1908] Oil Recent Earthquakes. 137 



become so flat that it is no longer recognisable. Should it, however, 

 run up a second estuary, we can imagine concentration taking place, 

 so that near the top of the second estuary, it would eventually become 

 instrumentally recordable. In these antipodean survivors, we see the 

 final efforts of a dying earthquake. It is only occasionally that the 

 precursors and the followers of these large waves have sufficient energy 

 to reach their antipodes. They die en route. The former, notwith- 

 standing their comparative feebleness, because they throw considerable 

 light upon the internal constitution of our earth, are the most 

 interesting feature in a seismogram. Tliey are of two kinds, a first 

 phase and a second phase. These are usually regarded as compres- 

 sional and distortional modes of wave propagation. The large waves 

 are probably quasi-elastic gravitational waves, something like an ocean 

 swell, which travel round the world with a constant velocity of about 

 3 km. per sec, causing continental surfaces to rise and fall like huge 

 rafts upon a heaving ocean. The precursors behave quite differently. 

 Phase I. may commence with a velocity of 3 or 4 km. per sec, but as 

 the length of the wave path increases, this quickly rises to 10 and 

 thence to a maximum of 12 km. per sec. These paths are assumed 

 to be along chords, and so long as these chords do not lie at a depth 

 greater than 20 or 30 miles, the speeds are such as we should expect 

 to find in materials like those composing the outer surface of our 

 earth. These waves, therefore, indicate a thickness for the earth's 

 crust, comparable to thicknesses which have been arrived at by 

 other lines of argument. The rapid approximation to uniform 

 speed suggests that below a depth of 20 or 30 miles we enter a 

 nucleus which is very rigid and fairly homogeneous. The second 

 phase waves, up to a distance of 120"^ from their origin, have a speed 

 of about 6 km. per sec. For longer paths, Mr. R. D. Oldham points 

 out that their velocity is apparently suddenly reduced. He seeks for 

 an explanation of this by postulating the existence of a central core 

 in the earth where waves are retarded and refracted with the result 

 that the wave paths no longer follow chords. These waves may, 

 therefore, emerge on the surface of the earth after having passed 

 relatively to their starting point on the farther side of its centre. 

 Whether we do or do not accept this central core, it is clear that the 

 new seismology has added in a very marked manner to the knowledge 

 we formerly possessed respecting the interior of the globe upon which 

 we live. Our ideas respecting its homogeneity and its great rigidity 

 have been changed by seismological investigations. 



When large earth waves sweep round the world, it is found that at 

 particular stations magnetic and electrometer needles have been dis- 

 turbed. Magnetometers, when installed at Toronto, do not appear to 

 have responded to the slow undulations of the earth's surface while 

 the same instruments after being removed to Agincourt, only 10 miles 

 distant, are now affected. The inference from this and observations 

 in other parts of the world is that the movements, rather than being 



