394 



EARTH. 



tendency of lava tides within the earth at times 

 of conjunction of the sun and moon must con- 

 spire with such differences, the crust of the 

 earth must at such places and tunes suffer rup- 

 ture, unless at least 140, and more probably 

 from 300 to 500 miles thick. To this, the 

 Kev. Mr. Haughton replied that Mr. Pratt 

 overlooks the principle of the arch, which 

 serves to increase practically the strength of 

 the strata, and also the very high crushing 

 strength of granite not less than 24,000 Ibs. 

 to the cubic inch. 



Prof. William Thomson presented before the 

 Royal Society, May 15, 1862, a paper on the 

 "Rigidity of the earth," considered in its 

 bearings on this question. Premising that he 

 would communicate also, on an early occasion, 

 a mathematical theory of the deformation of 

 elastic spheroids, and which would illustrate 

 his conclusions in respect to the effect of the 

 tide-generating influence of the sun and moon 

 on the mass of the earth, he proceeded to show 

 that unless the solid portions of the earth be 

 on the whole of extremely rigid material 

 more rigid than steel it must yield to the 

 attractions of those bodies, in the way of a 

 tide of the solid strata, and to such an extent 

 as very sensibly to diminish the actual oceanic 

 tides, as well as the well known changes in 

 direction of the earth's axis denoted under the 

 terms precession and nutation. A homogene- 

 ous and incompressible elastic spheroid, of the 

 same mass and volume with the earth, if it 

 have the rigidity of glass, would yield to the 

 luni-solar tide action to about ^, and if it have 

 the rigidity of steel, to about if, of the extent 

 to which a perfectly fluid globe of equal den- 

 sity must yield. In the former case, the actual 

 amounts of the water tides, and of the pre- 

 cession and nutation, would be only f , in the 

 latter J, of the amounts that would hold true 

 in case of a perfectly rigid spheroid of the like 

 dimensions, figure, and supposed uniform den- 

 sity. But the theory of precession and nutation 

 has always hitherto proceeded upon the sup- 

 position that the earth, in respect to its exterior 

 form, is practically a completely rigid body; 

 and the calculations deduced from such theory 

 have closely agreed with observation. It is 

 scarcely possible to admit that there can have 

 been between the theory and facts any such 

 discrepancy as required by the ratio of 3 to 5 ; 

 and the conclusion then almost necessarily fol- 

 lows, that the earth, as a whole, is much more 

 rigid than steel. The author suggests, as a fur- 

 ther test, careful observations on the lunar 

 fortnightly and the solar half-yearly tides, made 

 say at Iceland and Teneriffe, since the agency 

 of either the sun or moon in producing tides at 

 and near the poles is greatest when such body 

 has the greatest declination N. or S. of the 

 equator ; and by means of two such stations, 

 the actual diminution of the ocean tide by an 

 earth tide, if there be any, would more likely 

 be ascertained. From data already at command 

 Prof. Thomson regards it as certain that the 



general globe is far more rigid than glass, prob- 

 ably more so than steel. But in order to such 

 result, the interior must be even more rigid 

 than the superficial parts; and this is just 

 what would be expected, if, the interior being 

 solid, the enormous pressure upon it be taken 

 into account. The actual rigidity, however, 

 appears utterly inconsistent with the hypoth- 

 esis of many geologists, of the earth as mainly 

 a molten mass, inclosed in a shell 30 to 100 

 miles thick. These investigations confirm those 

 of Mr. Hopkins, who is led to conclude that 

 the solid crust cannot be at the least less than 

 800 miles in thickness; and in view of the 

 apparent absence of any interference with 

 ocean tides and precession, the author believes 

 it impossible the solid portion of the earth 

 should have,a depth less than 2,000 to 2,500 

 miles. 



Excentrality of the Earth. Mr. "W. Ogilby 

 (Brit. Assoc.), uses the term excentrality to 

 denote that quality in virtue of which the 

 earth's centre of gravity, as he believes he has 

 proved to be the case, is situated to one side 

 of the axis about which its rotation takes place. 

 He sets out with the principles that, in a freely 

 revolving body like the earth, the axis of 

 figure must, under all circumstances, coincide 

 with the axis of rotation, and the centre of 

 figure with the centre of the generating sphere. 

 This centre is a mathematical point, fixed and 

 immovable, whatever changes of revolution the 

 body may undergo. But the centre of gravity 

 is a physical point, its position dependent on 

 the constitution of the body, and changing 

 with alterations in the density or arrangement 

 of its materials. Since the earth is neither a 

 homogeneous body, nor composed of spherical 

 shells severally homogeneous, its centre of grav- 

 ity needs not coincide with its centre of figure ; 

 and facts lead to the conclusion that it does not, 

 and further, that it does not remain a fixed 

 point. These consequences follow from the ir- 

 regular density of the globe, and again from 

 the transportation always going on, of millions 

 of tons of solid materials from one part of the 

 earth to another, to say nothing of tides, rivers, 

 ocean currents, or of possible effects of electri- 

 cal currents within the earth. Reasoning 

 from the known forms, places, and dimensions 

 of the continents, and in like manner of the 

 oceans, with the average depths of the latter, 

 with other circumstances, Mr. Ogilby deduces 

 the place of the earth's centre of gravity as 

 full one statute mile toward the Asiatic side 

 from its centre of figure. 



If, as probable, the axis of rotation originally 

 passed through the centre of gravity i. e., if it 

 was a principal axis this, provided that the 

 centre of gravity had remained fixed, would 

 have been indefinitely maintained. But with 

 the revolution itself the transport of materials 

 would probably commence ; and the alteration 

 of the globe accumulating during a given geo- 

 logical epoch, the gravitative centre would in 

 the lapse of ages come to diverge materially 



