658 



NATURE 



{Oct. 31, i8»9 



may be the cause of the sound-phenomena of earthquakes, and 

 the movements which, although not felt by human beings, alarm 

 the lower animals. Why are the larger movements of an 

 earthquake preceded by tremors of this description? 



The rate at which earthquake motion is propagated is some- 

 times very high. From Toronto to Ontario in the Charleston 

 earthquake of 1886 the velocity was over I5,cxx) feet per second, 

 whilst at the destruction of Flood Rock, in 1885, velocities of 

 20,000 feet per second were observed. 



As was suggested by Sir William Thomson, observations like 

 these may mean that the rigidity of the Earth is greater than 

 that of surface rocks. If it is so, then, as Sir William suggested, 

 extended observations may lead to the determination of these 

 rigidities. In connection with this, it must be remarked, that, 

 both for artificial as well as for natural disturbances, the velocity 

 of transmission varies with the intensity of the initial disturb- 

 ance, the nature of the medium, and it becomes less as a 

 disturbance radiates. General Abbot, however, in one instance 

 noted an increase in velocity as a disturbance radiated. 



What is perhaps closely connected with the above, or what at 

 least is analogous, is the fact that at a given station the wave- 

 period becomes longer as a disturbance dies out, and it also 

 becomes longer as a disturbance radiates. One inference to be 

 drawn from this may be that long-period earthquakes originate 

 at a distance, a wave flattening out as it radiates, pretty much as 

 it does in the ocean ; but Sir William Thomson has suggested 

 that a long-period disturbance may be related to the dimensions 

 of the focal cavity. Recently, in Tokio, an earthquake having 

 a period of about eight seconds was observed. For seismographs 

 to record this, they must have been tipped from side to side. 



Another point of interest is that for sinall displacements 

 period increases with amplitude, but after a certain amplitude is 

 reached the period is either constant or only increases very 

 slowly. This observation apparently harmonizes earth motions 

 with those of ideally elastic bodies. 



Another set of interesting observations is the relationship 

 between normal and transverse movements. At a short dis- 

 tance from an origin the normal movement distinctly outraces the 

 transverse motion, but when the amplitude of the normal motion 

 has been decreased until it practically equals that of the trans- 

 verse motion the separation between the movements is tiil. A 

 curious feature, especially in normal movements, is the fact that 

 near an origin the movement inwards or towards the source of 

 the disturbance is greater than it is outwards ; further, as a 

 normal wave radiates it 'may be observed to gradually break up 

 into two waves, in consequence of which a diagram taken at one 

 station may be very different from one taken at another. 



In conclusion, I will point out a few observations which, on 

 account of the expense they involve, the difficulty of obtaining 

 •observers, &c., have not, or at least only partially, yet been 

 undertaken : — 



I. Velocity of Earthqtiake Propagation. 



The importance of determining the velocity of earthquake 

 propagation has already been noted, and it has been shown that 

 on account of the difference in the aspect of diagrams at neigh- 

 bouring stations it cannot be done on a small area. The Im- 

 perial Telegraph Department of Japan is at the present moment 

 giving its aysistance in this matter, but as so much depends upon 

 telegraph operators who have duties to attend to, a number of 

 specially constructed timepieces are required. 



2. A Gravity Observatory. 



Many years ago Sir William Thomson suggested^the import- 

 ance of an observatory to determine whether there are changes 

 in the value of G. In Japan we have a country where super- 

 ficial and probably subterranean and suboceanic changes are 

 taking place very rapidly. Last year the greater portion of a 

 mountain was blown away, and an area measuring 12 miles by 

 lO miles was, in ten minutes, or less, buried from 30 to loo feet 

 beneath a stream of earth and rock. What is going on beneath 

 our volcanoes we do not know, but every how and then they 

 pour out volumes of solid matter. Along the coast we have a 

 sharp and deep depression, perhaps the deepest on the face of 

 the globe. Do submarine landslips take place along this coast, 

 as they most certainly have done in other parts of the world ? 



