secure contradiction 

 ' to that hypothesis. We shall see, indeed, presently, that even 

 ' a continuous solid globe of the same mass and diameter as the 

 ' earth, would, if homogeneous and of the same rigidity as glass 

 ' or as steel, yield in its sliape to the tidal influences three-fifths 

 ' as much or one-third as much as a perfectly fluid globe ; and 

 ' further, it will be proved that the effect of such yielding in the 

 ' solid, according as its supposed rigidity is that of glass or that 

 ' of steel, would be to reduce the tides to about f or | of what 

 ' they would be if the rigidity were infinite." 



" § S34. To prove this, and to illustrate this question of elastic 

 ' tides m the solid earth, we shall work out explicitly the solu- 

 ' tion of the general problem of § 696 for the case of a homo- 

 ' geneous elastic solid sphere exposed to no surface traction ; 

 ' but deformed infinitesimally by an equilibrating system offerees 

 ' acting bodily through the interior, which we shall ultimately 

 ' make to agree with the tide generating influence of the moon 

 ' and sun " 



" § S47. We intend in our second volume to give a dynamical 

 ' investigation of precession and nutation, in which it will be 

 ' proved that the earth's elastic yielding influences these pheno- 

 ' mena in the same proportionate degree as it influences the 

 ' tides. We have seen already that the only datum wanted for a 

 ' comparison between their observed amounts and their theoreti- 

 ' cal amounts on the hypothesis of perfect rigidity, to an accuracy 

 ' of within one per cent., is a knowledge of the earth's moment 

 ' of inertia about any diameter within one per cent. Wc have 

 ' seen that the best theoretical estimates of precession hitherto 

 ' made, are in remarkable acordance with the observed amount. 

 ' But it is not at all improbable that better founded estimates of 

 ' the earth's moment of inertia, and more accurateknowledge than 

 ' we yet have from observation, of the liarmonic of the second 

 ' dfgree in the expression of external gravity, may show that 

 ' the true amount of precession (which is known at present with 

 ' extreme accuracy) is somewhat smaller than it would be if 

 ' the rigidity were infinite. Such a discrepancy, if genuine, 

 ' could only be explained by some small amount of deformation 

 ' experienced by the solid parts of the eatth under lunar and 

 ' solar influence. The agreement between theory on the hypo- 

 '' thesis of perfect rigidity, and observation as to precession and 

 ' nutation, are, however, on the whole so close as to allow us to 

 ' infer that the earth's elastic yielding to the disturbing influence 

 ' of the sun and moon is very small— much smaller, for in- 

 ' stance, than it would be if its effective rigidity were no more 

 ' than the rigidity of steel." 



" § 848. It is interesting to remark that the popular geological 

 ' hypothesis, that the earth is a thin shell of solid material, 

 ' having a hollow space within it filled with liquid, involves 

 ' two eflects of deviation from perfect rigidity, which could in- 

 ' fluence in opposite ways the amount of precession. The com- 

 ' paiatively easy yielding of the shell must, as we shall see in 

 * our second volume, render the effective moving couple, due 

 ' to sun and moon, much smaller than it would be if the whole 

 " interior were solid, and on this account must tend to diminish 

 ■' the amount of precession and nutation. But the effective 

 'moment of inertia of a thin solid shell containing fluid, 

 ' whether homogeneous or heterogeneous, in its interior, would 

 " be much less than that of the whole mass if solid throughout ; 

 " and the tendency would be to much greater amounts of pre- 

 " cession and imtation on tiiis account. It seems excessively 

 " improbable that the defect of moment of inertia due to fluid in 

 " the earth's interior, should bear at all approximately the same 

 " ratio to the whole moment of inertia, that the actual elattic 

 " yielding bcais to the perfectly easy yielding which would take 

 " place if the earth were quite fluid. But we must either admit 

 "this supposition, improbable as it seems, or conclude (from 

 " the close agreement of precession and nutation with what 

 " they would be if the earth were perfectly rigid) that the defect 

 " of moment of inertia, owing to fluid in the interior, is small in 

 " comparison with the whole amount of inertia of the eaith 

 " about any diameter ; and that the deformation experienced by 

 " the earth from lunar and solar influence is small in compari- 

 " son with what it would be if the earth were perfectly fluid. 

 " It is, however, certain that there is some fluid matter in the 

 *' interior of the earth ; witness eruptions of lava from vol- 

 " canoes. But this is probably quite local, ac has been urged 

 " by Mr. Hopkins, who first adduced the phenomena of pre- 

 " cession and nutation to disprove the hypothesis that the solid 

 " part of the earth's mass is merely a thin shell." 



The Kiltorcan Fossils 



I HAVE just seen Mr. Carruthers' letter in your number of 

 January 4th, to which I beg leave to reply. 



