R. D. Oldham — The Interior of the Earth. 25 



stony matter of the outer layers. This hypothesis was mathemati- 

 cally investigated by Professor S. Wiechert in 1897, 1 who found 

 that, within permissible variations, the earth might be regarded as 

 composed of a central core of density about 8 and an outer sheath 

 of density about 3, the dimension of the core being from about 

 three-quarters to less than four- fifths of the radius of the earth, 

 according to the actual densities adopted, and that such an earth 

 Avould satisfy the known conditions of mean density, as well as of 

 precession and mutation. On the latter I can offer no opinion or 

 criticism; the former is easy of verification, the densities are about 

 right for the stony casing and the mainly iron core, allowing for the 

 effects of pressure and compression, so that the hypothesis may be 

 accepted as at least a possible one, and it is noteworthy that th& 

 limit of the two parts of the earth lie just where the earthquake 

 records indicate a change in composition, unaccompanied by change 

 in state, of the material of which the earth is composed. 



There remains the central part of the earth, penetrated by wave 

 paths emerging beyond 120° from the epicentre. In 1906 it was 

 still doubtful whether the so-called second phase at these distances 

 represented the much delayed distortional waves, travelling by a 

 direct path, or was of a different character. In Professor Wiechert's 

 paper, already referred to, the slowing down of the rate of 

 transmission, at depths below a little more than half the radius, was 

 recognized, but the second phase was accepted without question as 

 representing the same phenomenon as at distances of 100° and less, 

 and this has remained the interpetation accepted by the Gottingen 

 school, up to the latest publication which has reached us. In 

 this country the trend of thought has been different; the first 

 noteworthy landmark was -the demonstration by Dr. G. W. Walker 

 that what was recorded as the second phase at these great distances 

 synchronized with the time at which waves reflected at, or near, the 

 surface of the earth would reach the place of record, 2 and this seems 

 still the most probable interpretation. Lately Professor H. EL Turner 

 has attacked the same problem and, in the latest reports of the 

 British Association and the Shide Bulletins, has shown, by statistical 

 methods, that the so-called second phase at distances beyond the 

 limit of 120° must be a different phenomenon from the second phase 

 at lesser distances. Meanwhile, by an entirely different path, I had 

 myself arrived at a similar conclusion ; the examination of records 

 of instruments of the type generally used in Italy, which give the 

 second phase in an exceptionally clearly marked and conspicuous 

 manner, showed that the so-called second phase at very long 

 distances was of a different type altogether, and a few years ago 

 I was able to examine some of the records of the Galitzin instrument 

 at Eskdalemuir, which gave just the same result. The typical 

 second phase, when well developed, shows a distinct commencement, 

 a well-marked maximum and a less rapid dying out; it is, in fact, 

 patently the record of a single group of waves of one character and 



1 Gottingen Nachrichten, 1897, pp. 221-43. 



2 Modem Seismology, 1913. 



