2/4 



NA TURE 



[January 20, 1898 



observatories : Bombay, Batavia and Hong Kong, about 

 the period of the eruption of Krakatoa in 1883. This 

 may be only a coincidence, but may it not also point 

 to the possibility that the changes below the surface of 

 the earth which culminated in that mighty explosion, 

 and may still be at work, have had and continues to have 

 magnetic effects which are recorded by the needles at 

 these observatories ? " (See also " The Volcanoes of 

 Japan," by J. Milne, Trans. Sets. Soc, vol. ix. part 2, 

 p. 178, 1886.) 



The seismograms of unfelt earthquakes tell the student 

 of dynamical geology that certain activities to which 

 he devotes attention are more pronounced beneath the 

 ocean than they are on land, whilst the location of the 

 origins from which these movements proceed indicate to 

 the cable engineer districts to be avoided. 



If we except the action of waves, the borings of teredo, 

 and other influences resulting in cable destruction in 

 shallow water, submarine earthquakes and their accom- 

 panying land-slides are responsible for very many inter- 

 ruptions of international communication. This form of 

 destruction is particularly marked along the west coast of 

 South America, and it is not unknown in the Atlantic, 

 the Mediterranean, and the Indian Ocean. In 1888 a 

 submarine earthquake cut off the Australian colonies 

 from the outer world for a period of nineteen days, and 

 the apprehension that the isolation was an act of war led 

 to the calling out of military and naval reserves. Had 

 Australia been provided with a single instrument to 

 record the unfelt movements, which must have reached it, 

 anxiety and expense would have been avoided. 



From an official notification I learn that two West 

 Indian cables gave way on December 31, 1897. I am 

 not certain, but the Isle of Wight seismograms indicate 

 that the failures probably took place at 11.30 a.m.. 

 G.M.T., on December 29. 



One immediate use of a seismogram is that it tells us 

 whenever a large earthquake occurs, and gives a locus for 

 its origin, which information has already on more than 

 one occasion been the means of correcting, confirming, 

 extending and disproving ordinary telegraphic informa- 

 tion. Sometimes messages have reached us which have 

 contained errors in their dates of one or two days, others 

 have been grossly exaggerated accounts of small disturb- 

 ances, whilst a third group, inasmuch as they create 

 feelings of anxiety without reason are the most repre- 

 hensible of all, have, if we except the feelings of satis- 

 faction they gave to their senders, been without founda- 

 tion. In all these instances the seismogram or its absence 

 has assisted in the interpretation of the telegram. 



Up to the present seismology has found foster-parents 

 in the meteorologist and the geologist — the one collecting 

 facts about earthquakes, whilst the other wrote about 

 them. Because it has grown to unwieldy proportions, 

 it seems time that the child should try and stand 

 alone and become guardian of its own discoveries. In 

 " diurnal waves " and " tremors " we see Romulus and 

 Remus seeking a mother, and although they are not 

 of the genus terrcF7notus, because they partially, at least, 

 represent earth-movements, Seismology has become their 

 guardian. 



By a diurnal wave we mean a slow tilting which takes 

 place in piers and buildings, especially on fine days, for 

 six or ten hours rapidly in one direction, and during the 

 remainder of the twenty-four hours, but 'more slowly, in an 

 opposite direction (Fig. 9). This movement may be found 

 underground where changes in temperature are insigni- 

 ficant. The actual causes of these movements are at 

 present matters for speculation, but the theory which 

 best explains the phenomena they present (as for 

 example, that on opposite sides of a valley, it has been 

 observed that movements take place simultaneously but 

 in opposite direction), is that these changes in the 

 vertical are due to differential changes on opposite sides 



NO. \Ar7i- VOL. 57] 



of a station in the loads removed during the day, or 

 acquired during the night by evaporation and conden- 

 sation of aqueous vapours. During the day it is assumed 

 that, by ordinary evaporation and the transpiration of 

 plants, the bottom of a valley loses more weight than 

 its comparatively drier and less clothed sides. 



During a hot day the stream at the bottom of such a 

 valley should discharge fewer and fewer gallons of water, 

 whilst the valley bed, because it is relieved of load, 

 should rise. For the remaining fourteen or eighteen 

 hours, because aqueous vapour is condensed beneath 

 the chilled surface of the ground, or as it emerges from 

 the ground on plant and other surfaces, the stream in 

 the bottom of the valley would increase its flow, and 

 relatively to the sides and bounding ridges of the valley, 

 where we may suppose the conditions for condensation 

 to be less favourable, the lower parts of the same would 

 become heavier and thereby sink. As to whether this 

 concertina-like opening and shutting of valleys repre- 

 senting changes of slope of one or two inches in three 

 or four miles in the average inclination of their boundaries 

 really exists, all we can say is that instruments have 

 given indications that can be explained on such a sup- 

 position. The fact that the piers carrying some of the 

 instruments have risen from the chalk, and not from the 

 alluvium, and that during long-continued wet weather 

 there is continuous creeping of a horizontal pendulum 

 towards the heavily loaded valley bottom, and that the 



Fig. 9.— Diurnal waves at Shide, I.W., i mm. deflection = tilt o"-5. 

 (Milne.) 



direction of greatest movement at the time of an earth- 

 quake appears to be at right angles to the dip, from 

 which it may be inferred that valleys due to geotectonic 

 folding exhibit a certain flexibihty, tend to support the 

 idea that the observed diurnal movements are due to 

 actual movements of more or less extensive areas. No 

 doubt some portion of the observed effects may be directly 

 attributable to solar radiation. 



In searching for an explanation of diurnal, annual arid 

 other changes in the vertical, the seismologist has had to 

 consult the records of the astronomer, the observations 

 of the hydraulic engineer, the botanist, and the farmer, 

 and to experiment and search for information in domains 

 far removed from anything supposed to be connected 

 with movements of the ground. 



Have changes in the vertical been most pronounced in 

 regions where it may be supposed that orogenic changes 

 are yet in progress ? How far are changes in the vertical 

 effected by seasonal and daily changes in temperature, 

 by fluctuations in barometrical pressure, by the rise and 

 fall of tide upon a coast line, and by lunar attraction ? 

 What tilting effects would result if the seasonal growth 

 and partial removal of foliage and herbage on one side 

 of an observing station were greater than those upon the 

 opposite side ? What is the rate at which alluvium may 

 creep down the face of steep slopes, carrying with it 

 perhaps a forest, and what is its cause? What is the 

 amount of moisture transpired by various plants per day, 

 per month, and per season ? Do not some plants and 



