Address by Prof. Sir T. H. Holland. 415 



Oldham's deductions are based confessedly on a small number of earthquake 

 records — he considered fourteen examples only — but the conclusions based on 

 a small number of trustworthy records, from which variations due to the different 

 methods of marking the phases are eliminated, are more reliable than those for 

 which there are imperfect distant records as well as doubts regarding the exact 

 times of the disturbances. If these observations, however, be confirmed by 

 further records, we are justified in assuming that below the heterogeneous 

 crust there is a thick shell of elastic material, fairly homogeneous to about 

 six-tenths of the radius, surrounding a central core, four-tenths in thickness, 

 which possesses physical properties utterly unlike those of the outer layers ; for 

 in this core the ' distortional ' waves are either damped completely or are 

 transmitted at very much lower speeds than in the shell. 



One cannot consider this interesting inference from the seismographic data 

 without being reminded of the contention of Kitter, Arrhenius, and Wilde 

 regarding the possibility of a persistent gaseous core still above the critical 

 temperature of the substances of which it is composed. According to Eitter,^ 

 the gaseous core is surrounded by a solid shell. Dr. Wilde" postulates the 

 existence of a liquid substratum and a gaseous core within a solid crust, the 

 two outer shells having a thickness that is ' not very considerable '. Arrhenius 

 assumes from purely physical considerations that the solid crust is only about 

 25 miles thick, that below this it is possibly in a molten condition for about 

 150 miles, and that the rest is a gas largely composed of iron under a pressure 

 so great that its compressibility is not much less than that of steel. 



The whole of these conclusions, being based on assumptions regarding the 

 physical properties of matter under conditions of temperature and pressure that 

 are well beyond those of actual experience, must be put on a plane of science 

 well below that occupied by the investigations initiated by Oldham, who opens 

 up a line of research in which, as said before, the seismograph may justifiably 

 be compared with the spectroscope as an instrument for observing some 

 inaccessible regions of Nature. 



The mathematician apparently finds it just as easy to prove that the Earth 

 is solid throughout as to show by extrapolation from known physical values that 

 it must be largely gaseous. As Huxley said in his Presidential Address to the 

 Geological Society in 1869, the mathematical mill is a mill which grinds you 

 stuff of any degree of fineness, but, nevertheless, it can grind only what is put 

 into it; and the seismograph thus offers a new source of substantial grist. Now 

 that it is fairly certain that some of the earthquake waves pass through the 

 deeper parts of the Earth, it is obvious that a fruitful development of science 

 will follow successful efforts to introduce precision in recording, and uniformity 

 of expression in reading, seismographic records. 



Oldham ^ has pointed out another way in which analysis of seismographic 

 records may lead to information regarding intra-telluric conditions by comparing 

 the records of waves that pass under the oceanic depressions with those that 

 are sub-continental for the whole or most of their paths. By comparing the 

 records in Europe of the Columbian earthquake of January 31, 1906, with those 

 of the San Francisco quake in the following April, there was a greater interval 



^ A. Eitter, " Untersuchungen iiber die Hohe der Atmosphare und die 

 Constitution gasformiger Weltkorper " : Wiedemann's Ann. d. I'hj/s. und 

 Cliem., vol. v, pp. 405, 543, 1878; vol. vi, p. 135, 1879; vol. vii, p. 304, 1879; 

 vol. viii, p. 157, 1879. 



- On the Causes of the Phenomena of Terrestrial Magnetism, pamphlet, 

 1890, p. 2. The idea that the Earth's magnetism is due to the electricity 

 generated by the friction between the shell and the core, rotating with a different 

 motion, was suggested by Dr. Wilde in 1902 (Mem. Manch. Lit. Phil. Soc, 

 vol. xlvi, pt. iv, p. 8, 1902). A similar suggestion based also on Halley's con- 

 ception of a separately rotating inner core was made previously by Capt. (Sir) 

 F. J. Evans in 1878 ("Eemarkable Changes in the Earth's Magnetism": 

 Nattire, vol. xviii, p. 80). 



2 Quart. Journ. Geol. Soc, vol. Ixiii, pp. 344-50, 1907. 



