EARTHQUAKE MOTION DEDUCED FEOM OBSERVATION. 77 



detected, and when it is recorded it is seldom more than a 

 millimetre. 



These results, which were put forward some years ago, 

 have since received confirmation by the use of a variety 

 of instruments in the hands of different observers. 



Mallet, in his account of the Neapolitan earthquake of 

 1857, approximated to the amplitude of an earth particle 

 by observing the width, at the level of the centre of 

 gravity, of fissures formed through and remaining in great 

 masses of very inelastic masonry. 



Taking stations situated on or very nearly on the same 

 line passing through the seismic vertical [epicentrum), 

 Mallet observed the amplitude increased as some function 

 of the distance, as will be seen from the following table: — 



station 



Polla 



La Sala 



Certosa 



Tramutola 



Sarconi 



Distance from Seismic x 

 Vertical in geographi- 

 cal miles . . . ' 



Amplitude in inches 



345 

 2-5 



11-60 

 3-5 



16-50 

 40 



20-60 

 4-5 



26-7 

 4-75 



The possibility of a law such as this having an exist- 

 ence for places at a distance from the seismic vertical 

 comparable with the vertical depth of the centrum will be 

 show^n farther on. 



With regard to the niaximum displacement of an 

 earth particle. Mallet was of opinion that there was evi- 

 dence to show that it had in some cases been over one 

 foot. M. Abella, in an earthquake which occurred in the 

 Philippines in 1881, made a rough observation of the 

 motion of the earth to a distance of about tiuo 'metres. 

 This, as might be expected, was beyond the elastic limits 

 of the material, and caused fissures to be formed, which 

 were seen to open and shut. 



Intensity of Earthquakes. — In speaking of the strength 



