O. Fisher — Rival Theories of Co&mogony. 4d1 



reverting to the expression for the compressibility, measured 

 in atmospheres per square foot, which would reduce matter of 

 the density of surface rock (s) to the density (p) which obtains 

 at a given depth, it is as already stated 3*6069 X 10~ 5 ~- p(p + s). 

 Now this gives a law of compressibility for all depths, and if 

 we follow it up to the surface, giving to p the value s, we 

 obtain the corresponding compressibility for surface rocks, viz., 



3-6069 X 10- 5 -=-2 (2-T5) 2 or 2'3847 X 1 0" G . 



It will be interesting to enquire whether this value, which has 

 been obtained without any reference to experiment, comes any- 

 where near the value of the compressibility of surface rock as 

 determined by actual trial. 



It does not appear that any direct measurements of the com- 

 pressibility of rocks have been carried out ; but the values of 

 Young's modulus (M), and of the modulus of rigidity (n), have 

 been obtained in some instances. From these the compressi- 

 bility 1/k can be found by a known formula." 5 * It is obvious 

 that any experiment which gives 1/k negative must be set 

 aside as fallacious. 



All the estimates which have been made have been recorded 

 in C. G. S. units. Dr. Knottf gives certain constants which he 

 considers a "fair approximation" in the case of "fairly solid 

 rocks." From these it is possible to calculate their compressi- 

 bility by the formula, and it comes out 6*67 X 10"". 



Dr. H. Nagaoka has also measured these constants for many 

 rocks and has found for six different specimens of granite the 

 following values of Young's modulus M, and the coefficient of 

 rigidity n.\ 



2 3 4 5 6 



19-63 14-98 28-48 10'93 X 10 10 



13-99 6-89 5-05 4'47 4-43 X 10 10 



Since k must be positive No. 5 must be rejected. ~No. 2 is 

 not complete. The values of 1/k (the compressibility) calcu- 

 lated for the remaining four are 





1 



M 



42-31 



n 



18-43 





In C. G. S. units. 



In atmospheres. 



(1) l/k= 4-99 X 10'" 



(3) l/&= 2-31 X 10-" 



(4) \jk— 5'U X 10~" 

 (6) l/k= 14-46 X 10-" 



and Knott's \/k— 6-67x10-" 



5 X 10-' 

 3 X 10"' 

 7X 10- 6 

 14 X 10-' 

 8 X 10-' 



* Itis i_ 1 _3 

 k M n 





f Transactions Seismological Soc. of Japan, vol. xii, p. 119, 1888. 

 jiSee Publications of Earthquake Investigation Committee in Foreign Lan- 

 guages, vol. iv, p. 58. Tokio, 1900. 



