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LII. Denudation and Deposition. — Part II. 

 By G. Johnstone Stoney, M.A., D.Sc, F.R.S* / I 



1. TN the May number of the Philosophical Magazine Dr. 

 A Charles Chree comments upon a paper in the preceding 

 number, in which the present writer suggested that Denuda- 

 tion and Deposition have probably taken a large part in 

 increasing and rendering permanent the elevations on land 

 and some of the depressions in the ocean which are found to 

 prevail over the earth. In reply to Dr. Chree's criticisms it 

 seems sufficient to call attention to the facts of nature with 

 which we have to deal, distinguishing those about which we 

 are justified in feeling confident from others of which we can 

 only affirm that they are probably what we suppose them to 

 be. To emphasize this distinction, the qualifying terms 

 certain and probable will in the following paragraphs be 

 applied to such facts of nature, or supposed facts, as we have 

 occasion to refer to. It will thus be seen that our information 

 about the compressibility of the earth is dependent almost 

 exclusively upon the known facts. 



2. It seems certain that the volume of a body of any given 

 material when it is compressed equally in all directions, 

 depends, so long as it does not undergo a change of state, 

 only on the temperature and pressure : that in fact, 



v=f(t,p), 



where v is its volume, t its temperature, and p the surface- 

 pressure to which it is exposed. The body is supposed to be 

 of such moderate size, that the variations of pressure arising 

 from gravity need not be taken into account. From this 

 expression we obtain the following expression for k, the coeffi- 

 cient of compressibility when the body is subjected to a small 

 increase of pressure without changing its temperature — 



where f is that function of t and p which is got by keeping 

 t constant and differentiating / by p only. We thus see 

 that for each kind of material the coefficient of compressibility 

 is a function of the temperature and pressure. 



3. The circumstance that the mean density of the earth is 

 only about 5-J times that of water, which we may take as 

 certain, supplies us with information which has significance 

 in our inquiry. The so-called solid part of the earth, by 

 which is to be understood the earth minus its atmosphere and 



* Communicated by the Author. 



