Vet. 26, 1882] 



NATURE 



629 



1. That the earthquakes in Tokio usually commence 

 gently, the motion is irregular, both as regards space and 

 time, increasing and decreasing. Finally it dies away as 

 it commenced. 



2. There are usually from two to three vibrations per 

 second. Occasionally there may be six or eight. 



3. The maximum amplitude of an earth particle is sel- 

 dom over one millimetre, although buildings may swing 

 through several inches. When the amplitude is four or 

 five millimetres, and the motion rapid, the shock may be 

 dangerous. 



4- During a given shock the direction of motion may 

 change, apparently showing the presence of normal and 

 transverse vibrations. 



5. The motion of the ground inwards towards the 

 origin of the disturbance has in certain cases been much 

 greater than the motion outwards. In this respect the 

 diagram obtained from an actual earthquake closely re- 

 sembles the diagram obtained when we explode a charge 

 of dynamite in a bore hole. 



6 The velocity, and with it the acceleration for the in- 

 ward motion is usually much greater than it is for the 

 outward motion. 



In addition to these characters, earthquake motion has 

 others which are more complex, and are now forming a 

 subject of examination. Thus, for instance, experiment 

 apparently indicates that two neighbouring points of 

 ground (say at the distance of two feet from each other) 

 do not synchronise in their motion. This would indicate 

 that a building, although it may be small, may not be 



\ 



S.35.W. x 



moved back and forth as a whole, but may suffer con- 

 siderable racking. 

 The intervals in time between the actual earthquakes, 



i h h T l aV6 SP J ° CCUr from six t0 ten times per 

 month, have been filled up with experiments upon artifici- 

 ally produced earthquakes, made by exploding charges of 

 dynamite and gunpowder in bore holes. These experi- 

 ments, in which the vibrations of the ground produced by 

 Lmh°r S '7A aVe beCn simulta "eously written down at 

 IT, 1 ff f ent St f 10ns ' have P erha P s b een more 

 instructs than the actual earthquakes. They have been ', 



to seismology what laboratory experiments on magnetism 

 have been to the student of the magnetic phenomena of the 

 earth. Not only have results similar to those which have 

 been enumerated for actual earthquakes been obtained, 

 but also many others. Thus it is found that in the allu- 

 vium of the Tokio plain a surface wave is produced, as 

 might be inferred from the fact that the observation of the 

 horizontal and vertical components of the motion of the 

 ground, do not enable us to deduce angles of emergence 

 for the shock and the depth of its origin. Normal and 

 transversal vibrations have been clearly separated. The 

 effect produced by inequalities in the surface of the ground 

 in cutting off the propagation of vibrations have been 

 studied. Small hills appear to produce but a slight 



effect, whilst cuttings (like a deep pond) are more or less 

 effective in interrupting a disturbance. 



By the comparison of a number of diagrams of earth 

 vibrations, taken simultaneously at different stations, it 

 has been an easy matter to investigate the relative ampli- 

 tudes of different vibrations. Near to the origin of a dis- 

 turbance the amplitude of the normal vibrations was 

 found to be greater than that of the transversal ones, but 

 the former, as they progressed outwards, died out more 

 quickly than the latter. The diminution in the period of 

 these vibrations as they died out at a single station, or as 

 they died out by propagation to distant stations, was also 

 a matter of considerable importance. In connection with 

 this subject it does not seem impossible that when we 

 have a large earthquake, say like that of Lisbon in 1755, 



N.IO.E 



N.60.W. 



that the vibrational period of the disturbance may gradually 

 be so far reduced, although its amplitude may be great, 

 that inhabitants in distant countries may be moved back- 

 wards so slowly that the only indication of the motion 

 would be a slow rising and falling in the waters of their 

 lakes and ponds. 



May not certain disturbances of this nature, like the 

 seiches of Switzerland, usually attributed to variations in 

 atmospheric pressure, be sometimes caused by slow oscil- 

 lations of this description ? 



Sir William Thomson has suggested that Mr. George 

 Darwin should employ the same reasoning to discuss these 

 phenomena as that which he has so well employed to dis- 

 cuss the elastic yielding of the earth in connection with 



