Correspondence — Mr. George Maw, 



149 



GRAVITATION, COMPKESSION, AND SLATY CLEAVAGE. 



Sir, — Dr. Sterry Hunt, in his paper in the February Number 

 of the Magazine, referring to the alleged condensing power of the 

 superincumbent mass on the central parts of the earth, remarks : 

 " The condensing effect of pressure was by Dr. Young estimated to 

 be sufficient to reduce a mass of granite at the earth's centre to the 

 eighth of its bulk at the surface, which would give the earth a mean 

 density equal to twelve or thirteen times that of water : this con- 

 sideration has led a recent writer in the London Athenoeum to 

 conclude with Herbert Spencer that our earth and the other planets 

 may be only shells of varying thickness, enclosing a central cavity 

 filled with vaporous matter, by which hypothesis we may explain 

 their comparatively feeble densities." Mr. David Forbes has also 

 noticed that the average density of the earth falls short of what it 

 would be, supposing it grew denser in descending, in proportion to 

 the superincumbent pressure ; and " That experimental research 

 tends to show that a limit is soon reached beyond which the com- 

 pression or increase of density becomes less and less in proportion 

 to the force employed." 



Do not the estimates of hypothetical increased central density fail 

 to consider the influence which the spherical form of the earth would 

 have in counteracting accumulating pressure, and diverting the fore© 

 of gravitation to a direction parallel with the circumference ? 



The case seems strictly ana- 

 logous to that of an arch, in 

 which the resulting force of gra- 

 vitation is diverted along the arch 

 to the abutments. If the earth 

 is hypothetically assumed to be 

 made up of a series of concentric 

 hollow spheres (see Woodcut) 

 A, B, C, D, it will be at once 

 evident that each of such spheres 

 would be self-supporting, just as 

 in the case of a bridge, the addi- 

 tion of each successive course of 

 brickwork composing the arch 

 adds no pressure to, but rather 

 increases, the resisting power of 

 the single course first laid; the 

 direction of the resistance of the gravitation of the mass being 

 accumulated on the spring of the arch. Again, if we go on filling 

 up this arch internally with successive courses of brickwork, we do 

 not interfere with the stability of the external arch, neither is the 

 weight of the first structure borne by the inner courses ; in fact 

 every zone of the arch or sphere is individually self-supporting. 

 The vertical pressure of gravitation, which in successive superimposed 

 layers of a plane would accumulate, is vertically neutralized in a 

 sphere, and instead of getting the sum of the weight of the con- 



