lO 



NATURE 



[September 3, 1914 



the solar system by the aggregation of innumerable 

 small bodies, " planetesimals," which have gathered 

 into knots to form the planets. Thus, the earth is 

 supposed to have grown gradually by the accretion of 

 meteoritic matter, and even now, although the process 

 has nearly ceased, it receives much meteoritic material 

 from outside. 



With the Chamberlin-Moulton theory there must 

 have been a time when the gravity of the earth was 

 insufficient to hold an atmosphere of any but the 

 heavier gases, such as carbon dioxide; later, the earth 

 became heavy enough to retain oxygen, then nitrogen, 

 water-vapour, and helium ; while even now it may not 

 be sufficiently attractive to prevent the light and agile 

 molecule of hydrogen from flying off into space. With 

 the growth of the young globe, the compression to- 

 wards the centre produced heat enough to melt the 

 accumulated fragments of meteoritic matter, and the 

 molten material thus formed welled out at the surface. 

 Such volcanic action is supposed to have predominated 

 at the surface until an appreciable atmosphere was 

 formed, and became charged with water, when the 

 now familiar processes of weathering, erosion, and 

 deposition produced the film of "rust" which geo- 

 logists know as sedimentary rocks. 



With this last addition to the variegated array of 

 theories about the physical condition of the earth and 

 about its genealogy, the scientific world began again 

 to settle down into serenity, comforted by the happy 

 feeling that all at any rate agree in regarding the 

 earth as a gradually cooling body, with many millions 

 of years still before it. Then came the discovery of 

 radium, and, with it at first, an assurance that geo- 

 logists were justified in claiming a long past, to be 

 followed by a longer future than the most optimistic 

 philosopher had dared before to assume with our 

 apparently limited store of earth-heat. Now, how- 

 ever, Prof. Joly warns us that if the deeper parts of 

 the globe contain anything near the proportion of 

 radio-active bodies found by him in the superficial 

 rocks, we may even be tending in the other direction ; 

 that, instead of a peaceful cooling, our descendants 

 may have to face a catastrophic heating; the now 

 inconspicuous little body known as the earth may 

 indeed yet become famous through the universe as a 

 new star.' 



To add to the variety of ideas regarding the present 

 state of the earth's interior. Prof. Schwarz, of 

 Grahamstown,* concludes that our volcanic pheno- 

 mena can be accounted for on the assumption that 

 the main mass of the earth below a superficial layer 

 is cold and solid throughout, being composed, like 

 the meteorites, largely of unaltered ferromagnesian 

 silicates and iron. 



Thus, we see, whole fleets of hypotheses have been 

 launched on this sea of controversy : some of the 

 craft have been decoyed by the cipher-signals of the 

 mathematician ; some have foundered after bombard- 

 ment by the heavy missiles classically reserved for use 

 by militant geologists; others, though built in the 

 dockyard of physicists, have suffered from the spon- 

 taneous combustion set up by an inadvertent ship- 

 ment of radium. Still, some of these hypotheses are 

 yet apparently seaworthy, and it may not be unprofit- 

 able to compare them with recently acquired data. 



The nearest approach to actual observation with 

 regard to the state of the earth's interior has been 

 obtained by the seismograph, designed to record the 

 movements of seismic waves at great distances from 

 the disturbing earthquake. Some of the waves sent 

 forth from an earthquake-centre travel through the 

 earth, and some travel around by the superficial crust, 

 the former reaching the distant seismograph before 



'■* J. Joly, " Radio-activity atid Geology," 1909, pp. 168-172. 

 S E. H. L. Schwarz, " Causal Geology." 1910. 



NO. 2340, VOL. 94] 



the latter. The seismograph, by its record of the 

 waves that travel through the earth, has thus given a 

 certain amount of information regarding the state of 

 the earth's interior which R. D. Oldham aptly regards 

 as analogous to that given by the spectroscope* with 

 regard to the inaccessible atmosphere of the sun. 



The existence of two groups of earthquake-waves 

 — those passing through, and those passing near the 

 surface around the earth — has long been recognised; 

 but R. D. Oldham * has shown that the waves pass- 

 ing through the earth are of two kinds, travelling at 

 two different speeds. 



The record on the distant seismograph thus shows 

 three well-marked phases : the first phase, due to 

 waves of compression passing through the earth's 

 interior; the second phase, due to waves of distortion,^ 

 also passing through the earth's interior; and the 

 third phase, recorded by the waves which pass around 

 the arc along the superficial crust. 



The third phase is always recorded at a time after 

 the occurrence of the shock proportional to the arcual 

 distance of the recording seismograph from the earth- 

 quake centre, the records of several large earthquakes 

 showing an average speed for the waves of about 

 three kilometres a second. The rates of propagation 

 of the waves giving the first and second phases are 

 both much greater than of those forming the third 

 phase ; and up to an arcual distance of about 120° 

 from the earthquake's centre the rate of their propaga- 

 tion increases with the distance. It is thus assumed 

 that the waves giving rise to the first and second 

 phases in each distant seismographic record, by follow- 

 ing approximately along the chord of the arc between 

 the place of origin and the instrument, pass through 

 deeper layers of the earth when the seismograph is 

 farther away, the material at greater depths being 

 presumably more elastic as well as denser. 



But Oldham '' has shown that when the seismograph 

 is as much as 150° from the earthquake centre there 

 is a remarkable decrease in the mean apparent rate of 

 propagation of the waves giving the second phase in 

 the record, from more than six to about four and a 

 half kilometres a second. There is also a drop, 

 although not nearly so marked, in the apparent speed 

 of the waves of the first phase when transmitted to a 

 seismograph 150° or more distant from the earthquake 

 origin. Oldham concludes that this decrease of ap- 

 parent rate for waves travelling through the earth 

 to places much more than 120° distant is due to their 

 passing into a central core, four-tenths of the radius 

 in thickness, composed of matter which transmits the 

 waves at a markedly slow speed. Thus the earth- 

 quake waves which emerge at a distance not greater 

 than 120° from their origin do not enter this central 

 core, while those which pass into the earth to a 

 greater depth than six-tenths of the radius are sup- 

 posed to be refracted on entering, and again on leav- 

 ing, the postulated core, in which the rate of trans- 

 mission of an elastic wave of distortion is very much 

 slower than in the main mass of the earth around. 

 In consequence of the refraction of these waves on 

 passing through the central core, places situated at 

 about 140° from an earthquake origin should be in 

 partial^ shadow, due to the great dispersion of the 

 distortional waves, and the few records made so far 

 by seismographs thus situated with regard to great 



■* In his presir'ential address to the Geological Society of London in 1909, 

 Prof. W. J. Sollas (Proc. Geol. .«oc.. igoo, p Ixxxvii.) credits H. Benndorf 

 (Rlitth. Geol. Gesellsch. Wien, I.. 1908, 336I with this pretty analogy, but 

 Oldham has the precedence ty just two years (c/. Quart. Journ. Geol. 

 Soc.. vol. Ixii., 1906, p. 456). 



5 Phil. Trans., Ser. A., vol. cxciv. (1900), pp. 135-74. 



6 There is more complete agreement regarding the fact that two distinct 

 sets of waves give rise to the .so-called preliminary tremors indicated by a 

 seismot;raphic record than about the nature of the waves. Confer. R. D. 

 Oldham, Phil. Trans., /oc. cit., and O. Fisher, Proc. Cambr. Phil. Soc, 

 vol. xii., pp. 354-61. 



" Quart. Journ. Geol. Sor., vol. Ixii., pp. 456-475 (1906). 



