178 Mr. E. L. Garbett on Popular Difficulties in Tide Theory. 



being more viscous, having a stiffer and deeper " surface visco- 

 sity," this being a surface property in all bodies, whether it have 

 a degree that leads us to call them solid or only fluid 9 . 



All this is apart from plutonic heat, and would apply if we 

 knew the temperature to be icy all through. Of course there 

 are two quite general facts (earthquakes and strata-dislocations) 

 that disprove the existence of any " crust/' a term implying 

 something capable, in parts at least, of self-support as a vault 

 or shell. The "solidity" really belongs to what can but be 

 compared to the silk of a balloon, or the finest grapeskin ; and 

 talking, in the face of geological facts, of mathematically dedu- 

 cing from precession or nutation, an adamantine "crust," some 

 hundreds of miles thick, is equivalent to producing mathema- 

 tical proofs, when a frozen pond is momentarily being roughened 

 with dislocatory cracks (and these only a foot long), that the 

 ice is so many fathoms thick ! The three newest dislocations of 

 which we have accounts, those in Cutch 10 and that in north New 

 Zealand n , barely extend a hundred miles each, and did not sen- 

 sibly shake places within a very few degrees of them. What 

 would be the least length of crack and area of shock in the "crust" 

 supposed to be deducible from precession ? To illustrate the 

 possibility of such deductions, Mr. Denison, in his ' Astronomy 

 without Mathematics/ observes that a pendulum formed of a 

 hollow globe filled with quicksilver would have a quicker rate 

 than if solid, and by the swinging the thickness of shell might be 

 found. I cannot see that there is any analogy ; for the difference 

 of rate would depend on the fluid slipping and escaping the in- 

 terrupted rotary movements of the shell. But in the continuous 

 rotations of the earth (partaken also by her external fluids) there 

 is no ground for assuming a slip of any part ; and if none is 

 supposed even as to the rapid diurnal rotation, much less may it 

 be in the slow nutation and precession movements. 



In any case, however ; seeing the earth must (unless we make 

 preternatural assumptions) undergo deformation by the same 

 forces as the sea, only as much less as a thin (< surface visco- 

 sity" might oblige, I perceived that the earth-tides must be as 

 simple as those of a hypothetic uniform sea. Their summits 

 cannot travel out of the tropical zone, nor have we reason to 

 assume any such anomalies as the land-barriers and varying 

 depths cause in the water-tides. Hence, though these latter pre- 

 sent, at different places, every variety of " establishment " from 



9 Phil. Mag. June 1866, p. 473. " There is therefore no evidence 

 that in the interior of large masses like the earth .... there can be any 

 atomic arrangements or conditions distinguishing the solid from the fluid 

 state." 



10 Lyell, 'Principles,' tenth edit. vol. ii. p. 100. 



11 Lyell, ibid. p. 86. 



