MODERN" SEISMOLOGY — HODGSON 355 



seconds. Moreover, the distances from each of three or more 

 stations being known, for any given earthquake, the position of the 

 epicenter can be determined with a surprising degree of accuracy. 

 And, finally, this work has been done, with ever-increasing accuracy, 

 by various agencies since 1899, supplementing previous catalogues 

 of felt earthquakes and giving us an analysis of the relative seis- 

 micity of the different parts of the earth's surface, which is very 

 good, indeed. This analysis is being continuously strengthened 

 to-day by the efforts of over 200 stations, regularly operating seismo- 

 graphs and publishing data. Our time distance curves are subject 

 to but minor corrections for distances between TOO miles and 7,000 

 miles. We know our seismic areas. The seismic history of many 

 of these areas is already long continued enough to be of value in 

 various lines of investigation. From this hard-won but firmly con- 

 solidated position we step forward into the front-line trenches, the 

 active sector of seismological research. 



To detail the various investigations which combine to give us our 

 present tentative conception of the internal structure of the earth is 

 beyond the limits of this address. It must suffice that, after outlining 

 that conception, the seismological evidence supporting it be briefly 

 inspected. The methods adopted for checking the details of the pro- 

 posed earth structure will serve to give us some idea of the procedure 

 by which that structure has been inferred. 



All seismologists agree that the earth has a spherically layered 

 structure, consisting of a central core surrounded by a series of shells 

 of different thickness, each with its own distinct properties. The 

 spherical surfaces separating the various shells are probably fairly 

 well defined ; that is to say, the transition is relatively sudden. They 

 are referred to as surfaces of discontinuity, or simply as disconti- 

 nuities. 



Let us consider first the central core. The surface of discontinuity 

 surrounding it is believed to lie about 2,900 km (1,800 miles) beneath 

 our feet, or a little less than half the distance to the center of the 

 earth. The material of the core is believed to be iron, with probably 

 some nickel as well. If our earth is made of the stuff of which other 

 worlds are made (and that seems a reasonable assumption) and if the 

 meteors which occasionally fall to the earth are to be regarded as 

 samples of that material then we may infer a mixture of iron and 

 nickel for the central core. Here we have a large part of the extra 

 density demanded by the physicist to make up for the light surface 

 rocks and average up the density of the earth to the 5.6 which he 

 demands. The existence of the core is rather well established. The 

 P waves are refracted into its surface in such a manner that they fail 

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