u 



KNOWLEDGE. 



[February 1, IIIOI. 



THE PROGRESS OF SEISMOLOGY DURING 

 THE NINETEENTH ChNIURY. 



By Charles Davison, sc.d., f.g p. 



Seismoloc4Y a<s a science is a product of tlu latter liaif 

 of the nineteentli century. Recalling earlier contri- 

 butions to the subject, that which stands out tlie most 

 prominently is John Michell's memoir on tl.e cau»3 and 

 phenomena of earthquakes, read before the Royal Society 

 in 1760. It is interesting to notice how Michell aiitici- 

 jjates some of the results of later workers. He recog- 

 nises, for instance, that the slow-period waves are 

 propagated to far greater distances along the surface 

 than the rapid vibrations which form the jjerceptible 

 shock, and that the velocity of seismic sea^waves in- 

 creases with the depth of the ocean. He suggests that 

 the position of the epicentre may be determined by 

 obsei-vations of the time of occuirenca or tlie 

 direction of the motion. And, though unable, f.s we 

 still are, to ascertain the depth of the fociis, he makes 

 a " random guess " that, in the case of the Lisbon 

 earthquake, it was nob less than a mile or a mile and a 

 half and probably not more than thi'ee miles. Michell's 

 theoretical views exercised no slight influence on those 

 of his successors. Their chief interest at present is to 

 show how far- we have travelled during the nineteenth 

 century. 



To describe adequately the progress that' has been 

 made would, therefore, be almost the same as to sum- 

 maaise our present knowledge. Nevertheless, there 

 are certain salient features in the liistoiy of the 

 subject to which at the beginning of a new century 

 it seems desirable to direct attention. 



(1) During the first half of the past century, the 

 contributions were comiDaratively few in number. 

 Among the most important may be mentioned Darwin's 

 memoir on the volcanic phenomena of South America, 

 and the commencement of the long series of earthquake- 

 catalogues compiled with unfailing industry by Alexis 

 Perrey of Dijon, between the years 1843 and 1874. It 

 would be difficult to investigate the seismic history of 

 any jjortioii of the earth without recourse to one or 

 more of Ferrey's valued works. 



(!') The foundation of seismology wa.s laid in 184G, 

 when Robert Mallet applied the known laws of wave- 

 motion in solids to the phenomena of earthquakes. 

 Obvious a.s such an application may seem at the present 

 day, and suggested as it had been by Michell, Thomas 

 Young and others, one cannot estimate Mallet's per- 

 formance too highly. Though his views on many points 

 are superseded, he threw fresh light on the bearing of 

 facts already known, invented much of the existing 

 terminology, and determined experimentally the velocity 

 of eaa-th-wavcs in several different rocks. The catalogue 

 of recorded earthquakes, prepai-ed with the help of his 

 son, will long be a book of reference (o seismologists. 

 But Mallet's greatest achievement was the investigation, 

 by methods duo almost entirely to himself, of the earth- 

 quake which devastated the kingdom of Naples in 

 December, 1857. No clearer evidence could be furnished 

 of the abiding influence of his labour's tlian the fact 

 that, out of about 3000 observers of the recent Hereford 

 earthquake, nearly .500 at once rccnrdcd tlie (lii-cctiun 

 lit' the shock. 



(3) To apjireciate the importaiice of the next step, 

 one has only to read first the article on " Earthquakes," 

 published in 1877 in the Enci/c/opdJIa lirltannica, and 

 t>hen that on the " Seismometer," which appeared nine 

 years later in the same work. During the interval. 



seismographs based on scientific principles were invented 

 by Profs. Ewing, Gray and Milne, and their value 

 tested by records of numerous earthquakes in Japan. 

 A modern seismological observatory, indeed, can hardly 

 be regarded as complete if it does not contain either 

 the Gray-Milne or Ewing's three-component seismo- 

 graph. 



While these instruments were expressly made to meet 

 a long-felt want, we are indebted almost to accident for 

 the use of the various forms of the horizontal pendulum 

 which have proved so serviceable in the investigation 

 of distant earthquakes. Hengeller in 1832, Gerard in 

 1851, Close and Zollner in 1869, the Darwins in 1880, 

 and von Rebeur-Paschwitz in 1887, all designed the 

 instrument for purposes foreign to seismology, but 

 nevertheless prepared the way for the detailed changes 

 inti-oduced during the last eight years by Milne, Ehlert, 

 Grablovitz, Cancani, and Omori. Good results have 

 also been obtained by means of the long and heavy 

 pendulums favoured by Italian observers. 



(4) Changes in the amplitude, period and direction 

 of earthquake^vibrations axe readily distinguished 

 without instrumental aid ; but seismographs have 

 done more than merely add precision to the evidence of 

 t)ui' senses. They have rendered manifest features of 

 the earthquaie-motion tliat would otherwise have passed 

 unnoticed. Still more interesting are the revelations 

 of the horizontal pendulum with regai'd to the pulsations 

 of distant earthcjuakes. By the disturbance of magneto- 

 graphs, levels, or lakes, the propagation of surface 

 undulations to immense distances had been known for 

 more than a century. For the fuller knowledge gained 

 during the last twelve years, we are indebted to the 

 late von Rebeur-Paisohwitz and those upon whom his 

 nuuitle has fallen — Pi-of. Milne, Dr. Agamennone, Mr. 

 Oldham and others. Much still remains to be learnt 

 in this fascinating field of inquiry, but it is no slight 

 feat to have proved tliat, in an eaathquake, two series 

 of elastic waves traverse the body of the eartih with 

 velocities of not less than 9 and 63 kilometres per 

 second respectively ; while the slow-period undulations 

 spread over the suHacc at the rate of 3 kilometres per 

 second, the latter having been traced to distances of 

 more than four-fifllis of the earth's circumference. It 

 IS an achievement, worthy of the last years of the 

 century. 



(5) 'While the more obvious earthquake-phenomena 

 were well-known fifty yeai's ago, closer study has 

 revealed others of equal importance. Statistical 

 inquiries have proved that earthqualies are fai' 

 more numerous than was formerly supposed, the most 

 modern estimate being that one takes place on an 

 average every half-hour. Harmonic analysis of the 

 seismic records of different countries indicates a distinct 

 perio^Ucity in the occiurence of eartlujuakcs, the 

 niaxinuiiii nl' the annual period being as a rule in the 

 local winter and that of the diurnal period at noon. 

 The latiCst seismic maps, in which ejjicentres are mai'ked 

 instead of disturbed areas, have led to the conclusion 

 that the most sensitive regions are those in, which the 

 mean surface-slope is greatest; while the Japaiie»<: 

 earthquake of 1891 and the Indian earthquake of 1897 

 have shown how ra))id may be the VAie of terrestrial 

 change. 



(6) Following the exjaiuple of Mallet, detailed his- 

 tories of important earthquakes have been publisihed 

 by various workers during the last twenty j'eai's ; par- 

 ticularly of the Ischian, Andalusian, Charlcstown. 

 Riviera, Zante, Laibach, Hereford and Indian eai'tli- 



