THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



SEPTEMBER 1886. 



XXVIII. Onthe Physical Structure of 'the Earth. By Henry 

 Hennbssy, F.R.S., Professor of Applied Mathematics in 

 the Royal College of Science for Ireland* 



THE structure of the Earth as a mechanical and physical 

 question is closely connected with the origin and for- 

 mation of its satellite, and of the planets and satellites belong- 

 ing to the same solar system. The brilliant results obtained 

 during the present and preceding century by the aid of 

 mathematical analysis, whereby the motions of these bodies 

 have been brought within the grasp of dynamical laws, may 

 have led to the notion that by similar methods many obscure 

 problems relating to the planet we inhabit might be accurately 

 solved. But, although the general configuration of the Earth 

 and planets has been treated mathematically with results 

 which leave little to be desired, when applications of analy- 

 tical methods are attempted to questions of detail in terrestrial 

 structure, the complication of the conditions is so great as to 

 impose the necessity on some investigators of so altering 

 these conditions as to make their results perfectly inapplicable 

 to the real state of the Earth. Physical Geology presents 

 problems the solution of which undoubtedly calls for mecha- 

 nical and physical considerations ; but these may in general, 

 under the complex nature of the phenomena, be often better 

 reasoned out without the employment of the symbolical 

 methods of analysis. In most cases the conditions are totally 

 unlike those above alluded to, which admit of precise nume- 



* Communicated by the Author. 

 Phil. Mag. S. 5. Vol. 22. No. 136. Sept. 1886. R 



