Dr. C. Chree on Denudation and Deposition. 157 



It appears, however, from his reply to my criticisms that 

 Dr. Stoney uses the terms elastic and plastic as in many 

 ways synonymous with Maxwell's viscous, and by no means 

 limits elastic to the sense defined in chap. xxi. of Maxwell's 

 ' Heat/ and employed in the ordinary mathematical text- 

 books. He was, it now appears, thinking of slow or secular 

 displacements — to which the terms viscous or plastic are 

 commonly applied — and the numerical result which attracted 

 my attention seems to bo based on soma reasoning as to the 

 behaviour of plastic bodies which was not given in his original 

 paper in your April number (see §§ 8 & 9 of Dr. Stoney's 

 second paper, pp. 561-2). Had I known this I should 

 certainly not have criticised the result from an elastic solid 

 standpoint, however disinclined 1 might have been to 

 accept it. . 



It is obvious from Dr. Stoney's reply that he has mis- 

 understood me on several points. The sense intended to be 

 conveyed by the term " killed " applied to the elasticity was 

 that the material might have become set, after being plastic, 

 under enormous stress, so as to behave like an elastic solid 

 — much less compressible than glass — under slight further 

 variations of stress. This seemed to me the hypothesis most 

 favourable to Dr. Stoney's application of what I supposed to 

 be the ordinary mathematical theory of elasticity. 



As to the earth's altering in volume if gravity ceased to 

 act, .1 expressed no opinion. That large viscous changes ot 

 volume would ensue is, I daresay, likely enough. 



Again, 1 do not advocate the view that an elastic solid can be 

 absolutely incompressible*. What 1 do claim to have shown is 

 that, if we regard the earth as perfectly elastic to the internal 

 gravitational stresses, we must treat it as very nearly incom- 

 pressible in applying the equations of mathematical elasticity. 

 When aiming at first approximations, it is legitimate to apply 

 to a nearly incompressible solid equations which assume 

 absolute in compressibility, so long as this is mathematically 

 equivalent to neglecting small terms in a converging series. 

 Dr. Stoney's arguments in his second paper seem really 

 directed against the earth's being treated as an elastic solid 

 of any description. I consider his arguments on this head 

 inconclusive, but the question is one which 1 had probably 

 better leave him to settle with Lord Kelvin. 



Dr. Stoney's remarks in his § 14 are evidently based on a 

 misconception of my phrase " locally loaded.'''' I was re- 

 ferring to the important results of Profs. Cerruti and Bous- 

 sinesq for an elastic solid bounded by an infinite plane over 

 * For explicit statements to the contrary, see Phil. Mag. Sept. 1891, 

 p. 235, and March 1897, pp. 174, 191, 195, & 200. 



