Section A.— ASTRONOMY, MATHEMATICS, PHYSICS, 

 METEOROLOGY, GEODESY, SURVEYING, EN- 

 GINEERING, ARCHITECTURE, AND IRRIGATION. 



President of the Section : — Professor J. T. Morrison, M. A., 



B.Sc, F.R.S.E. 



MONDAY. JULY 8. 



The President delivered the following address : — 



ON THE INTERNAL STRUCTURE OF THE EARTH. 



Some fourteen years ago, Parsons, of steam-turbine fame, 

 when President of the Engineering Section of the British Asso- 

 ciation, devoted a considerable part of his presidential address 

 to the question of the feasibility of the sinking of a mine-shaft 

 to the enormous depth of 12 miles. He went in some detail into 

 the methods of construction and probable cost of such a shaft, 

 basing his proposals largely on the experience gained in deep 

 mining on the Rand. His suggestions gave rise to a considerable 

 amount of discussion, which was directed more particularly to 

 the question whether the rocks surrounding such a shaft would 

 not flow like plastic solids at the pressures and temperatures 

 which must occur at so great a depth ; but at that time there 

 v/ere few accurate experimental data on which to base a well- 

 considered judgment on this question. 



Since the date of Parsons' address, a great deal of research 

 has been directed towards the elucidation of the conditions that 

 obtain at various depths in the earth's interior, and as much of 

 this research is geophysical rather than geological in character, 

 it has occurred to me that the subject of the internal structure 

 of the earth in its broad aspects would be a suitable one for the 

 presidential address to Section A of this Association, more espec- 

 ially at a town such as Johannesburg, whose material welfare is 

 so largely dependent on mining operations. 



The problem of the structure of the earth is one that pos- 

 sesses many elements of fascination and attractiveness. It has 

 important bearings on almost all the sciences. For the astrono- 

 mer, the earth sei'ves as the short and incessantly moving base 

 from which all astronomical positions and distances must be 

 measured, as well as the planet whose life-history is most likely 

 to throw light on the course of cosmical development. To the 

 physicist and physical chemist it has the interest that it must 

 contain materials under conditions of temperature and pressure, 

 such that their faint imitation calls for the highest resources of 

 experimental skill, and that their effect on the physical behaviour 

 of the materials will always present a wide field for the specula- 

 tive tendencies of molecular philosophers. For the geologist, 

 the knowledge of the internal structure of the earth is the only 



