ON SE18M0L0GICAL INVESTIGATIONS. 269 



iustruinents free of charge, there is no charge upon the Seismology Committee in 

 connection with the erection of the seismograph at Christmas Island.' 



As no records were received at Oxford, nor any answers to repeated inquiries 

 (principally made through Mr. Jones), application was ultimately made, as suggested 

 by Mr. Jones on March 17, 1925, to the Secretary of the Phosphate Company at the 

 London office, that the seismograph and clock might be returned to Mr. J. J. Shaw. 

 They were received by him free of charge, but in a very much damaged condition ; 

 corrosion had attacked many of the metal parts, and the wooden cases were ruined 

 by some form of dry rot. The instrument is being thoroughly repaired, and will now 

 be lent to the Colombo Observatory. 



The original Milne-Shaw instrument set up at Bidston, which has been replaced 

 by another with larger magnification, is to be set up at Oxford as aN.-S. component, 

 being similar to the E.-W. component already mounted (by the courtesy of Prof. 

 Lindemann) in the basement of the Clarendon Laboratorj-. It seems now probable 

 that a basement may bo constructed at the University Observator}"^ itself, to which 

 this pair of instruments will then be transferred. 



The two Milne-Sliaw seismographs sent to Entebbe, Uganda, in 1923, which 

 were awaiting suitable accommodation, have now been installed in a well-appointed 

 observatory. The ground floor is used as the office, beneath is the dark room, and 

 lower still (23 ft. below ground) is the instrument room. This should prove a very 

 efficient station in a locality where one was much needed. There are a number of 

 epicentres running down the African continent at which shocks occur which may not 

 reach more distant observatories. 



Seismographs have also been sent to Seiior Scipion Llona, Director of the Seismo. 

 logical Observatory, Lima; also a second component to Fordham University, New 

 York ; finally to Prof. N. E. Norlund, Copenhagen, with a view to the establishment 

 of a station in Greenland, which would be of great importance for the observation of 

 earthquakes in the Arctic regions. 



Bulletins and Tables. 



The ' International Seismological Summaries ' for April to December 1921 and 

 January to September 1922 have been printed and distributed. The number for 

 October to December 1922 is passed for press, with the exception of an appendix 

 dealing with some belated observations. There are thus now five years of the Summary, 

 1918-1922, and the residuals for P and S shov,Ti by the chief earthquakes have been 

 collected for discussion of the corrections to tables. The general nature of these 

 corrections has already been anticipated from partial study of the records, but it is 

 an important question whether we are in a position to make a definitive change, 

 for it is undesirable to multiply changes. The answer is, on the whole, encouraging ; 

 the corrections to the P and S tables are clearly shown within about 1 second as far 

 as A =80^. But beyond that point the correction to S is ambiguous ; it seems that 

 we are dealing with two phenomena and not one. The second phenomenon is easily 

 identified as that designated ScPcS by Gutenberg many years ago. It represents 

 the passage of a ray as S down to the liquid core ; its passage as P through the core, 

 and again as S on leaving the core. Attention was recently drawn to Gutenberg's 

 papers by Dr. Harold Jeffreys, and the accord between observation and this particular 

 part of his theory is very striking. (Other parts have not j'et been compared with 

 our records.) Accordingly Dr. Jeffreys has written the following paragraphs for this 

 Report : — ■ 



The Earth's Central Core. 

 By Dr. Harold Jeffreys. 

 The high mean density of the earth, in comparison with that of surface rocks, has 

 long been held to imply that the earth has a dense metallic core, probably mainly 

 of iron. From considerations relating to the figure of the earth Wiechert determined 

 the radius of this core as 0'78a, a being the outer radius, and its density as 8*2. Oldham 

 in 1906 found that the compressional waves from earthquakes showed a systematic 

 delay when observed at epicentral distances greater than 103^ or so, and inferred that 

 within a central region the velocities of these waves were lower than elsewhere. 

 Subsequent work of the greatest importance by Gutenberg has strongly confirmed 

 and extended Oldham's result. There is a central core of radius 0-55a, which transmits 

 P waves with about two-thirds of the velocity they have just outside it, but does not 

 transmit S waves at all. Several incidental phenomena implied by the existence of 



