136 MR. R. D. OLDHAM ON THE PROPAGATION OF 



shows that no separation of the condensational and distortions! waves is to be 

 expected in the records of seismographs. Wave motion of either kind, on passage 

 from one medium to another, is split up into refracted and reflected waves, while 

 each of these is again split up into condensational and distortional waves. The 

 result of this is that, even if earthquake motion were purely condensational or 

 distortional at first, it would soon become converted, in its passage through the 

 heterogeneous materials of the earth's crust, into an extremely complex disturbance, 

 analogous to that registered by seismographs. 



He also refers to Lord RAYLEIGH'S paper, but considers that the comparatively 

 large vertical and small horizontal displacements required by the theory are not in 

 accordance with the records of seismographs. Professor MILNE has, however, lately 

 suggested* that the records of the seismographs may be misleading, and the large 

 horizontal displacements registered by them be due to tilting of the instruments, and 

 not to the inertia of the supposed steady points. 



Another suggestion in the paper is that the principal disturbance in an earthquake 

 is not purely elastic, but quasi-elastic, and that what are known as the preliminary 

 tremors are truly elastic vibrations, set up by and outracing these. 



The same volume of ' Transactions' contains a paper by Professor MILNE, t in which 

 he suggests that the effect of compressional waves propagated direct from the origin 

 to the surface is confined to the neighbourhood of the epicentre, and that beyond 

 this the earthquake motion felt and recorded is due to surface waves, set up at the 

 outcrop of the waves of compression near the origin, and thence propagated 

 outwards. 



In 1894, Wertheim's memoir appears for the first time to have influenced seismo- 

 logical research, when it was recalled to notice by Dr. A. CANCANI,| who considered 

 that in the records of earthquakes near, and at a distance from, their origin, he 

 could trace the separation of the condensational and distortional waves, having rates 

 of propagation of about 5 and 2 '5 kiloms. per second respectively. In his earlier 

 paper he considered that it was only the latter which preserved sufficient energy to 

 make them recognisable, by instrumental aid, at a distance from the origin, while the 

 condensational waves were only recognisable in its vicinity. It is clear, however, 

 from his paper, and from two others subsequently written in defence of the ideas 

 promulgated in it, that the disturbance which he attributes to the distortional waves 

 is the phase of great surface undulations, travelling like the waves of the sea, which 



* 'Seismology,' 8vo., London, 1898, p. 117. 



t J. MILNE, "On certain Seismic Problems demanding Solution " ('Trans. Seismol. Soc. Japan,' vol. 12, 

 1888, pp. 107-113). 



I " Sulle ondulazioni provenienti dei centri sismici lontani " (' Ann. Ufficio Cent. Met. e Geodyn. 

 Italiano,' 2nd series, vol. 15, 1894, Part 1, pp. 13-24). 



" Intorno ad alcune obbiezioni relative alia velocita di propagazione delle onde sismiche " (' Atti, R., 

 Ace. Lincei,' vol. 3, 1894, Part 2, pp. 30-32); " Osservazioni e risultati recenti sulla forma e sul modo di 

 propagarsi delle ondulazioni sismiche" ('Bol. Soc. Sismol. Ital.,' vol. 2, 1896, Part I., pp. 125-137). 



