Theory of the Contraction of a Solid Earth, 391 



was employed, they obtained the minimum of sound in the 

 telephone with a resistance of 9,300. I have, therefore, 

 again tested my method for such effects : a coil of nearly a 

 thousand ohms was inserted in the fourth arm of my bridge, 

 and the values obtained with the ratio of the other arms, 

 10 : 10, 100 : 100, 1000 : 1000, were in the three cases the 

 same ; nor did they differ on varying the speed of alterna- 

 tion, the value being also the same as when the commutator 

 was not working. What results may be obtained with a 

 telephone as indicator I do not know, but with the galvano- 

 meter method the results obtained for dilute solutions need 

 not be considered as entirely illusory. 

 Cavendish Laboratory, Cambridge. 



L. A Reply to Objections raised by Mr. Charles Davison, 

 M.A., to the Argument on the Insufficiency of the Theory 

 of the Contraction of a Solid Earth to account for the 

 Inequalities or Elevations of the Surface. By Rev. O. 

 Fisher, J£ A, F.G.JS* 



A PAPER by Mr. Davison has appeared in the ' Philo- 

 sophical Transactions,' communicated by Professor 

 Bonney, treating of the distribution of strain in the Earth's 

 crust resulting from secular cooling f. It consists of two 

 parts. Part I. is of much interest, and shows that, upon the 

 hypothesis of the earth having cooled as a solid sphere, there 

 has always been a certain level, descending with the time, 

 above which the strata are suffering compression, and below 

 which they are being extended (" stretched "). An impor- 

 tant note upon the mathematical treatment of this question 

 has been appended by Professor Darwiu, who appears to have 

 acted as referee. The existence of such an unstrained level 

 had been already perceived by Mr. Meilard Readej:. 



Part II. of Mr. Davison's paper is devoted to a criticism of 

 my argument on the insufficiency of the contraction theory 

 to account for the existing inequalities of the Earth's surface, 

 and it is in defence of what I have published in this Magazine 

 upon this subject, that I offer the following remarks. 



The whole question will be made more clear by considering 

 first of all what would happen in two purely hypothetical 

 cases. 



1. If a sphere were to be cooled suddenly throughout by 



* Communicated by the Author. 



t Phil. Trans. Roy. Soc. vol. 178 (1887), pp. 231-249. 

 % ' The Origin of Mountain-Ranges,' chap. xi. (1886.) See Phil. Mag. 

 August 1887. 



