456 MR. E. D. OLDHAM ON THE CONSTITUTION" [Aug. I906, 



19. The Constitution of the Interior of the Earth, as revealed 

 hy Earthquakes. By Richard Dixon Oldham, E.G.S. (Read 

 February 21st, 1906.) 



I. Introductory. 



Op all regions of the earth none invites speculation more than 

 that which lies beneath our feet, and in none is speculation more 

 dangerous ; yet, apart from speculation, it is little that we can say 

 regarding the constitution of the interior of the earth. We know, 

 with sufficient accuracy for most purposes, its size and shape : we 

 know that its mean density is about o\ times that of water, that 

 the density must increase towards the centre, and that the tempera- 

 ture must be high, but beyond these facts little can be said to be 

 known . Many theories of the earth have been propounded at different 

 times : the central substance of the earth has been supposed to be 

 fiery, fluid, solid, and gaseous in turn, till geologists have turned in 

 despair from the subject, and become inclined to confine their 

 attention to the outermost crust of the earth, leaving its centre as 

 a playground for mathematicians. 



The object of this paper is not to introduce another speculation, 

 but to point out that the subject is, at least partly, removed from 

 the realm of speculation into that of knowledge by the instrument of 

 research which the modern seismograph has placed in our hands. 

 Just as the spectroscope opened up a new astronomy by enabling 

 the astronomer to determine some of the constituents of which 

 distant stars are composed, so the seismograph, recording the unfelt 

 motion of distant earthquakes, enables us to see into the earth and 

 determine its nature with as great a certainty, up to a certain point, 

 as if we could drive a tunnel through it and take samples of the 

 matter passed through. The subject is yet in its infancy, and much 

 may ultimately be expected of it ; already some interesting and 

 unexpected results have come out, which I propose to deal with in 

 this paper. 



So long ago as 1894 the late E. von Rebeur Paschwitz, recording 

 the Japanese earthquake x of March 22nd, found that the record 

 showed three separate disturbances or phases, but I believe that the 

 true character of this threefold disturbance was not established 

 until 1900, when I showed," by a study of the available data, that 

 the disturbance set up by a great earthquake was split up into three 

 distinct forms of wave- motion, propagated at different rates and 

 along different paths, giving rise to three distinct phases in its dis- 

 tant record. Of these, the third and latest was shown to be due to 

 surface-waves, that is to say, wave-motion propagated along, or close 

 to, the surface of the earth ; but it was also shown that the other 



1 Peterm. Mitth. vol. xli (1895) pp. 13-21 & 39-40. 



2 Phil. Trans. Roy. Soc. ser. A, vol. cxciv (1900) pp. 135-74. 



