12 Rev. 0. Fisher on Variations of Gravity and their 

 in question is nil at that point, we have approximately, 



E-4w{( C -|)V + («-*-|)V} 



^ = 7 t^ ' 



(«"*- I) 



Taking the mean density to be 5^, we have 

 4 lie E 



whence, neglecting terms in -^ 



c 



where a is a small fraction. 



We will first consider the conditions of hydrostatic equili- 

 brium in the case of a spherical cap as already defined, such as 

 has been made the basis of the calculations of Archdeacon Pratt 

 and of Colonel Herschel, making the additional hypothesis re- 

 specting the support of the mountain which has been proposed. 

 The elevated mass will therefore be considered as of the form of a 

 spherical cap, having its root also of that form. We will for the 

 present consider the root as contained within the same verticals 

 as the mountain ; that is, they are both frustra of the same cone, 

 whose vertex is the centre of the earth. The conditions of 

 equilibrium obtain for the cone. This supposition ignores 

 the rigidity of the crust, which may appear a violent assump- 

 tion to those who are not geologists. It is now, however, 

 generally admitted that the materials of the earth's crust have 

 yielded very freely to the stresses which have affected them ; 

 and that the phenomena of schistosity, cleavage, contortion, 

 and of other kinds of metamorphism, are evidences of this 

 fact. In short, the materials are of the nature of more rigid 

 portions imbedded in less rigid media, the shapes and magni- 

 tude of the more rigid, and the ductility of the less rigid 

 varying both absolutely and relatively in every possible 

 degree. Thus all rocks appear to be more less of a viscous 

 nature ; that is to say, when subjected to stress for a suffi- 

 ciently long period, they become sensibly strained out of their 

 original forms and relative positions. And under these cir- 

 cumstances chemical reactions are also set up. It will appear 

 in the sequel that rigidity does play some part in the equili- 

 brium of the crust ; but it may be for the present supposed 

 that the conditions of equilibrium are satisfied throughout 



