168 Notices of Memoirs — B. Forbes, F.R.S., 



portion as an eggshell, that it would be fully capable of sustaining 

 masses of rock equal in size and weight to many Himalayas, piled 

 up one atop of another, without any danger whatever to its stability. 



For the sake of comparison, another diagram was prepared, which 

 shows the actual proportions of the cross section of a fowl's egg, en- 

 larged to the same size as the ideal section of the earth's mass pre- 

 viously alluded to ; now as in this instance the crust of the earth was 

 assumed to be some 50 miles thick, which is in the ratio of -pJ-g- part 

 of its entire diameter, it naturally follows that the crust would com- 

 paratively be considerably stronger in proportion than the shell of the 

 egg in question, since this was found only to possess a thickness of 

 ■^-i-Q part of its mean diameter (two inches), or in other words, if the 

 earth's crust were proportionately as thick as the eggshell, it would 

 be only 40 miles in thickness, which is one-fifth less than previously 

 assumed. 



The next argument which has been advanced against the pro- 

 bability of the major part of the earth's substance being in a fluid 

 condition, is one based altogether upon astronomical considerations. It 

 having been demonstrated that when two clocks are set agoing, the 

 pendulums of which are similar to one another in all respects, except 

 that whilst the bob of the one is solid, that of the other is hollow 

 and filled with mercury, that the latter will swing somewhat faster, 

 and consequently this clock gain time upon the former. 



The late Mr. Hopkins, of Cambridge, applied this observation to 

 the consideration of the movements of the earth in space, and by a 

 very elaborate course of mathematical reasoning and calculation, de- 

 monstrated that the earth, if not quite solid, must be nearly so, since, 

 according to his results, if the earth was merely a comparatively thin 

 shell, filled with liquid matter, the ratio of certain of its movements 

 (precession and nutation) would differ considerably from what they 

 are actually known to be, and these conclusions appeared to be con- 

 firmed by the subsequent calculations of Sir William Thomson and 

 Archdeacon Pratt. 



Although grave doubts suggested themselves as to the correctness 

 of the values used in these calculations for two of their most im- 

 portant elements, viz. the condensing action of pressure, and the ex- 

 panding action of the very high temperatures within the globe itself, 

 neither of which have as yet been determined with any certainty ; — 

 and although it might also be surmised that the conditions of a pen- 

 dulum bob of polished glass filled with heavy slippery mercury 

 swinging at the end of a rod must be very different from those of a 

 nearly spherical globe filled with viscid sticky lava revolving on its 

 own axis; — still geologists felt themselves quite unable to answer the 

 arguments of the astronomers and mathematicians, and since none of 

 them appeared to be sufficiently versed in either astronomy or 

 mathematics as to be able to submit either the mode of reasoning or 

 the calculations themselves to any strict scrutiny, they felt them- 

 selves, reluctantly no doubt, compelled to bow to the decision of such 

 eminent authorities. 



So stood the matter until the summer of 1868, when, fortunately 