Lastly, may there not be an apparent change in the value of 

 G, dependent upon the time at which the observations have been 

 made ? At certain seasons tremor storms are very marked, and 

 may not their minute movements have a cumulative effect upon 



the small swings of pendulums used in gravity determinations 

 Pendulum observations of the ordinary kind have been made in 

 many parts of Japan, from the summit of Fujiyama to the Bonin 

 Islands, but neither in japan nor in any other portion of the 

 world — so far as the writer is aware — has a pendulum been 

 swung at a given point for a considerable period of time. 



3, Observations on Submarine Changes, 



Many of the Japanese earthquakes originate near the deep 

 submarine depression which has been spoken about. If any of 

 these are due to suboceanic land slides or sinkings, as have been 

 observed in the Mediterranean and in the Atlantic, such changes 

 might be noted by periodical soundings, and perhaps even by 

 the records£of tide-gauges. 



4. Magnetic Obseriations. 



Dr. Edmund Naumann called attention to the fact that nea. 

 certain Japanese volcanoes there have been abnormal changes in 

 declination. The volcanic rocks of Japan are exceedingly 

 magnetic, and they are of enormous extent. Many of the rocks 

 in Fujiyama will deflect an ordinary compass needle through 180°. 

 Now, as these rocks are sometimes hot and sometimes cold, 

 whilst at the time of an eruption, and possibly at other times, 

 there are subterranean shiftings in the positions of these mag- 

 netic masses, could not these changes be noted by establishing 

 a magnetic observatory on the side, say, of a recent volcano like 

 Fujiyama? 



5. Bending of the Earth Critst due to Tidal Load. 



Some )'ears ago, in conjunction with Mr. John Stoddarl, 

 Manager of the Takashima Colliery, the workings of which ex- 

 tend a considerable distance beneath the Pacific Ocean, the 

 writer, by means of a simple apparatus, endeavoured to measure 

 any movement of the roof of the mine which might be occasioned 

 by the rise and fall in the tide. Unfortunately, the apparatus 

 together with other instruments were lost by a complete sub- 

 sidence in one portion of the workings, and these observations, if 

 they are of any value, have to be recommenced. 



6. Tromomctric Observations. 



A continuous automatic observation of earth tremors has 

 hitherto only been made in Tokio. Might not these observations 

 be continued in a coal-mining region to determine whether 

 these minute earth movements, which are certainly connected 

 with barometrical jchanges, hold any relation to the outflow of 

 fire-damp. ' 



7. Earth Currents. o 



Have earth currents any relationship to earth tremors and to, 

 earthquakes ? Earth currents are produced when the ground is, 

 shaken by an explosion of dynamite, but this might be due to 

 the increase or diminution of pressure in the earth plates causing 

 changes in chemical activity. Have earth currents been observed 

 in the vicinity of an active volcano, or in relation to some large 

 earth fracture ? 



8. Earth Oscillations. 



In Japan the borings of marine shells, 10 or 12 feet above high- 

 water mark, in very soft rock which easily disintegrates, shows 

 that there has been a rapid movement in the earth's crust rela- 

 tively to sea-level. Is it likely that this could be measured, and 

 the axis of the movement be determined, by repeating, at 

 intervals of twelve months, the levelling of two lines running as 

 nearly as possible at right angles to each other? It has been 

 suggested that, if the movement is rapid, say i inch per year in a 

 large bay like that of Tokio when the rise and fall of the tide is 

 small, the isochronous observation of records obtained under ap- 

 proximately similar conditions from a series of tide-gauges, the 

 level at each gauge relative to some fixed point on the neighbour- 

 ing rocks being known, might furnish data which would be of 

 value in the measuring of earth oscillations. 



These, then, are a few suggestions respecting work which might 

 perhaps be better carried out in Japan than in most other 

 countries. They are laid before this meeting for the purpose 

 of obtaining an opinion as to how far they may be regarded as 

 legitimate subjects for investigation ; and if the members of the 

 meeting will freely criticize them, or suggest other lines of re- 

 search, a benefit will be conferred upon workers in Japan, and 

 on all who are interested in earth physics. 



