348 TRANSACTIONS OF SECTION C. 
One cannot consider this interesting inference from the seismographic data 
without being reminded of the contention of Ritter, Arrhenius, and Wilde 
regarding the possibility of a persistent gaseous core still above the critical tem- 
perature of the substances of which it is composed. According to Ritter,® 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 twenty-five 
miles thick, that below this it is possibly in a molten condition for about a 
hundred and fifty miles, and that the resé is a gas largely composed of iron 
aig 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 inacces- 
sible 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 Colombian earthquake of January 31, 1906, with those 
of the San Francisco quake in the following April, there was a greater interval 
noticed between the first and second phases of the Californian earthquake— 
an interval greater than can be accounted for by mere difference of distance 
between the origin of the shock and the recording instruments. The seismic 
waves which passed from Colombia to Europe must have travelled under the 
broadest and deepest part of the North Atlantic basin, while those from 
California ran under the continent of North America, crossed the North Atlantic 
not far south of Iceland, and approached Europe from the north-west, the wave 
paths throughout being under continents or the continental shelf of the North 
Atlantic. There is thus suggested some difference between the elastic con- 
ditions of the sub-oceanic and the sub-continental parts of the crust—a difference 
which, judging by the particular instances discussed, may extend to a depth of 
one-quarter of the radius, but is not noticeable in the waves which penetrate 
to one-third of the radius below the surface. 
Obviously these data must be multiplied many times before they can be 
regarded as a reliable index to a natural law; but it is significant that this 
* A. Ritter, ‘Untersuchungen tiber die Hohe der Atmosphare und die 
Constitution gasformiger Weltkérper,’ Wiedemann’s Ann. d. Phys. und Chem., 
vol. v. 405, 543 (1878); vol. vi. 135 (1879) ; vii. 304 (1879) ; viii. 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. and Phil. Soc., 
vol. 46, Part IV. p. 8, 1902). A similar suggestion based also on Halley’s con- 
ception of a separately rotating inner core was made previously by Sir F. J. 
Evans in 1878 (‘Remarkable Changes in the Earth’s Magnetism,’ Nature, 
vol. xviii. p. 80). 
10 Quart. Journ, Geol. Soc., vol. 63, 344-3850 (1907), 