In this communication it now appears that Mr. Carruthers' 

 former remarks in the discussion upon Prof. Heer's paper were 

 intended as a personal attack upon me ; as he now states that on 

 me alone rests the credit of misleading Prof. Heer by my erro- 

 neous determination of the Kiltorcan plant 



I have no hesitation in acknowledging to having referred the 

 Kiltorcan plant in question to Sagcnariavdtheimiana, and I think 

 it very possible I may even now be correct. I will however now 

 state the reason for my afterwards adopting Professor Schimpcr's 

 name in preference. When that gentleman was in Ireland he 

 spent some time in the examination of the Kiltorcan fossils, and 

 did not then object to my determination of the species ; it was 

 afterwards, on my sending him a collection, that his further study 

 of these fossils and comparison with the original species (of which 

 I had only seen figures) enabled him to announce to me what lie 

 believed to be the distinctive characters in relation to the fruit 

 which accompanied it, of those I had named Sagenaiia vdthci- 

 niiana ; these fossils in his letter to me he referred to Sagcnaria, 

 and afterwards in his work " Traite Paleontologie Vegeiale," to 

 Kiioryia under the name of A', hailyana. In the meantime I had 

 read my report on these fossils at the British Association, and 

 naturally adopted the generic name first applied to it by Prof. 

 Schimper, which I afterwards corrected to Knorria, on his autho- 

 rity, in my " Figures of British Fossils," as Mr. Carruthers 

 states. 



In my letter to Professor Heer (June 1S70) accompanying the 

 specimens which I was requested to send him for his comparison 

 with the Bear Island flora, I named those from Kiltorcan Sagc- 

 naria bailyana in accordance with Prof. Schimper's determina- 

 tion, whilst others from Tallow Bridge, co. VVaterford, which 

 he specially wished to see, I still referred to S. vclthcimiana. I 

 made him aware of Prof. Schimper's views on these plants, 

 stating distinctly that they were originally referred by me to .S". 

 vtitlicimiaiia, but that Prof. Schimper, in consequence of his 

 being enabled to compare the fruit accompanying it with that of 

 the true S. vcltliciinia)!a, had arrived at the conclusion that it 

 could not be that specres, and therefore he had named it as a 

 distinct one. Under these circumstances I cannot see how Mr. 

 Carruthers can charge me with misleading Prof. Heer, who harl 

 the whole facts, with examples of the specimens from both 

 localities, to draw his own conclusions from ; with his acknow- 

 ledged powers of discrimination, surely he was fully competent 

 to judge for himself as to their correct identity. 



The amount of Mr. Carruthers' knowledge on the subject 

 about which he writes, is evidenced from his intimation that the 

 fossil figured by me in the explanation to Sheet 187, &c., of the 

 Irish Survey maps, is from Kiltorcan (co. Kilkenny), whereas it 

 was sketched by me, on the spot, at Tallow Bridge (co. Water- 

 ford), where the section exposed exhibited a profusion of these 

 plants in various conditions and stages of growili. The character 

 of the rock in which they occur is totally different from that at 

 Kiltorcan, the former being a grey shale, corresponding with the 

 Lower Carboniferous shales, the latter a fine-grained greenish 

 sandstone ; neither has any of the associated Krltorcan fossils, 

 including the fish which are of typical Devonian or Old Red 

 sandstone genera, ever been found at Tallow Bridge. I did how- 

 ever state in this memoir my belief that the .S'. veltJuiniiaita^ as 

 identified by me at Tallow Bridge, was similar to the Kiltorcan 

 plant in question, and also that it corresponded with the so-called 

 Knorria of the Marwood beds, N. Devon. 



With reference to Mr. Carruthers' announcement that Sagnia- 

 ria vc-llheiiniana is a " coal measure plant," I may remark that 

 it is a particularly abundant fossil, occurring in various conditions, 

 but seldom, if ever, met with in the typical coal series of Great 

 Britain; I have ideniified it from the sandstones of the lower 

 coal measures in the North of Ireland, as well as at various 

 localities in the Lower Carboniferous shales of the Counties of 

 Cork and Kerry. On the Continent, especially in Germany, it 

 appears to be still more universal, and has been recorded under 

 various names by fossil botanists, as Dr. H. R. Goeppert, in hi:; 

 " Fossile Flora der .Silurischen der Devonischen," &c., mentions 

 more than twenty synonyms for this species ; moreover the same 

 author states its occurrence to be " In der Kulmgrauwacke, dcm 

 Kohlenkalke und in Aex Jiiiigs/t'n Gran'oacic:" Dr. F. Unger 

 and Dr. H. B. Geinitz, the latter ot whom personally inspected 

 the collections from Kiltorcan and Tallow Bridge, also mentions 

 similar lower geological horizons at which it occurs ; and Dr. 



