3 68 



NA TURE 



{August 16, 1883 



internal pressure it will be observed both tend in the same 

 direction, namely, to an elevation of the earth's crust. 



This explanation of the increased activity of earth 

 tremors which is I believe due to M. di Rossi, is here 

 only advanced as a speculation — more probable perhaps 

 than many others. We know how a mass of sulphur 

 which has been fused in the presence of water, in a closed 

 boiler, gives up in the form of steam the occluded moisture 

 upon the relief of pressure. In a similar manner we see 

 steam escaping from volcanic vents and cooling streams 

 of lava. We also know how gas escapes from the pores 

 and cavities in a seam of coal on the fall of the baro- 

 metrical column. We also know that certain wells 

 i lcrease the height of their column under like conditions. 

 The latter of these phenomena may be added to that 

 which we have already mentioned, as a result consequent 

 on diminution of atmospheric pressure, which, by its ten- 

 dency to render an area of less weight, facilitates its rise. 

 The next question is as to whether we have any direct 

 evidence of such heavings and sinkings in our earth's 

 crust. 



Although some of the proofs which are brought forward 

 to show that slow pulsations like these are phenomena 

 which have been repeatedly observed are unsatisfactory, 

 taking them one with another they indicate that these 

 pulsatory phenomena have a real existence. 



Pendulums for instance which have been suspended for 

 the purposes of seismometrical observations, have, both 

 by observers in Italy and Japan, been seen to have moved 

 a short distance out from and then back to their normal 

 position. 



This motion has simply taken place on one side of their 

 central position, and is not due to a swing. The character 

 of these records is such that we might imagine the soil 

 on which the support of the pendulum had rented to have 

 been slowly tilted and slowly lowered. They are the 

 most marked on those pendulums provided with an index 

 writing a record of its motions on a smok.d glass plate, 

 which index is so arranged that it gives a multiplied 

 representation of the relative motion between it and the 

 earth. As motions of this sort might be possibly due to 

 the action of moisture in the soil tilting the support of 

 the pendulum, and to a variety of other accidental causes, 

 we cannot insist on them as being certain indications that 

 t'lere are slow tips in the soil, but for the present allow 

 them to remain as possible proofs of such phenomena. 



Evidences of displacements of the vertical which are 

 more definite than the above are tho-e made by Bertelli, 

 Rossi, Count Malvasi, and other Italian observers, who, 

 whilst recording earth tremors, have spent so much time 

 in watching the vibrations of stiles of delicate pendulums 

 by means of microscopes. As a result of these observa- 

 tions we are told that the point about which the stile of a 

 pendulum oscillates is variable. These displacements 

 tike place in various azimuths, and they appear to be 

 connected with changes of the barometer. 



From this and from the fact that it is found that a 

 number of different pendulums differently situated on the 

 same area give similar evidence of these movements, it 

 would hardly seem that this phenomena could be attri- 

 l.uted to changes in temperature, moisture, and the like. 

 M. S. di Rossi lays stress on this point, especially in con- 

 i.eciion with his microseismograph, where there are a 

 number of pendulums of unequal length which give indi- 

 cations of a like character. The directions in which these 

 tips ot the soil take place, which phenomena are notice- 

 able in seismic as well as microseismic motions, Rossi 

 sta'es are related to the direction of certain lines of 

 /aulting. 



Bubbles of delicate levels when examined by a micro- 

 scope change their position with meteorological variations, 

 Out Rossi also tells us that they change their position, 

 sometimes not to return for a long time during a micro- 

 seismic storm. Here again we have another phenomena 



pointing to the fact that microseismic disturbances are 

 the companions of slow alterations in level. 1 



The more definite kinds of information which we have 

 to bring forward, tending to prove the existence of earth 

 pulsations too slow in period to be felt, are those which 

 appear to be resultant phenomena of great earthquakes. 



The phenomena that we are certain of in connection 

 with earth vibrations, whether these vibrations are produced 

 artificially by explosions of dynamite in bore holes, or 

 whether they are produced naturally by earthquakes, are, 

 firstly, that a disturbance as it dies out at a given point 

 often shows in the diagrams obtained by seismographs a 

 decrease in period ; and secondly, a similar decrease in 

 the period of the disturbance takes place as the disturbance 

 spreads. 



As examples of these actions I will refer to the diagrams 

 which I have given in a paper on the " Systematic Obser- 

 vation of Earthquakes" in vol. iv. of the Transactions of 

 the Seismological Society of Japan. 



In a diagram of the disturbance of March 1, 1882, it 

 seems that the vibrations at the commencement of the 

 disturbance had a period of about 3 per second, near the 

 middle of the disturbance the period is about ri, whilst 

 near the end the period has decreased to "46. That is to 

 say, the back and forth motion of the ground at the 

 commencement of the earthquake was six times as great 

 as it was near the end, when to make one complete oscilla- 

 tion it took between two and three seconds. Probably the 

 period became still less, but was not recorded owing to 

 the insensibility of the instruments to such slow motions. 

 We have not yet the means of comparing together 

 diagrams of two or more earthquakes, one having been 

 taken near to the origin and the other at a distance. The 

 only comparisons which I have been enabled to make 

 have been those of diagrams taken of the same earth- 

 quake — one in Tokio and the other in Yokohama. As 

 this base is only sixteen miles, and the earthquake may 

 have originated at a distance of several hundreds of miles, 

 comparisons like these can be of but little value. 



The best diagrams to illustrate the point I wish to bring 

 forward are those at the end of the paper just referred to. 

 These are the results obtained at three stations in a 

 straight line, but at different distances from the origin, of 

 a disturbance produced by exploding a charge of dynamite 

 in a bore hole. A simple inspection of the diagrams 

 shows that at the near station the disturbance consisted 

 of back and forth motions which, compared with the same 

 disturbance as recorded at a more distant station, were 

 very rapid. Further, by examining the diagram of the 

 motions, say at the near station, it is clearly evident that 

 the period of the back and forth motion rapidly decreased 

 as the motion died out. 



Then illustrations are given, as examples out of a large 

 series of other records, all showing like results. 



Although we must draw a distinction between earth 

 waves and water waves, we yet see that in these points 

 they present a striking likeness. Let us take, for example, 

 any of the large earthquake waves which have originated 

 off the coast of South America, and then radiated out- 

 wards, until they spread across the Pacific, to be recorded 

 in Japan and other countries perhaps twenty-five hours 

 afterwards, at a distance of nearly 9000 miles from their 

 origin. Near this origin they appeared as walls of water, 

 which were seen rapidly advancing towards the coast. 

 These have been from 20 to 200 feet in height, and they 

 succeeded each other at rapid intervals, until finally they 

 died out as gentle waves. By the time these walls of 

 water traversed the Pacific to, let us say, Japan, they 



1 Since my return to Japan in January, 18S3, I may mention that I have 

 commenced series uf observations on earth tremors and earth pulsations, 

 and on several occasions have observed very marked coincidences between 

 barometrical depressions and these movements. Not only are these atmo- 

 spheric changes accompanied with microseismic storms, but there are deflec- 

 tions in the stile of a pendulum, and changes in the p .siti »n of the bulbs of 

 delicate levels, which at such times can be seen with the naked eye to SURGE 

 back and forth through a small range. 



